Use drain instead of clear (#123)

This will, at least on x86_64, compile down to simpler, shorter assembly that uses a zeroed allocation instead of a regular allocation, a memset and various `raw_vec` methods.

.github/workflows/rust.yml

@@ -37,6 +37,16 @@ jobs:
     - name: Check with all features
       run: cargo check --all-features
 
+  # Make sure the code and its dependencies compile without std.
+  no_std:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - name: Install thumbv6m-none-eabi target
+      run: rustup target add thumbv6m-none-eabi
+    - name: Check no_std build
+      run: cargo check --target thumbv6m-none-eabi --no-default-features --features trace-log,checker,enable-serde
+
   # Lint dependency graph for security advisories, duplicate versions, and
   # incompatible licences.
   cargo_deny:

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "regalloc2"
-version = "0.5.1"
+version = "0.6.1"
 authors = [
     "Chris Fallin <chris@cfallin.org>",
     "Mozilla SpiderMonkey Developers",
@@ -13,11 +13,15 @@ repository = "https://github.com/bytecodealliance/regalloc2"
 [dependencies]
 log = { version = "0.4.8", default-features = false }
 smallvec = { version = "1.6.1", features = ["union"] }
-fxhash = "0.2.1"
-slice-group-by = "0.3.0"
+rustc-hash = { version = "1.1.0", default-features = false }
+slice-group-by = { version = "0.3.0", default-features = false }
+hashbrown = "0.13.2"
 
 # Optional serde support, enabled by feature below.
-serde = { version = "1.0.136", features = ["derive"], optional = true }
+serde = { version = "1.0.136", features = [
+    "derive",
+    "alloc",
+], default-features = false, optional = true }
 
 # The below are only needed for fuzzing.
 libfuzzer-sys = { version = "0.4.2", optional = true }
@@ -29,7 +33,10 @@ debug-assertions = true
 overflow-checks = true
 
 [features]
-default = []
+default = ["std"]
+
+# Enables std-specific features such as the Error trait for RegAllocError.
+std = []
 
 # Enables generation of DefAlloc edits for the checker.
 checker = []

README.md

@@ -3,10 +3,7 @@
 This is a register allocator that started life as, and is about 50%
 still, a port of IonMonkey's backtracking register allocator to
 Rust. In many regards, it has been generalized, optimized, and
-improved since the initial port, and now supports both SSA and non-SSA
-use-cases. (However, non-SSA should be considered deprecated; we want to
-move to SSA-only in the future, to enable some performance improvements.
-See #4.)
+improved since the initial port.
 
 In addition, it contains substantial amounts of testing infrastructure
 (fuzzing harnesses and checkers) that does not exist in the original

src/cfg.rs

@@ -6,6 +6,8 @@
 //! Lightweight CFG analyses.
 
 use crate::{domtree, postorder, Block, Function, Inst, ProgPoint, RegAllocError};
+use alloc::vec;
+use alloc::vec::Vec;
 use smallvec::{smallvec, SmallVec};
 
 #[derive(Clone, Debug)]

src/checker.rs

@@ -96,14 +96,16 @@
 #![allow(dead_code)]
 
 use crate::{
-    Allocation, AllocationKind, Block, Edit, Function, Inst, InstOrEdit, InstPosition, MachineEnv,
-    Operand, OperandConstraint, OperandKind, OperandPos, Output, PReg, PRegSet, VReg,
+    Allocation, AllocationKind, Block, Edit, Function, FxHashMap, FxHashSet, Inst, InstOrEdit,
+    InstPosition, MachineEnv, Operand, OperandConstraint, OperandKind, OperandPos, Output, PReg,
+    PRegSet, VReg,
 };
-use fxhash::{FxHashMap, FxHashSet};
+use alloc::vec::Vec;
+use alloc::{format, vec};
+use core::default::Default;
+use core::hash::Hash;
+use core::result::Result;
 use smallvec::{smallvec, SmallVec};
-use std::default::Default;
-use std::hash::Hash;
-use std::result::Result;
 
 /// A set of errors detected by the regalloc checker.
 #[derive(Clone, Debug)]
@@ -230,7 +232,7 @@ impl CheckerValue {
     }
 
     fn from_reg(reg: VReg) -> CheckerValue {
-        CheckerValue::VRegs(std::iter::once(reg).collect())
+        CheckerValue::VRegs(core::iter::once(reg).collect())
     }
 
     fn remove_vreg(&mut self, reg: VReg) {
@@ -269,10 +271,6 @@ fn visit_all_vregs<F: Function, V: FnMut(VReg)>(f: &F, mut v: V) {
             for op in f.inst_operands(inst) {
                 v(op.vreg());
             }
-            if let Some((src, dst)) = f.is_move(inst) {
-                v(src.vreg());
-                v(dst.vreg());
-            }
             if f.is_branch(inst) {
                 for succ_idx in 0..f.block_succs(block).len() {
                     for &param in f.branch_blockparams(block, inst, succ_idx) {
@@ -377,8 +375,8 @@ impl Default for CheckerState {
     }
 }
 
-impl std::fmt::Display for CheckerValue {
-    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Display for CheckerValue {
+    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
         match self {
             CheckerValue::Universe => {
                 write!(f, "top")
@@ -565,25 +563,6 @@ impl CheckerState {
                 // according to the move semantics in the step
                 // function below.
             }
-            &CheckerInst::ProgramMove { inst, src, dst: _ } => {
-                // Validate that the fixed-reg constraint, if any, on
-                // `src` is satisfied.
-                if let OperandConstraint::FixedReg(preg) = src.constraint() {
-                    let alloc = Allocation::reg(preg);
-                    let val = self.get_value(&alloc).unwrap_or(&default_val);
-                    trace!(
-                        "checker: checkinst {:?}: cheker value in {:?} is {:?}",
-                        checkinst,
-                        alloc,
-                        val
-                    );
-                    self.check_val(inst, src, alloc, val, &[alloc], checker)?;
-                }
-                // Note that we don't do anything with `dst`
-                // here. That is implicitly checked whenever `dst` is
-                // used; the `update()` step below adds the symbolic
-                // vreg for `dst` into wherever `src` may be stored.
-            }
         }
         Ok(())
     }
@@ -686,15 +665,6 @@ impl CheckerState {
                     }
                 }
             }
-            &CheckerInst::ProgramMove { inst: _, src, dst } => {
-                // Remove all earlier instances of `dst`: this vreg is
-                // now stale (it is being overwritten).
-                self.remove_vreg(dst.vreg());
-                // Define `dst` wherever `src` occurs.
-                for (_, value) in self.get_mappings_mut() {
-                    value.copy_vreg(src.vreg(), dst.vreg());
-                }
-            }
         }
     }
 
@@ -786,23 +756,6 @@ pub(crate) enum CheckerInst {
     /// A safepoint, with the given Allocations specified as containing
     /// reftyped values. All other reftyped values become invalid.
     Safepoint { inst: Inst, allocs: Vec<Allocation> },
-
-    /// An op with one source operand, and one dest operand, that
-    /// copies any symbolic values from the source to the dest, in
-    /// addition to adding the symbolic value of the dest vreg to the
-    /// set. This "program move" is distinguished from the above
-    /// `Move` by being semantically relevant in the original
-    /// (pre-regalloc) program.
-    ///
-    /// We transform checker values as follows: for any vreg-set that
-    /// contains `dst`'s vreg, we first delete that vreg (because it
-    /// is being redefined). Then, for any vreg-set with `src`
-    /// present, we add `dst`.
-    ProgramMove {
-        inst: Inst,
-        src: Operand,
-        dst: Operand,
-    },
 }
 
 #[derive(Debug)]
@@ -903,35 +856,10 @@ impl<'a, F: Function> Checker<'a, F> {
             self.bb_insts.get_mut(&block).unwrap().push(checkinst);
         }
 
