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cmd/compile: add intrinsic HasCPUFeature for checking cpu features

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Before using some CPU instructions, we must check for their presence.
We use global variables in the runtime package to record features.

Prior to this CL, we issued a regular memory load for these features.
The downside to this is that, because it is a regular memory load,
it cannot be hoisted out of loops or otherwise reordered with other loads.

This CL introduces a new intrinsic just for checking cpu features.
It still ends up resulting in a memory load, but that memory load can
now be floated to the entry block and rematerialized as needed.

One downside is that the regular load could be combined with the comparison
into a CMPBconstload+NE. This new intrinsic cannot; it generates MOVB+TESTB+NE.
(It is possible that MOVBQZX+TESTQ+NE would be better.)

This CL does only amd64. It is easy to extend to other architectures.

For the benchmark in #36196, on my machine, this offers a mild speedup.

name      old time/op  new time/op  delta
FMA-8     1.39ns ± 6%  1.29ns ± 9%  -7.19%  (p=0.000 n=97+96)
NonFMA-8  2.03ns ±11%  2.04ns ±12%    ~     (p=0.618 n=99+98)

Updates #15808
Updates #36196

Change-Id: I75e2fcfcf5a6df1bdb80657a7143bed69fca6deb
Reviewed-on: https://go-review.googlesource.com/c/go/+/212360
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Giovanni Bajo <rasky@develer.com>
This commit is contained in:
Josh Bleecher Snyder 2019-12-19 10:58:28 -08:00
parent ed7a8332c4
commit fff7509d47
8 changed files with 46 additions and 10 deletions
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6
src/cmd/compile/internal/amd64/ssa.go
View file

@ -902,6 +902,12 @@ func ssaGenValue(s *gc.SSAGenState, v *ssa.Value) {
p.From.Type = obj.TYPE_REG
p.From.Reg = v.Args[0].Reg()
gc.AddrAuto(&p.To, v)
case ssa.OpAMD64LoweredHasCPUFeature:
p := s.Prog(x86.AMOVB)
p.From.Type = obj.TYPE_MEM
gc.AddAux(&p.From, v)
p.To.Type = obj.TYPE_REG
p.To.Reg = v.Reg()
case ssa.OpAMD64LoweredGetClosurePtr:
// Closure pointer is DX.
gc.CheckLoweredGetClosurePtr(v)