Reduce arm64 Inst enum size

This reduces the size of the Inst enum from 112 bytes to 48 bytes.

Using DHAT on a regex-rs.wasm benchmark, `valgrind --tool=dhat clif-util compile --target aarch64`

The total number of allocated bytes, drops by around 170 MB.
At t-gmax drops by 3 MB.

Using `perf stat clif-util compile --target aarch64`, the instructions count dropped by 0.6%. Cache misses dropped by 6%. Cycles dropped by 2.3%.

cranelift/codegen/src/isa/aarch64/abi.rs

@@ -68,6 +68,7 @@ use crate::isa::aarch64::{self, inst::*};
 use crate::machinst::*;
 use crate::settings;
 
+use alloc::boxed::Box;
 use alloc::vec::Vec;
 
 use regalloc::{RealReg, Reg, RegClass, Set, SpillSlot, Writable};
@@ -1275,9 +1276,9 @@ impl ABICall for AArch64ABICall {
         );
         match &self.dest {
             &CallDest::ExtName(ref name, RelocDistance::Near) => ctx.emit(Inst::Call {
-                dest: name.clone(),
-                uses,
-                defs,
+                dest: Box::new(name.clone()),
+                uses: Box::new(uses),
+                defs: Box::new(defs),
                 loc: self.loc,
                 opcode: self.opcode,
             }),
@@ -1290,16 +1291,16 @@ impl ABICall for AArch64ABICall {
                 });
                 ctx.emit(Inst::CallInd {
                     rn: spilltmp_reg(),
-                    uses,
-                    defs,
+                    uses: Box::new(uses),
+                    defs: Box::new(defs),
                     loc: self.loc,
                     opcode: self.opcode,
                 });
             }
             &CallDest::Reg(reg) => ctx.emit(Inst::CallInd {
                 rn: reg,
-                uses,
-                defs,
+                uses: Box::new(uses),
+                defs: Box::new(defs),
                 loc: self.loc,
                 opcode: self.opcode,
             }),

cranelift/codegen/src/isa/aarch64/inst/emit.rs

@@ -1303,7 +1303,7 @@ impl<O: MachSectionOutput> MachInstEmit<O> for Inst {
                 };
                 inst.emit(sink, flags, state);
                 // Emit jump table (table of 32-bit offsets).
-                for target in targets {
+                for target in targets.iter() {
                     let off = target.as_offset_words() * 4;
                     let off = i32::try_from(off).unwrap();
                     // cast i32 to u32 (two's-complement)

cranelift/codegen/src/isa/aarch64/inst/emit_tests.rs

@@ -3,6 +3,7 @@ use crate::isa::aarch64::inst::*;
 use crate::isa::test_utils;
 use crate::settings;
 
+use alloc::boxed::Box;
 use alloc::vec::Vec;
 
 #[test]
@@ -2112,9 +2113,9 @@ fn test_aarch64_binemit() {
 
     insns.push((
         Inst::Call {
-            dest: ExternalName::testcase("test0"),
-            uses: Set::empty(),
-            defs: Set::empty(),
+            dest: Box::new(ExternalName::testcase("test0")),
+            uses: Box::new(Set::empty()),
+            defs: Box::new(Set::empty()),
             loc: SourceLoc::default(),
             opcode: Opcode::Call,
         },
@@ -2125,8 +2126,8 @@ fn test_aarch64_binemit() {
     insns.push((
         Inst::CallInd {
             rn: xreg(10),
-            uses: Set::empty(),
-            defs: Set::empty(),
+            uses: Box::new(Set::empty()),
+            defs: Box::new(Set::empty()),
             loc: SourceLoc::default(),
             opcode: Opcode::CallIndirect,
         },

cranelift/codegen/src/isa/aarch64/inst/mod.rs

@@ -12,6 +12,7 @@ use crate::{settings, CodegenError, CodegenResult};
 use regalloc::{RealRegUniverse, Reg, RegClass, SpillSlot, VirtualReg, Writable};
 use regalloc::{RegUsageCollector, RegUsageMapper, Set};
 