-        // If this is a move, handle specially. Note that the
-        // regalloc2-inserted moves are not semantically present in
-        // the original program and so do not modify the sets of
-        // symbolic values at all, but rather just move them around;
-        // but "program moves" *are* present, and have the following
-        // semantics: they define the destination vreg, but also
-        // retain any symbolic values in the source.
-        //
-        // regalloc2 reifies all moves into edits in its unified
-        // move/edit framework, so we don't get allocs for these moves
-        // in the post-regalloc output, and the embedder is not
-        // supposed to emit the moves. But we *do* want to check the
-        // semantic implications, namely definition of new vregs. So
-        // we emit `ProgramMove` ops that do just this.
-        if let Some((src, dst)) = self.f.is_move(inst) {
-            let src_op = Operand::any_use(src.vreg());
-            let dst_op = Operand::any_def(dst.vreg());
-            let checkinst = CheckerInst::ProgramMove {
-                inst,
-                src: src_op,
-                dst: dst_op,
-            };
-            trace!("checker: adding inst {:?}", checkinst);
-            self.bb_insts.get_mut(&block).unwrap().push(checkinst);
-        }
         // Skip normal checks if this is a branch: the blockparams do
         // not exist in post-regalloc code, and the edge-moves have to
         // be inserted before the branch rather than after.
-        else if !self.f.is_branch(inst) {
+        if !self.f.is_branch(inst) {
             let operands: Vec<_> = self.f.inst_operands(inst).iter().cloned().collect();
             let allocs: Vec<_> = out.inst_allocs(inst).iter().cloned().collect();
             let clobbers: Vec<_> = self.f.inst_clobbers(inst).into_iter().collect();
@@ -987,11 +915,21 @@ impl<'a, F: Function> Checker<'a, F> {
         let mut queue = Vec::new();
         let mut queue_set = FxHashSet::default();
 
-        queue.push(self.f.entry_block());
-        queue_set.insert(self.f.entry_block());
+        // Put every block in the queue to start with, to ensure
+        // everything is visited even if the initial state remains
+        // `Top` after preds update it.
+        //
+        // We add blocks in reverse order so that when we process
+        // back-to-front below, we do our initial pass in input block
+        // order, which is (usually) RPO order or at least a
+        // reasonable visit order.
+        for block in (0..self.f.num_blocks()).rev() {
+            let block = Block::new(block);
+            queue.push(block);
+            queue_set.insert(block);
+        }
 
-        while !queue.is_empty() {
-            let block = queue.pop().unwrap();
+        while let Some(block) = queue.pop() {
             queue_set.remove(&block);
             let mut state = self.bb_in.get(&block).cloned().unwrap();
             trace!("analyze: block {} has state {:?}", block.index(), state);
@@ -1032,9 +970,8 @@ impl<'a, F: Function> Checker<'a, F> {
                         new_state
                     );
                     self.bb_in.insert(succ, new_state);
-                    if !queue_set.contains(&succ) {
+                    if queue_set.insert(succ) {
                         queue.push(succ);
-                        queue_set.insert(succ);
                     }
                 }
             }
@@ -1119,9 +1056,6 @@ impl<'a, F: Function> Checker<'a, F> {
                         }
                         trace!("    safepoint: {}", slotargs.join(", "));
                     }
-                    &CheckerInst::ProgramMove { inst, src, dst } => {
-                        trace!("    inst{}: prog_move {} -> {}", inst.index(), src, dst);
-                    }
                     &CheckerInst::ParallelMove { .. } => {
                         panic!("unexpected parallel_move in body (non-edge)")
                     }

src/domtree.rs

@@ -12,6 +12,9 @@
 //   TR-06-33870
 //   https://www.cs.rice.edu/~keith/EMBED/dom.pdf
 
+use alloc::vec;
+use alloc::vec::Vec;
+
 use crate::Block;
 
 // Helper

src/fuzzing/func.rs

@@ -8,6 +8,9 @@ use crate::{
     OperandConstraint, OperandKind, OperandPos, PReg, PRegSet, RegClass, VReg,
 };
 
+use alloc::vec::Vec;
+use alloc::{format, vec};
+
 use super::arbitrary::Result as ArbitraryResult;
 use super::arbitrary::{Arbitrary, Unstructured};
 
@@ -124,10 +127,6 @@ impl Function for Func {
         &self.debug_value_labels[..]
     }
 
-    fn is_move(&self, _: Inst) -> Option<(Operand, Operand)> {
-        None
-    }
-
     fn inst_operands(&self, insn: Inst) -> &[Operand] {
         &self.insts[insn.index()].operands[..]
     }
@@ -279,7 +278,7 @@ pub struct Options {
     pub reftypes: bool,
 }
 
-impl std::default::Default for Options {
+impl core::default::Default for Options {
     fn default() -> Self {
         Options {
             reused_inputs: false,
@@ -408,7 +407,7 @@ impl Func {
             }
             vregs_by_block.push(vregs.clone());
             vregs_by_block_to_be_defined.push(vec![]);
-            let mut max_block_params = u.int_in_range(0..=std::cmp::min(3, vregs.len() / 3))?;
+            let mut max_block_params = u.int_in_range(0..=core::cmp::min(3, vregs.len() / 3))?;
             for &vreg in &vregs {
                 if block > 0 && opts.block_params && bool::arbitrary(u)? && max_block_params > 0 {
                     block_params[block].push(vreg);
@@ -595,8 +594,8 @@ impl Func {
     }
 }
 
-impl std::fmt::Debug for Func {
-    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Debug for Func {
+    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
         write!(f, "{{\n")?;
         for vreg in self.reftype_vregs() {
             write!(f, "  REF: {}\n", vreg)?;
@@ -657,16 +656,18 @@ impl std::fmt::Debug for Func {
 }
 
 pub fn machine_env() -> MachineEnv {
-    fn regs(r: std::ops::Range<usize>) -> Vec<PReg> {
+    fn regs(r: core::ops::Range<usize>) -> Vec<PReg> {
         r.map(|i| PReg::new(i, RegClass::Int)).collect()
     }
     let preferred_regs_by_class: [Vec<PReg>; 2] = [regs(0..24), vec![]];
     let non_preferred_regs_by_class: [Vec<PReg>; 2] = [regs(24..32), vec![]];
+    let scratch_by_class: [Option<PReg>; 2] = [None, None];
     let fixed_stack_slots = regs(32..63);
     // Register 63 is reserved for use as a fixed non-allocatable register.
     MachineEnv {
         preferred_regs_by_class,
         non_preferred_regs_by_class,
+        scratch_by_class,
         fixed_stack_slots,
     }
 }

src/index.rs

@@ -50,11 +50,11 @@ macro_rules! define_index {
     };
 }
 
-pub trait ContainerIndex: Clone + Copy + std::fmt::Debug + PartialEq + Eq {}
+pub trait ContainerIndex: Clone + Copy + core::fmt::Debug + PartialEq + Eq {}
 
 pub trait ContainerComparator {
     type Ix: ContainerIndex;
-    fn compare(&self, a: Self::Ix, b: Self::Ix) -> std::cmp::Ordering;
+    fn compare(&self, a: Self::Ix, b: Self::Ix) -> core::cmp::Ordering;
 }
 
 define_index!(Inst);
@@ -146,6 +146,9 @@ impl Iterator for InstRangeIter {
 
 #[cfg(test)]
 mod test {
+    use alloc::vec;
+    use alloc::vec::Vec;
+
     use super::*;
 
     #[test]

src/indexset.rs

@@ -5,8 +5,10 @@
 
 //! Index sets: sets of integers that represent indices into a space.
 
