52818a7ed6
* Extend fuzzer to generate cases like #53. Currently, the fuzz testcase generator will add at most one fixed-register constraint to an instruction per physical register. This avoids impossible situations, such as specifying that both `v0` and `v1` must be placed into the same `p0`. However, it *should* be possible to say that `v0` is in `p0` before the instruction, and `v1` is in `p0` after the instruction (i.e., at `Early` and `Late` operand positions). This in fact exposes a limitation in the current allocator design: when `v0` is live downward, with the above constraints, it will result in an impossible allocation situation because we cannot split in the middle of an instruction. A subsequent fix will rectify this by using the multi-fixed-reg fixup mechanism. * Handle conflicting Before and After fixed-reg constraints with a copy. This fixes #53. Previously, if two operands on an instruction specified *different* vregs constrained to the same physical register at the Before (Early) and After (Late) points of the instruction, and the Before was live downward as well, we would panic: we can't insert a move into the middle of an instruction, so putting the first vreg in the preg at Early implies we have an unsolveable conflict at Late. We can solve this issue by adding some new logic to insert a copy, and rewrite the constraint. This reuses the multi-fixed-reg-constraint fixup logic. While that logic handles the case where the *same* vreg has multiple *different* fixed-reg constraints, this new logic handles *different* vregs with the *same* fixed-reg constraints, but at different *program points*; so the two are complementary. This addresses the specific test case in #53, and also fuzzes cleanly with the change to the fuzz testcase generator to generate these cases (which also immediately found the bug). * Add a reservation to the PReg when rewriting constraint so it is not doubly-allocated. * Distinguish initial fixup moves from secondary moves. * Use `trace` macro, not `log::trace`, to avoid trace output when feature is disabled. * Rework operand rewriting to properly handle bundle-merging edge case. When the liverange for the defined vreg with fixed constraint at Late is *merged* with the liverange for the used vreg with fixed constraint at Early, the strategy of putting a fixed reservation on the preg at Early fails, because the whole bundle is minimal (if it spans just the instruction's Early and Late and nothing else). This could happen if e.g. the def flows into a blockparam arg that merges with a blockparam defining the used value. Instead we move the def one halfstep earlier, to the Early point, with its fixed-reg constraint still in place. This has the same effect but works when the two are merged. * Fix checker issue: make more flexible in the presence of victim-register saves.
429 lines
17 KiB
Rust
429 lines
17 KiB
Rust
/*
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* Released under the terms of the Apache 2.0 license with LLVM
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* exception. See `LICENSE` for details.
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*/
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use crate::{ion::data_structures::u64_key, Allocation, PReg};
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use smallvec::{smallvec, SmallVec};
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use std::fmt::Debug;
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/// A list of moves to be performed in sequence, with auxiliary data
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/// attached to each.
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pub type MoveVec<T> = SmallVec<[(Allocation, Allocation, T); 16]>;
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/// A list of moves to be performance in sequence, like a
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/// `MoveVec<T>`, except that an unchosen scratch space may occur as
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/// well, represented by `Allocation::none()`.
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#[derive(Clone, Debug)]
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pub enum MoveVecWithScratch<T> {
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/// No scratch was actually used.
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NoScratch(MoveVec<T>),
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/// A scratch space was used.
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Scratch(MoveVec<T>),
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}
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/// A `ParallelMoves` represents a list of alloc-to-alloc moves that
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/// must happen in parallel -- i.e., all reads of sources semantically
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/// happen before all writes of destinations, and destinations are
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/// allowed to overwrite sources. It can compute a list of sequential
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/// moves that will produce the equivalent data movement, possibly
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/// using a scratch register if one is necessary.
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pub struct ParallelMoves<T: Clone + Copy + Default> {
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parallel_moves: MoveVec<T>,
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}
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impl<T: Clone + Copy + Default> ParallelMoves<T> {
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pub fn new() -> Self {
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Self {
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parallel_moves: smallvec![],
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}
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}
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pub fn add(&mut self, from: Allocation, to: Allocation, t: T) {
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self.parallel_moves.push((from, to, t));
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}
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fn sources_overlap_dests(&self) -> bool {
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// Assumes `parallel_moves` has already been sorted in `resolve()` below.
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for &(_, dst, _) in &self.parallel_moves {
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if self
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.parallel_moves
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.binary_search_by_key(&dst, |&(src, _, _)| src)
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.is_ok()
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{
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return true;
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}
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}
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false
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}
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/// Resolve the parallel-moves problem to a sequence of separate
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/// moves, such that the combined effect of the sequential moves
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/// is as-if all of the moves added to this `ParallelMoves`
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/// resolver happened in parallel.
