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cmd/compile: prefer fixed registers for values

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For this code:

func f() (int, int) {
	return 0, 0
}

We currently generate on arm64:

   MOVD ZR, R0
   MOVD R0, R1

This CL changes that to

   MOVD ZR, R0
   MOVD ZR, R1

Probably not a big performance difference, but it makes the generated
code clearer.

A followup-ish CL from 633075 when the zero fixed register was exposed
to the register allocator.

Change-Id: I869a92817dcbbca46c900999fab538e76e10ed05
Reviewed-on: https://go-review.googlesource.com/c/go/+/721440
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
This commit is contained in:
Keith Randall 2025-11-17 15:33:01 -08:00
parent ba634ca5c7
commit 4d0658bb08
1 changed files with 5 additions and 4 deletions
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9
src/cmd/compile/internal/ssa/regalloc.go
View file

@ -596,17 +596,18 @@ func (s *regAllocState) allocValToReg(v *Value, mask regMask, nospill bool, pos
var c *Value
if vi.regs != 0 {
// Copy from a register that v is already in.
r2 := pickReg(vi.regs)
var current *Value
if !s.allocatable.contains(r2) {
current = v // v is in a fixed register
if vi.regs&^s.allocatable != 0 {
// v is in a fixed register, prefer that
current = v
} else {
r2 := pickReg(vi.regs)
if s.regs[r2].v != v {
panic("bad register state")
}
current = s.regs[r2].c
s.usedSinceBlockStart |= regMask(1) << r2
}
s.usedSinceBlockStart |= regMask(1) << r2
c = s.curBlock.NewValue1(pos, OpCopy, v.Type, current)
} else if v.rematerializeable() {
// Rematerialize instead of loading from the spill location.