-use fxhash::FxHashMap;
-use std::cell::Cell;
+use alloc::vec::Vec;
+use core::cell::Cell;
+
+use crate::FxHashMap;
 
 const SMALL_ELEMS: usize = 12;
 
@@ -151,10 +153,10 @@ impl AdaptiveMap {
 
 enum AdaptiveMapIter<'a> {
     Small(&'a [u32], &'a [u64]),
-    Large(std::collections::hash_map::Iter<'a, u32, u64>),
+    Large(hashbrown::hash_map::Iter<'a, u32, u64>),
 }
 
-impl<'a> std::iter::Iterator for AdaptiveMapIter<'a> {
+impl<'a> core::iter::Iterator for AdaptiveMapIter<'a> {
     type Item = (u32, u64);
 
     #[inline]
@@ -292,7 +294,7 @@ impl Iterator for SetBitsIter {
         // Build an `Option<NonZeroU64>` so that on the nonzero path,
         // the compiler can optimize the trailing-zeroes operator
         // using that knowledge.
-        std::num::NonZeroU64::new(self.0).map(|nz| {
+        core::num::NonZeroU64::new(self.0).map(|nz| {
             let bitidx = nz.trailing_zeros();
             self.0 &= self.0 - 1; // clear highest set bit
             bitidx as usize
@@ -300,8 +302,8 @@ impl Iterator for SetBitsIter {
     }
 }
 
-impl std::fmt::Debug for IndexSet {
-    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Debug for IndexSet {
+    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
         let vals = self.iter().collect::<Vec<_>>();
         write!(f, "{:?}", vals)
     }

src/ion/data_structures.rs

@@ -17,14 +17,16 @@ use crate::cfg::CFGInfo;
 use crate::index::ContainerComparator;
 use crate::indexset::IndexSet;
 use crate::{
-    define_index, Allocation, Block, Edit, Function, Inst, MachineEnv, Operand, PReg, ProgPoint,
-    RegClass, VReg,
+    define_index, Allocation, Block, Edit, Function, FxHashSet, Inst, MachineEnv, Operand, PReg,
+    ProgPoint, RegClass, VReg,
 };
-use fxhash::FxHashSet;
+use alloc::collections::BTreeMap;
+use alloc::string::String;
+use alloc::vec::Vec;
+use core::cmp::Ordering;
+use core::fmt::Debug;
+use hashbrown::{HashMap, HashSet};
 use smallvec::SmallVec;
-use std::cmp::Ordering;
-use std::collections::{BTreeMap, HashMap, HashSet};
-use std::fmt::Debug;
 
 /// A range from `from` (inclusive) to `to` (exclusive).
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
@@ -64,13 +66,13 @@ impl CodeRange {
     }
 }
 
-impl std::cmp::PartialOrd for CodeRange {
+impl core::cmp::PartialOrd for CodeRange {
     #[inline(always)]
     fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
         Some(self.cmp(other))
     }
 }
-impl std::cmp::Ord for CodeRange {
+impl core::cmp::Ord for CodeRange {
     #[inline(always)]
     fn cmp(&self, other: &Self) -> Ordering {
         if self.to <= other.from {
@@ -278,7 +280,7 @@ const fn no_bloat_capacity<T>() -> usize {
     //
     // So if `size_of([T; N]) == size_of(pointer) + size_of(capacity)` then we
     // get the maximum inline capacity without bloat.
-    std::mem::size_of::<usize>() * 2 / std::mem::size_of::<T>()
+    core::mem::size_of::<usize>() * 2 / core::mem::size_of::<T>()
 }
 
 #[derive(Clone, Debug)]
@@ -405,21 +407,6 @@ pub struct Env<'a, F: Function> {
     pub extra_spillslots_by_class: [SmallVec<[Allocation; 2]>; 2],
     pub preferred_victim_by_class: [PReg; 2],
 
-    // Program moves: these are moves in the provided program that we
-    // handle with our internal machinery, in order to avoid the
-    // overhead of ordinary operand processing. We expect the client
-    // to not generate any code for instructions that return
-    // `Some(..)` for `.is_move()`, and instead use the edits that we
-    // provide to implement those moves (or some simplified version of
-    // them) post-regalloc.
-    //
-    // (from-vreg, inst, from-alloc), sorted by (from-vreg, inst)
-    pub prog_move_srcs: Vec<((VRegIndex, Inst), Allocation)>,
-    // (to-vreg, inst, to-alloc), sorted by (to-vreg, inst)
-    pub prog_move_dsts: Vec<((VRegIndex, Inst), Allocation)>,
-    // (from-vreg, to-vreg) for bundle-merging.
-    pub prog_move_merges: Vec<(LiveRangeIndex, LiveRangeIndex)>,
-
     // When multiple fixed-register constraints are present on a
     // single VReg at a single program point (this can happen for,
     // e.g., call args that use the same value multiple times), we
@@ -446,7 +433,7 @@ pub struct Env<'a, F: Function> {
 
     // For debug output only: a list of textual annotations at every
     // ProgPoint to insert into the final allocated program listing.
-    pub debug_annotations: std::collections::HashMap<ProgPoint, Vec<String>>,
+    pub debug_annotations: hashbrown::HashMap<ProgPoint, Vec<String>>,
     pub annotations_enabled: bool,
 
     // Cached allocation for `try_to_allocate_bundle_to_reg` to avoid allocating
@@ -507,7 +494,7 @@ impl SpillSlotList {
 
 #[derive(Clone, Debug)]
 pub struct PrioQueue {
-    pub heap: std::collections::BinaryHeap<PrioQueueEntry>,
+    pub heap: alloc::collections::BinaryHeap<PrioQueueEntry>,
 }
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
@@ -546,28 +533,28 @@ impl LiveRangeKey {
     }
 }
 
-impl std::cmp::PartialEq for LiveRangeKey {
+impl core::cmp::PartialEq for LiveRangeKey {
     #[inline(always)]
     fn eq(&self, other: &Self) -> bool {
         self.to > other.from && self.from < other.to
     }
 }
-impl std::cmp::Eq for LiveRangeKey {}
-impl std::cmp::PartialOrd for LiveRangeKey {
+impl core::cmp::Eq for LiveRangeKey {}
+impl core::cmp::PartialOrd for LiveRangeKey {
     #[inline(always)]
-    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+    fn partial_cmp(&self, other: &Self) -> Option<core::cmp::Ordering> {
         Some(self.cmp(other))
     }
 }
-impl std::cmp::Ord for LiveRangeKey {
+impl core::cmp::Ord for LiveRangeKey {
     #[inline(always)]
-    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+    fn cmp(&self, other: &Self) -> core::cmp::Ordering {
         if self.to <= other.from {
-            std::cmp::Ordering::Less
+            core::cmp::Ordering::Less
         } else if self.from >= other.to {
-            std::cmp::Ordering::Greater
+            core::cmp::Ordering::Greater
         } else {
-            std::cmp::Ordering::Equal
+            core::cmp::Ordering::Equal
         }
     }
 }
@@ -577,7 +564,7 @@ pub struct PrioQueueComparator<'a> {
 }
 impl<'a> ContainerComparator for PrioQueueComparator<'a> {
     type Ix = LiveBundleIndex;
-    fn compare(&self, a: Self::Ix, b: Self::Ix) -> std::cmp::Ordering {
+    fn compare(&self, a: Self::Ix, b: Self::Ix) -> core::cmp::Ordering {
         self.prios[a.index()].cmp(&self.prios[b.index()])
     }
 }
@@ -585,7 +572,7 @@ impl<'a> ContainerComparator for PrioQueueComparator<'a> {
 impl PrioQueue {
     pub fn new() -> Self {
         PrioQueue {
-            heap: std::collections::BinaryHeap::new(),
+            heap: alloc::collections::BinaryHeap::new(),
         }
     }
 
@@ -628,9 +615,7 @@ pub struct InsertedMove {
 #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
 pub enum InsertMovePrio {
     InEdgeMoves,
-    BlockParam,
     Regular,
-    PostRegular,
     MultiFixedRegInitial,
     MultiFixedRegSecondary,
     ReusedInput,
@@ -660,10 +645,6 @@ pub struct Stats {
     pub livein_iterations: usize,
     pub initial_liverange_count: usize,
     pub merged_bundle_count: usize,
-    pub prog_moves: usize,
-    pub prog_moves_dead_src: usize,
-    pub prog_move_merge_attempt: usize,
-    pub prog_move_merge_success: usize,
     pub process_bundle_count: usize,
     pub process_bundle_reg_probes_fixed: usize,
     pub process_bundle_reg_success_fixed: usize,

src/ion/dump.rs

@@ -1,5 +1,9 @@
 //! Debugging output.
 