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///
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/// Sometimes, if there is a cycle, a scratch register is
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/// necessary to allow the moves to occur sequentially. In this
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/// case, `Allocation::none()` is returned to represent the
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/// scratch register. The caller may choose to always hold a
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/// separate scratch register unused to allow this to be trivially
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/// rewritten; or may dynamically search for or create a free
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/// register as needed, if none are available.
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pub fn resolve(mut self) -> MoveVecWithScratch<T> {
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// Easy case: zero or one move. Just return our vec.
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if self.parallel_moves.len() <= 1 {
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return MoveVecWithScratch::NoScratch(self.parallel_moves);
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}
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// Sort moves by source so that we can efficiently test for
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// presence.
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self.parallel_moves
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.sort_by_key(|&(src, dst, _)| u64_key(src.bits(), dst.bits()));
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// Do any dests overlap sources? If not, we can also just
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// return the list.
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if !self.sources_overlap_dests() {
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return MoveVecWithScratch::NoScratch(self.parallel_moves);
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}
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// General case: some moves overwrite dests that other moves
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// read as sources. We'll use a general algorithm.
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//
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// *Important property*: because we expect that each register
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// has only one writer (otherwise the effect of the parallel
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// move is undefined), each move can only block one other move
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// (with its one source corresponding to the one writer of
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// that source). Thus, we *can only have simple cycles* (those
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// that are a ring of nodes, i.e., with only one path from a
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// node back to itself); there are no SCCs that are more
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// complex than that. We leverage this fact below to avoid
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// having to do a full Tarjan SCC DFS (with lowest-index
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// computation, etc.): instead, as soon as we find a cycle, we
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// know we have the full cycle and we can do a cyclic move
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// sequence and continue.
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// Sort moves by destination and check that each destination
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// has only one writer.
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self.parallel_moves.sort_by_key(|&(_, dst, _)| dst);
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if cfg!(debug_assertions) {
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let mut last_dst = None;
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for &(_, dst, _) in &self.parallel_moves {
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if last_dst.is_some() {
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debug_assert!(last_dst.unwrap() != dst);
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}
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last_dst = Some(dst);
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}
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}
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// Construct a mapping from move indices to moves they must
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// come before. Any given move must come before a move that
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// overwrites its destination; we have moves sorted by dest
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// above so we can efficiently find such a move, if any.
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let mut must_come_before: SmallVec<[Option<usize>; 16]> =
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smallvec![None; self.parallel_moves.len()];
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for (i, &(src, _, _)) in self.parallel_moves.iter().enumerate() {
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if let Ok(move_to_dst_idx) = self
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.parallel_moves
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.binary_search_by_key(&src, |&(_, dst, _)| dst)
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{
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must_come_before[i] = Some(move_to_dst_idx);
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}
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}
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// Do a simple stack-based DFS and emit moves in postorder,
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// then reverse at the end for RPO. Unlike Tarjan's SCC
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// algorithm, we can emit a cycle as soon as we find one, as
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// noted above.