+use alloc::boxed::Box;
 use alloc::vec::Vec;
 use smallvec::{smallvec, SmallVec};
 use std::string::{String, ToString};
@@ -616,17 +617,17 @@ pub enum Inst {
     /// code should use a `LoadExtName` / `CallInd` sequence instead, allowing an arbitrary 64-bit
     /// target.
     Call {
-        dest: ExternalName,
-        uses: Set<Reg>,
-        defs: Set<Writable<Reg>>,
+        dest: Box<ExternalName>,
+        uses: Box<Set<Reg>>,
+        defs: Box<Set<Writable<Reg>>>,
         loc: SourceLoc,
         opcode: Opcode,
     },
     /// A machine indirect-call instruction.
     CallInd {
         rn: Reg,
-        uses: Set<Reg>,
-        defs: Set<Writable<Reg>>,
+        uses: Box<Set<Reg>>,
+        defs: Box<Set<Writable<Reg>>>,
         loc: SourceLoc,
         opcode: Opcode,
     },
@@ -704,8 +705,8 @@ pub enum Inst {
     /// Jump-table sequence, as one compound instruction (see note in lower.rs
     /// for rationale).
     JTSequence {
-        targets: Vec<BranchTarget>,
-        targets_for_term: Vec<BlockIndex>, // needed for MachTerminator.
+        targets: Box<[BranchTarget]>,
+        targets_for_term: Box<[BlockIndex]>, // needed for MachTerminator.
         ridx: Reg,
         rtmp1: Writable<Reg>,
         rtmp2: Writable<Reg>,
@@ -760,6 +761,13 @@ fn count_zero_half_words(mut value: u64) -> usize {
     count
 }
 
+#[test]
+fn inst_size_test() {
+    // This test will help with unintentionally growing the size
+    // of the Inst enum.
+    assert_eq!(48, std::mem::size_of::<Inst>());
+}
+
 impl Inst {
     /// Create a move instruction.
     pub fn mov(to_reg: Writable<Reg>, from_reg: Reg) -> Inst {
@@ -1090,8 +1098,8 @@ fn aarch64_get_regs(inst: &Inst, collector: &mut RegUsageCollector) {
         &Inst::Call {
             ref uses, ref defs, ..
         } => {
-            collector.add_uses(uses);
-            collector.add_defs(defs);
+            collector.add_uses(&*uses);
+            collector.add_defs(&*defs);
         }
         &Inst::CallInd {
             ref uses,
@@ -1099,8 +1107,8 @@ fn aarch64_get_regs(inst: &Inst, collector: &mut RegUsageCollector) {
             rn,
             ..
         } => {
-            collector.add_uses(uses);
-            collector.add_defs(defs);
+            collector.add_uses(&*uses);
+            collector.add_defs(&*defs);
             collector.add_use(rn);
         }
         &Inst::CondBr { ref kind, .. }
@@ -1643,8 +1651,8 @@ fn aarch64_map_regs(inst: &mut Inst, mapper: &RegUsageMapper) {
                 map_def(mapper, &mut r);
                 r
             });
-            *uses = new_uses;
-            *defs = new_defs;
+            *uses = Box::new(new_uses);
+            *defs = Box::new(new_defs);
         }
         &mut Inst::Ret | &mut Inst::EpiloguePlaceholder => {}
         &mut Inst::CallInd {
@@ -1664,8 +1672,8 @@ fn aarch64_map_regs(inst: &mut Inst, mapper: &RegUsageMapper) {
                 map_def(mapper, &mut r);
                 r
             });
-            *uses = new_uses;
-            *defs = new_defs;
+            *uses = Box::new(new_uses);
+            *defs = Box::new(new_defs);
             map_use(mapper, rn);
         }
         &mut Inst::CondBr { ref mut kind, .. } => {
@@ -1895,7 +1903,7 @@ impl MachInst for Inst {
             &mut Inst::JTSequence {
                 targets: ref mut t, ..
             } => {
-                for target in t {
+                for target in t.iter_mut() {
                     // offset+20: jumptable is 20 bytes into compound sequence.
                     target.lower(targets, my_offset + 20);
                 }

cranelift/codegen/src/isa/aarch64/lower_inst.rs

@@ -2132,8 +2132,8 @@ pub(crate) fn lower_branch<C: LowerCtx<I = Inst>>(
                     ridx,
                     rtmp1,
                     rtmp2,
-                    targets: jt_targets,
-                    targets_for_term,
+                    targets: jt_targets.into_boxed_slice(),
+                    targets_for_term: targets_for_term.into_boxed_slice(),
                 });
             }