+use alloc::string::ToString;
+use alloc::{format, vec};
+use alloc::{string::String, vec::Vec};
+
 use super::Env;
 use crate::{Block, Function, ProgPoint};
 

src/ion/liveranges.rs

@@ -22,13 +22,15 @@ use crate::ion::data_structures::{
     BlockparamIn, BlockparamOut, FixedRegFixupLevel, MultiFixedRegFixup,
 };
 use crate::{
-    Allocation, Block, Function, Inst, InstPosition, Operand, OperandConstraint, OperandKind,
-    OperandPos, PReg, ProgPoint, RegAllocError, VReg,
+    Allocation, Block, Function, FxHashMap, FxHashSet, Inst, InstPosition, Operand,
+    OperandConstraint, OperandKind, OperandPos, PReg, ProgPoint, RegAllocError, VReg,
 };
-use fxhash::{FxHashMap, FxHashSet};
+use alloc::collections::VecDeque;
+use alloc::vec;
+use alloc::vec::Vec;
+use hashbrown::HashSet;
 use slice_group_by::GroupByMut;
 use smallvec::{smallvec, SmallVec};
-use std::collections::{HashSet, VecDeque};
 
 /// A spill weight computed for a certain Use.
 #[derive(Clone, Copy, Debug)]
@@ -42,7 +44,7 @@ pub fn spill_weight_from_constraint(
 ) -> SpillWeight {
     // A bonus of 1000 for one loop level, 4000 for two loop levels,
     // 16000 for three loop levels, etc. Avoids exponentiation.
-    let loop_depth = std::cmp::min(10, loop_depth);
+    let loop_depth = core::cmp::min(10, loop_depth);
     let hot_bonus: f32 = (0..loop_depth).fold(1000.0, |a, _| a * 4.0);
     let def_bonus: f32 = if is_def { 2000.0 } else { 0.0 };
     let constraint_bonus: f32 = match constraint {
@@ -91,7 +93,7 @@ impl SpillWeight {
     }
 }
 
-impl std::ops::Add<SpillWeight> for SpillWeight {
+impl core::ops::Add<SpillWeight> for SpillWeight {
     type Output = SpillWeight;
     fn add(self, other: SpillWeight) -> Self {
         SpillWeight(self.0 + other.0)
@@ -334,8 +336,7 @@ impl<'a, F: Function> Env<'a, F> {
             workqueue_set.insert(block);
         }
 
-        while !workqueue.is_empty() {
-            let block = workqueue.pop_front().unwrap();
+        while let Some(block) = workqueue.pop_front() {
             workqueue_set.remove(&block);
             let insns = self.func.block_insns(block);
 
@@ -357,13 +358,6 @@ impl<'a, F: Function> Env<'a, F> {
             }
 
             for inst in insns.rev().iter() {
-                if let Some((src, dst)) = self.func.is_move(inst) {
-                    live.set(dst.vreg().vreg(), false);
-                    live.set(src.vreg().vreg(), true);
-                    self.observe_vreg_class(src.vreg());
-                    self.observe_vreg_class(dst.vreg());
-                }
-
                 for pos in &[OperandPos::Late, OperandPos::Early] {
                     for op in self.func.inst_operands(inst) {
                         if op.as_fixed_nonallocatable().is_some() {
@@ -520,147 +514,6 @@ impl<'a, F: Function> Env<'a, F> {
                     }
                 }
 
-                // If this is a move, handle specially.
-                if let Some((src, dst)) = self.func.is_move(inst) {
-                    // We can completely skip the move if it is
-                    // trivial (vreg to same vreg).
-                    if src.vreg() != dst.vreg() {
-                        trace!(" -> move inst{}: src {} -> dst {}", inst.index(), src, dst);
-
-                        debug_assert_eq!(src.class(), dst.class());
-                        debug_assert_eq!(src.kind(), OperandKind::Use);
-                        debug_assert_eq!(src.pos(), OperandPos::Early);
-                        debug_assert_eq!(dst.kind(), OperandKind::Def);
-                        debug_assert_eq!(dst.pos(), OperandPos::Late);
-
-                        // Redefine src and dst operands to have
-                        // positions of After and Before respectively
-                        // (see note below), and to have Any
-                        // constraints if they were originally Reg.
-                        let src_constraint = match src.constraint() {
-                            OperandConstraint::Reg => OperandConstraint::Any,
-                            x => x,
-                        };
-                        let dst_constraint = match dst.constraint() {
-                            OperandConstraint::Reg => OperandConstraint::Any,
-                            x => x,
-                        };
-                        let src = Operand::new(
-                            src.vreg(),
-                            src_constraint,
-                            OperandKind::Use,
-                            OperandPos::Late,
-                        );
-                        let dst = Operand::new(
-                            dst.vreg(),
-                            dst_constraint,
-                            OperandKind::Def,
-                            OperandPos::Early,
-                        );
-
-                        if self.annotations_enabled {
-                            self.annotate(
-                                ProgPoint::after(inst),
-                                format!(
-                                    " prog-move v{} ({:?}) -> v{} ({:?})",
-                                    src.vreg().vreg(),
-                                    src_constraint,
-                                    dst.vreg().vreg(),
-                                    dst_constraint,
-                                ),
-                            );
-                        }
-
-                        // N.B.: in order to integrate with the move
-                        // resolution that joins LRs in general, we
-                        // conceptually treat the move as happening
-                        // between the move inst's After and the next
-                        // inst's Before. Thus the src LR goes up to
-                        // (exclusive) next-inst-pre, and the dst LR
-                        // starts at next-inst-pre. We have to take
-                        // care in our move insertion to handle this
-                        // like other inter-inst moves, i.e., at
-                        // `Regular` priority, so it properly happens
-                        // in parallel with other inter-LR moves.
-                        //
-                        // Why the progpoint between move and next
-                        // inst, and not the progpoint between prev
-                        // inst and move? Because a move can be the
-                        // first inst in a block, but cannot be the
-                        // last; so the following progpoint is always
-                        // within the same block, while the previous
-                        // one may be an inter-block point (and the
-                        // After of the prev inst in a different
-                        // block).
-
-                        // Handle the def w.r.t. liveranges: trim the
-                        // start of the range and mark it dead at this
-                        // point in our backward scan.
-                        let pos = ProgPoint::before(inst.next());
-                        let mut dst_lr = vreg_ranges[dst.vreg().vreg()];
-                        if !live.get(dst.vreg().vreg()) {
-                            let from = pos;
-                            let to = pos.next();
-                            dst_lr = self.add_liverange_to_vreg(
-                                VRegIndex::new(dst.vreg().vreg()),
-                                CodeRange { from, to },
-                            );
-                            trace!(" -> invalid LR for def; created {:?}", dst_lr);
-                        }
-                        trace!(" -> has existing LR {:?}", dst_lr);
-                        // Trim the LR to start here.
-                        if self.ranges[dst_lr.index()].range.from
-                            == self.cfginfo.block_entry[block.index()]
-                        {
-                            trace!(" -> started at block start; trimming to {:?}", pos);
-                            self.ranges[dst_lr.index()].range.from = pos;
-                        }
-                        self.ranges[dst_lr.index()].set_flag(LiveRangeFlag::StartsAtDef);
-                        live.set(dst.vreg().vreg(), false);
-                        vreg_ranges[dst.vreg().vreg()] = LiveRangeIndex::invalid();
-
-                        // Handle the use w.r.t. liveranges: make it live
-                        // and create an initial LR back to the start of
-                        // the block.
-                        let pos = ProgPoint::after(inst);
-                        let src_lr = if !live.get(src.vreg().vreg()) {
-                            let range = CodeRange {
-                                from: self.cfginfo.block_entry[block.index()],
-                                to: pos.next(),
-                            };
-                            let src_lr = self
-                                .add_liverange_to_vreg(VRegIndex::new(src.vreg().vreg()), range);
-                            vreg_ranges[src.vreg().vreg()] = src_lr;
-                            src_lr
-                        } else {
-                            vreg_ranges[src.vreg().vreg()]
-                        };
-
-                        trace!(" -> src LR {:?}", src_lr);
-
-                        // Add to live-set.
-                        let src_is_dead_after_move = !live.get(src.vreg().vreg());
-                        live.set(src.vreg().vreg(), true);
-
-                        // Add to program-moves lists.
-                        self.prog_move_srcs.push((
-                            (VRegIndex::new(src.vreg().vreg()), inst),
-                            Allocation::none(),
-                        ));
-                        self.prog_move_dsts.push((
-                            (VRegIndex::new(dst.vreg().vreg()), inst.next()),
-                            Allocation::none(),
-                        ));
-                        self.stats.prog_moves += 1;
-                        if src_is_dead_after_move {
-                            self.stats.prog_moves_dead_src += 1;
-                            self.prog_move_merges.push((src_lr, dst_lr));
-                        }
-                    }
-
-                    continue;
-                }
-
                 // Preprocess defs and uses. Specifically, if there
                 // are any fixed-reg-constrained defs at Late position
                 // and fixed-reg-constrained uses at Early position
@@ -959,12 +812,9 @@ impl<'a, F: Function> Env<'a, F> {
             }
         }
 