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let mut ret: MoveVec<T> = smallvec![];
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let mut stack: SmallVec<[usize; 16]> = smallvec![];
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let mut visited: SmallVec<[bool; 16]> = smallvec![false; self.parallel_moves.len()];
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let mut onstack: SmallVec<[bool; 16]> = smallvec![false; self.parallel_moves.len()];
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let mut scratch_used = false;
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stack.push(0);
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onstack[0] = true;
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loop {
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if stack.is_empty() {
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if let Some(next) = visited.iter().position(|&flag| !flag) {
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stack.push(next);
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onstack[next] = true;
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} else {
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break;
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}
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}
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let top = *stack.last().unwrap();
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visited[top] = true;
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match must_come_before[top] {
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None => {
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ret.push(self.parallel_moves[top]);
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onstack[top] = false;
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stack.pop();
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while let Some(top) = stack.pop() {
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ret.push(self.parallel_moves[top]);
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onstack[top] = false;
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}
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}
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Some(next) if visited[next] && !onstack[next] => {
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ret.push(self.parallel_moves[top]);
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onstack[top] = false;
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stack.pop();
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while let Some(top) = stack.pop() {
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ret.push(self.parallel_moves[top]);
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onstack[top] = false;
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}
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}
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Some(next) if !visited[next] && !onstack[next] => {
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stack.push(next);
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onstack[next] = true;
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continue;
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}
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Some(next) => {
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// Found a cycle -- emit a cyclic-move sequence
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// for the cycle on the top of stack, then normal
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// moves below it. Recall that these moves will be
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// reversed in sequence, so from the original
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// parallel move set
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//
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// { B := A, C := B, A := B }
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//
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// we will generate something like:
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//
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// A := scratch
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// B := A
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// C := B
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// scratch := C
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//
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// which will become:
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//
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// scratch := C
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// C := B
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// B := A
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// A := scratch
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let mut last_dst = None;
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let mut scratch_src = None;
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while let Some(move_idx) = stack.pop() {
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onstack[move_idx] = false;
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let (mut src, dst, dst_t) = self.parallel_moves[move_idx];
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if last_dst.is_none() {
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scratch_src = Some(src);
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src = Allocation::none();
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scratch_used = true;
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} else {
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debug_assert_eq!(last_dst.unwrap(), src);
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}
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ret.push((src, dst, dst_t));
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last_dst = Some(dst);
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if move_idx == next {
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break;
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}
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}
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if let Some(src) = scratch_src {
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ret.push((src, Allocation::none(), T::default()));
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}
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}
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}
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}
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ret.reverse();
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if scratch_used {
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MoveVecWithScratch::Scratch(ret)
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} else {
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MoveVecWithScratch::NoScratch(ret)
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}
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}
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}
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impl<T> MoveVecWithScratch<T> {
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/// Fills in the scratch space, if needed, with the given
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/// register/allocation and returns a final list of moves. The
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/// scratch register must not occur anywhere in the parallel-move
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/// problem given to the resolver that produced this
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/// `MoveVecWithScratch`.
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pub fn with_scratch(self, scratch: Allocation) -> MoveVec<T> {
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match self {
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MoveVecWithScratch::NoScratch(moves) => moves,
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MoveVecWithScratch::Scratch(mut moves) => {
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for (src, dst, _) in &mut moves {
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debug_assert!(
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*src != scratch && *dst != scratch,
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"Scratch register should not also be an actual source or dest of moves"
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);
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debug_assert!(
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!(src.is_none() && dst.is_none()),
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"Move resolution should not have produced a scratch-to-scratch move"
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);
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if src.is_none() {
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*src = scratch;
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}
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if dst.is_none() {
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*dst = scratch;
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}
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}
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moves
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}
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}
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}
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/// Unwrap without a scratch register.
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pub fn without_scratch(self) -> Option<MoveVec<T>> {
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match self {
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MoveVecWithScratch::NoScratch(moves) => Some(moves),
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MoveVecWithScratch::Scratch(..) => None,
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}
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}
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/// Do we need a scratch register?
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pub fn needs_scratch(&self) -> bool {
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match self {
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MoveVecWithScratch::NoScratch(..) => false,
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MoveVecWithScratch::Scratch(..) => true,
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}
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}
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/// Do any moves go from stack to stack?
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pub fn stack_to_stack(&self) -> bool {
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match self {
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MoveVecWithScratch::NoScratch(moves) | MoveVecWithScratch::Scratch(moves) => moves
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.iter()
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.any(|(src, dst, _)| src.is_stack() && dst.is_stack()),
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}
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}
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}
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/// Final stage of move resolution: finding or using scratch
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/// registers, creating them if necessary by using stackslots, and
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/// ensuring that the final list of moves contains no stack-to-stack
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/// moves.
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///
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/// The resolved list of moves may need one or two scratch registers,
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/// and maybe a stackslot, to ensure these conditions. Our general
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/// strategy is in two steps.
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///
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/// First, we find a scratch register, so we only have to worry about
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/// a list of moves, all with real locations as src and dest. If we're
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/// lucky and there are any registers not allocated at this
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/// program-point, we can use a real register. Otherwise, we use an
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/// extra stackslot. This is fine, because at this step,
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/// stack-to-stack moves are OK.
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///
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/// Then, we resolve stack-to-stack moves into stack-to-reg /
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/// reg-to-stack pairs. For this, we try to allocate a second free
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/// register. If unavailable, we create another scratch stackslot, and
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/// we pick a "victim" register in the appropriate class, and we
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/// resolve into: victim -> extra-stackslot; stack-src -> victim;
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/// victim -> stack-dst; extra-stackslot -> victim.
