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cmd/compile: update comment about x86 nop instruction generator

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The comment about losing the high bits is incorrect.  We now use these
nops in places where they really need to be a nop.  (Before inline
marks, we used them just before deferreturn calls, so they could
clobber any caller-saved values.)

Change-Id: I433d1ec455aa37dab8fef6eb7d407f3737dbb97f
Reviewed-on: https://go-review.googlesource.com/c/158057
Reviewed-by: Ilya Tocar <ilya.tocar@intel.com>
This commit is contained in:
Keith Randall 2019-01-15 15:00:43 -08:00 committed by Keith Randall
parent 5cf4e442a5
commit 57976fe2b4
2 changed files with 7 additions and 3 deletions
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9
src/cmd/compile/internal/amd64/ggen.go
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@ -142,9 +142,12 @@ func zeroAuto(pp *gc.Progs, n *gc.Node) {
} }
func ginsnop(pp *gc.Progs) *obj.Prog { func ginsnop(pp *gc.Progs) *obj.Prog {
// This is actually not the x86 NOP anymore, // This is a hardware nop (1-byte 0x90) instruction,
// but at the point where it gets used, AX is dead // even though we describe it as an explicit XCHGL here.
// so it's okay if we lose the high bits. // Particularly, this does not zero the high 32 bits
// like typical *L opcodes.
// (gas assembles "xchg %eax,%eax" to 0x87 0xc0, which
// does zero the high 32 bits.)
p := pp.Prog(x86.AXCHGL) p := pp.Prog(x86.AXCHGL)
p.From.Type = obj.TYPE_REG p.From.Type = obj.TYPE_REG
p.From.Reg = x86.REG_AX p.From.Reg = x86.REG_AX

1
src/cmd/compile/internal/x86/ggen.go
View File

@ -54,6 +54,7 @@ func zeroAuto(pp *gc.Progs, n *gc.Node) {
} }
func ginsnop(pp *gc.Progs) *obj.Prog { func ginsnop(pp *gc.Progs) *obj.Prog {
// See comment in ../amd64/ggen.go.
p := pp.Prog(x86.AXCHGL) p := pp.Prog(x86.AXCHGL)
p.From.Type = obj.TYPE_REG p.From.Type = obj.TYPE_REG
p.From.Reg = x86.REG_AX p.From.Reg = x86.REG_AX