-        for range in 0..self.ranges.len() {
-            self.ranges[range].uses.reverse();
-            debug_assert!(self.ranges[range]
-                .uses
-                .windows(2)
-                .all(|win| win[0].pos <= win[1].pos));
+        for range in &mut self.ranges {
+            range.uses.reverse();
+            debug_assert!(range.uses.windows(2).all(|win| win[0].pos <= win[1].pos));
         }
 
         // Insert safepoint virtual stack uses, if needed.
@@ -1019,11 +869,6 @@ impl<'a, F: Function> Env<'a, F> {
 
         self.blockparam_ins.sort_unstable_by_key(|x| x.key());
         self.blockparam_outs.sort_unstable_by_key(|x| x.key());
-        self.prog_move_srcs.sort_unstable_by_key(|(pos, _)| *pos);
-        self.prog_move_dsts.sort_unstable_by_key(|(pos, _)| *pos);
-
-        trace!("prog_move_srcs = {:?}", self.prog_move_srcs);
-        trace!("prog_move_dsts = {:?}", self.prog_move_dsts);
 
         self.stats.initial_liverange_count = self.ranges.len();
         self.stats.blockparam_ins_count = self.blockparam_ins.len();
@@ -1032,7 +877,7 @@ impl<'a, F: Function> Env<'a, F> {
 
     pub fn fixup_multi_fixed_vregs(&mut self) {
         // Do a fixed-reg cleanup pass: if there are any LiveRanges with
-        // multiple uses (or defs) at the same ProgPoint and there is
+        // multiple uses at the same ProgPoint and there is
         // more than one FixedReg constraint at that ProgPoint, we
         // need to record all but one of them in a special fixup list
         // and handle them later; otherwise, bundle-splitting to
@@ -1154,15 +999,13 @@ impl<'a, F: Function> Env<'a, F> {
                     }
                 }
 
-                for &(clobber, pos) in &extra_clobbers {
+                for (clobber, pos) in extra_clobbers.drain(..) {
                     let range = CodeRange {
                         from: pos,
                         to: pos.next(),
                     };
                     self.add_liverange_to_preg(range, clobber);
                 }
-
-                extra_clobbers.clear();
             }
         }
     }

src/ion/merge.rs

@@ -18,6 +18,7 @@ use super::{
 use crate::{
     ion::data_structures::BlockparamOut, Function, Inst, OperandConstraint, OperandKind, PReg,
 };
+use alloc::format;
 use smallvec::smallvec;
 
 impl<'a, F: Function> Env<'a, F> {
@@ -132,7 +133,7 @@ impl<'a, F: Function> Env<'a, F> {
             // `to` bundle is empty -- just move the list over from
             // `from` and set `bundle` up-link on all ranges.
             trace!(" -> to bundle{} is empty; trivial merge", to.index());
-            let list = std::mem::replace(&mut self.bundles[from.index()].ranges, smallvec![]);
+            let list = core::mem::replace(&mut self.bundles[from.index()].ranges, smallvec![]);
             for entry in &list {
                 self.ranges[entry.index.index()].bundle = to;
 
@@ -170,7 +171,7 @@ impl<'a, F: Function> Env<'a, F> {
         // Two non-empty lists of LiveRanges: concatenate and
         // sort. This is faster than a mergesort-like merge into a new
         // list, empirically.
-        let from_list = std::mem::replace(&mut self.bundles[from.index()].ranges, smallvec![]);
+        let from_list = core::mem::replace(&mut self.bundles[from.index()].ranges, smallvec![]);
         for entry in &from_list {
             self.ranges[entry.index.index()].bundle = to;
         }
@@ -213,7 +214,7 @@ impl<'a, F: Function> Env<'a, F> {
         }
 
         if self.bundles[from.index()].spillset != self.bundles[to.index()].spillset {
-            let from_vregs = std::mem::replace(
+            let from_vregs = core::mem::replace(
                 &mut self.spillsets[self.bundles[from.index()].spillset.index()].vregs,
                 smallvec![],
             );
@@ -351,28 +352,6 @@ impl<'a, F: Function> Env<'a, F> {
             self.merge_bundles(from_bundle, to_bundle);
         }
 
-        // Attempt to merge move srcs/dsts.
-        for i in 0..self.prog_move_merges.len() {
-            let (src, dst) = self.prog_move_merges[i];
-            trace!("trying to merge move src LR {:?} to dst LR {:?}", src, dst);
-            let src = self.resolve_merged_lr(src);
-            let dst = self.resolve_merged_lr(dst);
-            trace!(
-                "resolved LR-construction merging chains: move-merge is now src LR {:?} to dst LR {:?}",
-                src,
-                dst
-            );
-
-            let src_bundle = self.ranges[src.index()].bundle;
-            debug_assert!(src_bundle.is_valid());
-            let dest_bundle = self.ranges[dst.index()].bundle;
-            debug_assert!(dest_bundle.is_valid());
-            self.stats.prog_move_merge_attempt += 1;
-            if self.merge_bundles(/* from */ dest_bundle, /* to */ src_bundle) {
-                self.stats.prog_move_merge_success += 1;
-            }
-        }
-
         trace!("done merging bundles");
     }
 

src/ion/mod.rs

@@ -16,7 +16,9 @@
 use crate::cfg::CFGInfo;
 use crate::ssa::validate_ssa;
 use crate::{Function, MachineEnv, Output, PReg, ProgPoint, RegAllocError, RegClass};
-use std::collections::HashMap;
+use alloc::vec;
+use alloc::vec::Vec;
+use hashbrown::HashMap;
 
 pub(crate) mod data_structures;
 pub use data_structures::Stats;
@@ -71,10 +73,6 @@ impl<'a, F: Function> Env<'a, F> {
             extra_spillslots_by_class: [smallvec![], smallvec![]],
             preferred_victim_by_class: [PReg::invalid(), PReg::invalid()],
 
-            prog_move_srcs: Vec::with_capacity(n / 2),
-            prog_move_dsts: Vec::with_capacity(n / 2),
-            prog_move_merges: Vec::with_capacity(n / 2),
-
             multi_fixed_reg_fixups: vec![],
             inserted_moves: vec![],
             edits: Vec::with_capacity(n),
@@ -86,7 +84,7 @@ impl<'a, F: Function> Env<'a, F> {
 
             stats: Stats::default(),
 
-            debug_annotations: std::collections::HashMap::new(),
+            debug_annotations: hashbrown::HashMap::new(),
             annotations_enabled,
 
             conflict_set: Default::default(),

src/ion/moves.rs

@@ -22,12 +22,13 @@ use crate::ion::data_structures::{
 use crate::ion::reg_traversal::RegTraversalIter;
 use crate::moves::{MoveAndScratchResolver, ParallelMoves};
 use crate::{
-    Allocation, Block, Edit, Function, Inst, InstPosition, OperandConstraint, OperandKind,
-    OperandPos, PReg, ProgPoint, RegClass, SpillSlot, VReg,
+    Allocation, Block, Edit, Function, FxHashMap, Inst, InstPosition, OperandConstraint,
+    OperandKind, OperandPos, PReg, ProgPoint, RegClass, SpillSlot, VReg,
 };
-use fxhash::FxHashMap;
+use alloc::vec::Vec;
+use alloc::{format, vec};
+use core::fmt::Debug;
 use smallvec::{smallvec, SmallVec};
-use std::fmt::Debug;
 
