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cmd/compile/internal/amd64: update popcnt code generation

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Popcnt has false dependency on output register and generates
MOVQ $0, reg to break it. But recently we switched MOVQ $0, reg
encoding from xor reg, reg  to actual mov $0, reg. This CL updates
code generation for popcnt to use actual XOR.

Change-Id: I4c1fc11e85758b53ba2679165fa55614ec54b27d
Reviewed-on: https://go-review.googlesource.com/82516
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
This commit is contained in:
Ilya Tocar 2017-12-07 10:56:45 -06:00
parent 9c4fd4626c
commit de4edf3de7
1 changed files with 4 additions and 4 deletions
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8
src/cmd/compile/internal/amd64/ssa.go
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@ -867,10 +867,10 @@ func ssaGenValue(s *gc.SSAGenState, v *ssa.Value) {
case ssa.OpAMD64POPCNTQ, ssa.OpAMD64POPCNTL:
if v.Args[0].Reg() != v.Reg() {
// POPCNT on Intel has a false dependency on the destination register.
// Zero the destination to break the dependency.
p := s.Prog(x86.AMOVQ)
p.From.Type = obj.TYPE_CONST
p.From.Offset = 0
// Xor register with itself to break the dependency.
p := s.Prog(x86.AXORQ)
p.From.Type = obj.TYPE_REG
p.From.Reg = v.Reg()
p.To.Type = obj.TYPE_REG
p.To.Reg = v.Reg()
}