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cmd/internal/obj/x86: move MOV->XOR rewriting into compiler

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Fixes #20986.

Change-Id: Ic3cf5c0ab260f259ecff7b92cfdf5f4ae432aef3
Reviewed-on: https://go-review.googlesource.com/73072
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
This commit is contained in:
Matthew Dempsky 2017-10-24 13:24:14 -07:00
parent 73f1a1a1a7
commit a5868a47c6
5 changed files with 28 additions and 30 deletions
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17
src/cmd/compile/internal/x86/ssa.go
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@ -426,16 +426,23 @@ func ssaGenValue(s *gc.SSAGenState, v *ssa.Value) {
p.To.Reg = v.Args[0].Reg()
case ssa.Op386MOVLconst:
x := v.Reg()
// If flags aren't live (indicated by v.Aux == nil),
// then we can rewrite MOV $0, AX into XOR AX, AX.
if v.AuxInt == 0 && v.Aux == nil {
p := s.Prog(x86.AXORL)
p.From.Type = obj.TYPE_REG
p.From.Reg = x
p.To.Type = obj.TYPE_REG
p.To.Reg = x
break
}
p := s.Prog(v.Op.Asm())
p.From.Type = obj.TYPE_CONST
p.From.Offset = v.AuxInt
p.To.Type = obj.TYPE_REG
p.To.Reg = x
// If flags are live at this instruction, suppress the
// MOV $0,AX -> XOR AX,AX optimization.
if v.Aux != nil {
p.Mark |= x86.PRESERVEFLAGS
}
case ssa.Op386MOVSSconst, ssa.Op386MOVSDconst:
x := v.Reg()
p := s.Prog(v.Op.Asm())