 impl<'a, F: Function> Env<'a, F> {
     pub fn is_start_of_block(&self, pos: ProgPoint) -> bool {
@@ -179,8 +180,6 @@ impl<'a, F: Function> Env<'a, F> {
 
         let mut blockparam_in_idx = 0;
         let mut blockparam_out_idx = 0;
-        let mut prog_move_src_idx = 0;
-        let mut prog_move_dst_idx = 0;
         for vreg in 0..self.vregs.len() {
             let vreg = VRegIndex::new(vreg);
             if !self.is_vreg_used(vreg) {
@@ -190,7 +189,7 @@ impl<'a, F: Function> Env<'a, F> {
             // For each range in each vreg, insert moves or
             // half-moves.  We also scan over `blockparam_ins` and
             // `blockparam_outs`, which are sorted by (block, vreg),
-            // and over program-move srcs/dsts to fill in allocations.
+            // to fill in allocations.
             let mut prev = LiveRangeIndex::invalid();
             for range_idx in 0..self.vregs[vreg.index()].ranges.len() {
                 let entry = self.vregs[vreg.index()].ranges[range_idx];
@@ -496,9 +495,9 @@ impl<'a, F: Function> Env<'a, F> {
                             // this case returns the index of the first
                             // entry that is greater as an `Err`.
                             if label_vreg.vreg() < vreg.index() {
-                                std::cmp::Ordering::Less
+                                core::cmp::Ordering::Less
                             } else {
-                                std::cmp::Ordering::Greater
+                                core::cmp::Ordering::Greater
                             }
                         })
                         .unwrap_err();
@@ -517,96 +516,13 @@ impl<'a, F: Function> Env<'a, F> {
                             continue;
                         }
 
-                        let from = std::cmp::max(label_from, range.from);
-                        let to = std::cmp::min(label_to, range.to);
+                        let from = core::cmp::max(label_from, range.from);
+                        let to = core::cmp::min(label_to, range.to);
 
                         self.debug_locations.push((label, from, to, alloc));
                     }
                 }
 
-                // Scan over program move srcs/dsts to fill in allocations.
-
-                // Move srcs happen at `After` of a given
-                // inst. Compute [from, to) semi-inclusive range of
-                // inst indices for which we should fill in the source
-                // with this LR's allocation.
-                //
-                // range from inst-Before or inst-After covers cur
-                // inst's After; so includes move srcs from inst.
-                let move_src_start = (vreg, range.from.inst());
-                // range to (exclusive) inst-Before or inst-After
-                // covers only prev inst's After; so includes move
-                // srcs to (exclusive) inst.
-                let move_src_end = (vreg, range.to.inst());
-                trace!(
-                    "vreg {:?} range {:?}: looking for program-move sources from {:?} to {:?}",
-                    vreg,
-                    range,
-                    move_src_start,
-                    move_src_end
-                );
-                while prog_move_src_idx < self.prog_move_srcs.len()
-                    && self.prog_move_srcs[prog_move_src_idx].0 < move_src_start
-                {
-                    trace!(" -> skipping idx {}", prog_move_src_idx);
-                    prog_move_src_idx += 1;
-                }
-                while prog_move_src_idx < self.prog_move_srcs.len()
-                    && self.prog_move_srcs[prog_move_src_idx].0 < move_src_end
-                {
-                    trace!(
-                        " -> setting idx {} ({:?}) to alloc {:?}",
-                        prog_move_src_idx,
-                        self.prog_move_srcs[prog_move_src_idx].0,
-                        alloc
-                    );
-                    self.prog_move_srcs[prog_move_src_idx].1 = alloc;
-                    prog_move_src_idx += 1;
-                }
-
-                // move dsts happen at Before point.
-                //
-                // Range from inst-Before includes cur inst, while inst-After includes only next inst.
-                let move_dst_start = if range.from.pos() == InstPosition::Before {
-                    (vreg, range.from.inst())
-                } else {
-                    (vreg, range.from.inst().next())
-                };
-                // Range to (exclusive) inst-Before includes prev
-                // inst, so to (exclusive) cur inst; range to
-                // (exclusive) inst-After includes cur inst, so to
-                // (exclusive) next inst.
-                let move_dst_end = if range.to.pos() == InstPosition::Before {
-                    (vreg, range.to.inst())
-                } else {
-                    (vreg, range.to.inst().next())
-                };
-                trace!(
-                    "vreg {:?} range {:?}: looking for program-move dests from {:?} to {:?}",
-                    vreg,
-                    range,
-                    move_dst_start,
-                    move_dst_end
-                );
-                while prog_move_dst_idx < self.prog_move_dsts.len()
-                    && self.prog_move_dsts[prog_move_dst_idx].0 < move_dst_start
-                {
-                    trace!(" -> skipping idx {}", prog_move_dst_idx);
-                    prog_move_dst_idx += 1;
-                }
-                while prog_move_dst_idx < self.prog_move_dsts.len()
-                    && self.prog_move_dsts[prog_move_dst_idx].0 < move_dst_end
-                {
-                    trace!(
-                        " -> setting idx {} ({:?}) to alloc {:?}",
-                        prog_move_dst_idx,
-                        self.prog_move_dsts[prog_move_dst_idx].0,
-                        alloc
-                    );
-                    self.prog_move_dsts[prog_move_dst_idx].1 = alloc;
-                    prog_move_dst_idx += 1;
-                }
-
                 prev = entry.index;
             }
         }
@@ -714,7 +630,7 @@ impl<'a, F: Function> Env<'a, F> {
         }
 
         // Handle multi-fixed-reg constraints by copying.
-        for fixup in std::mem::replace(&mut self.multi_fixed_reg_fixups, vec![]) {
+        for fixup in core::mem::replace(&mut self.multi_fixed_reg_fixups, vec![]) {
             let from_alloc = self.get_alloc(fixup.pos.inst(), fixup.from_slot as usize);
             let to_alloc = Allocation::reg(PReg::from_index(fixup.to_preg.index()));
             trace!(
@@ -820,42 +736,6 @@ impl<'a, F: Function> Env<'a, F> {
             }
         }
 
-        // Sort the prog-moves lists and insert moves to reify the
-        // input program's move operations.
-        self.prog_move_srcs
-            .sort_unstable_by_key(|((_, inst), _)| *inst);
-        self.prog_move_dsts
-            .sort_unstable_by_key(|((_, inst), _)| inst.prev());
-        let prog_move_srcs = std::mem::replace(&mut self.prog_move_srcs, vec![]);
-        let prog_move_dsts = std::mem::replace(&mut self.prog_move_dsts, vec![]);
-        debug_assert_eq!(prog_move_srcs.len(), prog_move_dsts.len());
-        for (&((_, from_inst), from_alloc), &((to_vreg, to_inst), to_alloc)) in
-            prog_move_srcs.iter().zip(prog_move_dsts.iter())
-        {
-            trace!(
-                "program move at inst {:?}: alloc {:?} -> {:?} (v{})",
-                from_inst,
-                from_alloc,
-                to_alloc,
-                to_vreg.index(),
-            );
-            debug_assert!(from_alloc.is_some());
-            debug_assert!(to_alloc.is_some());
-            debug_assert_eq!(from_inst, to_inst.prev());
-            // N.B.: these moves happen with the *same* priority as
-            // LR-to-LR moves, because they work just like them: they
-            // connect a use at one progpoint (move-After) with a def
-            // at an adjacent progpoint (move+1-Before), so they must
-            // happen in parallel with all other LR-to-LR moves.
-            self.insert_move(
-                ProgPoint::before(to_inst),
-                InsertMovePrio::Regular,
-                from_alloc,
-                to_alloc,
-                self.vreg(to_vreg),
-            );
-        }
-
         // Sort the debug-locations vector; we provide this
         // invariant to the client.
         self.debug_locations.sort_unstable();
@@ -986,6 +866,9 @@ impl<'a, F: Function> Env<'a, F> {
                     to: pos_prio.pos.next(),
                 });
                 let get_reg = || {
+                    if let Some(reg) = self.env.scratch_by_class[regclass as usize] {
+                        return Some(Allocation::reg(reg));
+                    }
                     while let Some(preg) = scratch_iter.next() {
                         if !self.pregs[preg.index()]
                             .allocations

src/ion/process.rs

@@ -22,12 +22,11 @@ use crate::{
         CodeRange, BUNDLE_MAX_NORMAL_SPILL_WEIGHT, MAX_SPLITS_PER_SPILLSET,
         MINIMAL_BUNDLE_SPILL_WEIGHT, MINIMAL_FIXED_BUNDLE_SPILL_WEIGHT,
     },
-    Allocation, Function, Inst, InstPosition, OperandConstraint, OperandKind, PReg, ProgPoint,
-    RegAllocError,
+    Allocation, Function, FxHashSet, Inst, InstPosition, OperandConstraint, OperandKind, PReg,
+    ProgPoint, RegAllocError,
 };
-use fxhash::FxHashSet;
+use core::fmt::Debug;
 use smallvec::{smallvec, SmallVec};
-use std::fmt::Debug;
 