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///
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/// Sometimes move elision will be able to clean this up a bit. But,
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/// for simplicity reasons, let's keep the concerns separated! So we
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/// always do the full expansion above.
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pub struct MoveAndScratchResolver<GetReg, GetStackSlot>
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where
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GetReg: FnMut() -> Option<Allocation>,
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GetStackSlot: FnMut() -> Allocation,
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{
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/// Scratch register for stack-to-stack move expansion.
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stack_stack_scratch_reg: Option<Allocation>,
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/// Stackslot into which we need to save the stack-to-stack
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/// scratch reg before doing any stack-to-stack moves, if we stole
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/// the reg.
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stack_stack_scratch_reg_save: Option<Allocation>,
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/// Closure that finds us a PReg at the current location.
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find_free_reg: GetReg,
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/// Closure that gets us a stackslot, if needed.
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get_stackslot: GetStackSlot,
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/// The victim PReg to evict to another stackslot at every
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/// stack-to-stack move if a free PReg is not otherwise
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/// available. Provided by caller and statically chosen. This is a
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/// very last-ditch option, so static choice is OK.
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victim: PReg,
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}
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impl<GetReg, GetStackSlot> MoveAndScratchResolver<GetReg, GetStackSlot>
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where
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GetReg: FnMut() -> Option<Allocation>,
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GetStackSlot: FnMut() -> Allocation,
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{
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pub fn new(find_free_reg: GetReg, get_stackslot: GetStackSlot, victim: PReg) -> Self {
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Self {
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stack_stack_scratch_reg: None,
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stack_stack_scratch_reg_save: None,
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find_free_reg,
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get_stackslot,
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victim,
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}
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}
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pub fn compute<T: Debug + Copy>(mut self, moves: MoveVecWithScratch<T>) -> MoveVec<T> {
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// First, do we have a vec with no stack-to-stack moves or use
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// of a scratch register? Fast return if so.
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if !moves.needs_scratch() && !moves.stack_to_stack() {
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return moves.without_scratch().unwrap();
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}
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let mut result = smallvec![];
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// Now, find a scratch allocation in order to resolve cycles.
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let scratch = (self.find_free_reg)().unwrap_or_else(|| (self.get_stackslot)());
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trace!("scratch resolver: scratch alloc {:?}", scratch);
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let moves = moves.with_scratch(scratch);
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for &(src, dst, data) in &moves {
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// Do we have a stack-to-stack move? If so, resolve.
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if src.is_stack() && dst.is_stack() {
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trace!("scratch resolver: stack to stack: {:?} -> {:?}", src, dst);
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// Lazily allocate a stack-to-stack scratch.
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if self.stack_stack_scratch_reg.is_none() {
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if let Some(reg) = (self.find_free_reg)() {
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trace!(
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"scratch resolver: have free stack-to-stack scratch preg: {:?}",
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reg
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);
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self.stack_stack_scratch_reg = Some(reg);
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} else {
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self.stack_stack_scratch_reg = Some(Allocation::reg(self.victim));
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self.stack_stack_scratch_reg_save = Some((self.get_stackslot)());
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trace!("scratch resolver: stack-to-stack using victim {:?} with save stackslot {:?}",
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self.stack_stack_scratch_reg,
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self.stack_stack_scratch_reg_save);
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}
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}
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// If we have a "victimless scratch", then do a
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// stack-to-scratch / scratch-to-stack sequence.
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if self.stack_stack_scratch_reg_save.is_none() {
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result.push((src, self.stack_stack_scratch_reg.unwrap(), data));
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result.push((self.stack_stack_scratch_reg.unwrap(), dst, data));
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}
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// Otherwise, save the current value in the
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// stack-to-stack scratch reg (which is our victim) to
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// the extra stackslot, then do the stack-to-scratch /
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// scratch-to-stack sequence, then restore it.
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else {
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result.push((
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self.stack_stack_scratch_reg.unwrap(),
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self.stack_stack_scratch_reg_save.unwrap(),
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data,
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));
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result.push((src, self.stack_stack_scratch_reg.unwrap(), data));
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result.push((self.stack_stack_scratch_reg.unwrap(), dst, data));
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result.push((
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self.stack_stack_scratch_reg_save.unwrap(),
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self.stack_stack_scratch_reg.unwrap(),
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data,
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));
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}
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} else {
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// Normal move.
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result.push((src, dst, data));
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}
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}
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trace!("scratch resolver: got {:?}", result);
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result
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}
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}
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