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub enum AllocRegResult {
@@ -159,7 +158,7 @@ impl<'a, F: Function> Env<'a, F> {
                     trace!("   -> conflict bundle {:?}", conflict_bundle);
                     if self.conflict_set.insert(conflict_bundle) {
                         conflicts.push(conflict_bundle);
-                        max_conflict_weight = std::cmp::max(
+                        max_conflict_weight = core::cmp::max(
                             max_conflict_weight,
                             self.bundles[conflict_bundle.index()].cached_spill_weight(),
                         );
@@ -172,7 +171,7 @@ impl<'a, F: Function> Env<'a, F> {
                     }
 
                     if first_conflict.is_none() {
-                        first_conflict = Some(ProgPoint::from_index(std::cmp::max(
+                        first_conflict = Some(ProgPoint::from_index(core::cmp::max(
                             preg_key.from,
                             key.from,
                         )));
@@ -334,7 +333,7 @@ impl<'a, F: Function> Env<'a, F> {
                     self.bundles[bundle.index()].prio,
                     final_weight
                 );
-                std::cmp::min(BUNDLE_MAX_NORMAL_SPILL_WEIGHT, final_weight)
+                core::cmp::min(BUNDLE_MAX_NORMAL_SPILL_WEIGHT, final_weight)
             } else {
                 0
             }
@@ -824,7 +823,7 @@ impl<'a, F: Function> Env<'a, F> {
                 // (up to the Before of the next inst), *unless*
                 // the original LR was only over the Before (up to
                 // the After) of this inst.
-                let to = std::cmp::min(ProgPoint::before(u.pos.inst().next()), lr_to);
+                let to = core::cmp::min(ProgPoint::before(u.pos.inst().next()), lr_to);
 
                 // If the last bundle was at the same inst, add a new
                 // LR to the same bundle; otherwise, create a LR and a
@@ -863,7 +862,7 @@ impl<'a, F: Function> Env<'a, F> {
 
                 // Otherwise, create a new LR.
                 let pos = ProgPoint::before(u.pos.inst());
-                let pos = std::cmp::max(lr_from, pos);
+                let pos = core::cmp::max(lr_from, pos);
                 let cr = CodeRange { from: pos, to };
                 let lr = self.create_liverange(cr);
                 new_lrs.push((vreg, lr));
@@ -1036,7 +1035,7 @@ impl<'a, F: Function> Env<'a, F> {
                     self.get_or_create_spill_bundle(bundle, /* create_if_absent = */ false)
                 {
                     let mut list =
-                        std::mem::replace(&mut self.bundles[bundle.index()].ranges, smallvec![]);
+                        core::mem::replace(&mut self.bundles[bundle.index()].ranges, smallvec![]);
                     for entry in &list {
                         self.ranges[entry.index.index()].bundle = spill;
                     }
@@ -1107,7 +1106,7 @@ impl<'a, F: Function> Env<'a, F> {
                     lowest_cost_evict_conflict_cost,
                     lowest_cost_split_conflict_cost,
                 ) {
-                    (Some(a), Some(b)) => Some(std::cmp::max(a, b)),
+                    (Some(a), Some(b)) => Some(core::cmp::max(a, b)),
                     _ => None,
                 };
                 match self.try_to_allocate_bundle_to_reg(bundle, preg_idx, scan_limit_cost) {
@@ -1291,7 +1290,7 @@ impl<'a, F: Function> Env<'a, F> {
                 );
                 let bundle_start = self.bundles[bundle.index()].ranges[0].range.from;
                 let mut split_at_point =
-                    std::cmp::max(lowest_cost_split_conflict_point, bundle_start);
+                    core::cmp::max(lowest_cost_split_conflict_point, bundle_start);
                 let requeue_with_reg = lowest_cost_split_conflict_reg;
 
                 // Adjust `split_at_point` if it is within a deeper loop

src/ion/redundant_moves.rs

@@ -1,7 +1,6 @@
 //! Redundant-move elimination.
 
-use crate::{Allocation, VReg};
-use fxhash::FxHashMap;
+use crate::{Allocation, FxHashMap, VReg};
 use smallvec::{smallvec, SmallVec};
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
@@ -112,9 +111,9 @@ impl RedundantMoveEliminator {
     pub fn clear_alloc(&mut self, alloc: Allocation) {
         trace!("   redundant move eliminator: clear {:?}", alloc);
         if let Some(ref mut existing_copies) = self.reverse_allocs.get_mut(&alloc) {
-            for to_inval in existing_copies.iter() {
+            for to_inval in existing_copies.drain(..) {
                 trace!("     -> clear existing copy: {:?}", to_inval);
-                if let Some(val) = self.allocs.get_mut(to_inval) {
+                if let Some(val) = self.allocs.get_mut(&to_inval) {
                     match val {
                         RedundantMoveState::Copy(_, Some(vreg)) => {
                             *val = RedundantMoveState::Orig(*vreg);
@@ -122,9 +121,8 @@ impl RedundantMoveEliminator {
                         _ => *val = RedundantMoveState::None,
                     }
                 }
-                self.allocs.remove(to_inval);
+                self.allocs.remove(&to_inval);
             }
-            existing_copies.clear();
         }
         self.allocs.remove(&alloc);
     }

src/ion/reg_traversal.rs

@@ -78,7 +78,7 @@ impl<'a> RegTraversalIter<'a> {
     }
 }
 
-impl<'a> std::iter::Iterator for RegTraversalIter<'a> {
+impl<'a> core::iter::Iterator for RegTraversalIter<'a> {
     type Item = PReg;
 
     fn next(&mut self) -> Option<PReg> {

src/ion/spill.rs

@@ -138,7 +138,7 @@ impl<'a, F: Function> Env<'a, F> {
             let mut success = false;
             // Never probe the same element more than once: limit the
             // attempt count to the number of slots in existence.
-            for _attempt in 0..std::cmp::min(self.slots_by_size[size].slots.len(), MAX_ATTEMPTS) {
+            for _attempt in 0..core::cmp::min(self.slots_by_size[size].slots.len(), MAX_ATTEMPTS) {
                 // Note: this indexing of `slots` is always valid
                 // because either the `slots` list is empty and the
                 // iteration limit above consequently means we don't

src/ion/stackmap.rs

@@ -12,6 +12,8 @@
 
 //! Stackmap computation.
 
+use alloc::vec::Vec;
+
 use super::{Env, ProgPoint, VRegIndex};
 use crate::{ion::data_structures::u64_key, Function};
 

src/lib.rs

@@ -11,6 +11,12 @@
  */
 
 #![allow(dead_code)]
+#![no_std]
+
+#[cfg(feature = "std")]
+extern crate std;
+
+extern crate alloc;
 
 // Even when trace logging is disabled, the trace macro has a significant
 // performance cost so we disable it in release builds.
@@ -28,6 +34,11 @@ macro_rules! trace_enabled {
     };
 }
 
+use core::hash::BuildHasherDefault;
+use rustc_hash::FxHasher;
+type FxHashMap<K, V> = hashbrown::HashMap<K, V, BuildHasherDefault<FxHasher>>;
+type FxHashSet<V> = hashbrown::HashSet<V, BuildHasherDefault<FxHasher>>;
+
 pub(crate) mod cfg;
 pub(crate) mod domtree;
 pub mod indexset;
@@ -38,6 +49,8 @@ pub mod ssa;
 
 #[macro_use]
 mod index;
+
+use alloc::vec::Vec;
 pub use index::{Block, Inst, InstRange, InstRangeIter};
 
 pub mod checker;
@@ -142,8 +155,8 @@ impl PReg {
     }
 }
 
-impl std::fmt::Debug for PReg {
-    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Debug for PReg {
+    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
         write!(
             f,
             "PReg(hw = {}, class = {:?}, index = {})",
@@ -154,8 +167,8 @@ impl std::fmt::Debug for PReg {
     }
 }
 
-impl std::fmt::Display for PReg {
-    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Display for PReg {
+    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
         let class = match self.class() {
             RegClass::Int => "i",
             RegClass::Float => "f",
@@ -266,8 +279,7 @@ impl From<&MachineEnv> for PRegSet {
 /// A virtual register. Contains a virtual register number and a
 /// class.
 ///
-/// A virtual register ("vreg") corresponds to an SSA value for SSA
-/// input, or just a register when we allow for non-SSA input. All
+/// A virtual register ("vreg") corresponds to an SSA value. All
 /// dataflow in the input program is specified via flow through a
 /// virtual register; even uses of specially-constrained locations,
 /// such as fixed physical registers, are done by using vregs, because
@@ -312,8 +324,8 @@ impl VReg {
     }
 }
 
-impl std::fmt::Debug for VReg {
-    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Debug for VReg {
+    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
         write!(
             f,
             "VReg(vreg = {}, class = {:?})",
@@ -323,8 +335,8 @@ impl std::fmt::Debug for VReg {
     }
 }
 
-impl std::fmt::Display for VReg {
-    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Display for VReg {
+    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
         write!(f, "v{}", self.vreg())
     }
 }
@@ -383,8 +395,8 @@ impl SpillSlot {
     }
 }
 
-impl std::fmt::Display for SpillSlot {
-    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Display for SpillSlot {
+    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
         write!(f, "stack{}", self.index())
     }
 }
@@ -414,8 +426,8 @@ pub enum OperandConstraint {
     Reuse(usize),
 }
 
-impl std::fmt::Display for OperandConstraint {
-    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Display for OperandConstraint {
+    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
         match self {
             Self::Any => write!(f, "any"),
             Self::Reg => write!(f, "reg"),
@@ -797,14 +809,14 @@ impl Operand {
     }
 }
 
-impl std::fmt::Debug for Operand {
-    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
-        std::fmt::Display::fmt(self, f)
+impl core::fmt::Debug for Operand {
+    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
+        core::fmt::Display::fmt(self, f)
     }
 }
 
-impl std::fmt::Display for Operand {
-    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Display for Operand {
+    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
         match (self.kind(), self.pos()) {
             (OperandKind::Def, OperandPos::Late) | (OperandKind::Use, OperandPos::Early) => {
                 write!(f, "{:?}", self.kind())?;
@@ -837,14 +849,14 @@ pub struct Allocation {
     bits: u32,
 }
 
-impl std::fmt::Debug for Allocation {
-    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
-        std::fmt::Display::fmt(self, f)
+impl core::fmt::Debug for Allocation {
+    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
+        core::fmt::Display::fmt(self, f)
     }
 }
 
-impl std::fmt::Display for Allocation {
-    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Display for Allocation {
+    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
         match self.kind() {
             AllocationKind::None => write!(f, "none"),
             AllocationKind::Reg => write!(f, "{}", self.as_reg().unwrap()),
@@ -1029,10 +1041,6 @@ pub trait Function {
         false
     }
 
-    /// Determine whether an instruction is a move; if so, return the
-    /// Operands for (src, dst).
-    fn is_move(&self, insn: Inst) -> Option<(Operand, Operand)>;
-
     // --------------------------
     // Instruction register slots
     // --------------------------
@@ -1178,8 +1186,8 @@ pub struct ProgPoint {
     bits: u32,
 }
 
-impl std::fmt::Debug for ProgPoint {
-    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Debug for ProgPoint {
+    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
         write!(
             f,
             "progpoint{}{}",
@@ -1326,19 +1334,43 @@ impl<'a> Iterator for OutputIter<'a> {
 pub struct MachineEnv {
     /// Preferred physical registers for each class. These are the
     /// registers that will be allocated first, if free.
+    ///
+    /// If an explicit scratch register is provided in `scratch_by_class` then
+    /// it must not appear in this list.
     pub preferred_regs_by_class: [Vec<PReg>; 2],
 
     /// Non-preferred physical registers for each class. These are the
     /// registers that will be allocated if a preferred register is
     /// not available; using one of these is considered suboptimal,
     /// but still better than spilling.
+    ///
+    /// If an explicit scratch register is provided in `scratch_by_class` then
+    /// it must not appear in this list.
     pub non_preferred_regs_by_class: [Vec<PReg>; 2],
 
+    /// Optional dedicated scratch register per class. This is needed to perform
+    /// moves between registers when cyclic move patterns occur. The
+    /// register should not be placed in either the preferred or
+    /// non-preferred list (i.e., it is not otherwise allocatable).
+    ///
+    /// Note that the register allocator will freely use this register
+    /// between instructions, but *within* the machine code generated
+    /// by a single (regalloc-level) instruction, the client is free
+    /// to use the scratch register. E.g., if one "instruction" causes
+    /// the emission of two machine-code instructions, this lowering
+    /// can use the scratch register between them.
+    ///
+    /// If a scratch register is not provided then the register allocator will
+    /// automatically allocate one as needed, spilling a value to the stack if
+    /// necessary.
+    pub scratch_by_class: [Option<PReg>; 2],
+
     /// Some `PReg`s can be designated as locations on the stack rather than
     /// actual registers. These can be used to tell the register allocator about
     /// pre-defined stack slots used for function arguments and return values.
     ///
-    /// `PReg`s in this list cannot be used as an allocatable register.
+    /// `PReg`s in this list cannot be used as an allocatable or scratch
+    /// register.
     pub fixed_stack_slots: Vec<PReg>,
 }
 
@@ -1403,9 +1435,9 @@ impl Output {
                 // binary_search_by returns the index of where it would have
                 // been inserted in Err.
                 if pos < ProgPoint::before(inst_range.first()) {
-                    std::cmp::Ordering::Less
+                    core::cmp::Ordering::Less
                 } else {
-                    std::cmp::Ordering::Greater
+                    core::cmp::Ordering::Greater
                 }
             })
             .unwrap_err();
@@ -1444,12 +1476,13 @@ pub enum RegAllocError {
     TooManyLiveRegs,
 }
 
-impl std::fmt::Display for RegAllocError {
-    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Display for RegAllocError {
+    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
         write!(f, "{:?}", self)
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for RegAllocError {}
 
 /// Run the allocator.

src/moves.rs

@@ -4,8 +4,8 @@
  */
 
 use crate::{ion::data_structures::u64_key, Allocation, PReg};
+use core::fmt::Debug;
 use smallvec::{smallvec, SmallVec};
-use std::fmt::Debug;
 
 /// A list of moves to be performed in sequence, with auxiliary data
 /// attached to each.

src/postorder.rs

@@ -6,6 +6,8 @@
 //! Fast postorder computation.
 
 use crate::Block;
+use alloc::vec;
+use alloc::vec::Vec;
 use smallvec::{smallvec, SmallVec};
 
 pub fn calculate<'a, SuccFn: Fn(Block) -> &'a [Block]>(
@@ -16,8 +18,7 @@ pub fn calculate<'a, SuccFn: Fn(Block) -> &'a [Block]>(
     let mut ret = vec![];
 
     // State: visited-block map, and explicit DFS stack.
-    let mut visited = vec![];
-    visited.resize(num_blocks, false);
+    let mut visited = vec![false; num_blocks];
 
     struct State<'a> {
         block: Block,

src/ssa.rs

@@ -5,7 +5,8 @@
 
 //! SSA-related utilities.
 
-use std::collections::HashSet;
+use alloc::vec;
+use hashbrown::HashSet;
 
 use crate::cfg::CFGInfo;
 use crate::{Block, Function, Inst, OperandKind, RegAllocError, VReg};