Stowage/go
2
0
Fork 0
mirror of https://github.com/golang/go.git synced 2025-12-08 06:10:04 +00:00
Code Issues Projects Releases Packages Wiki Activity

cmd/compile: use Block.Likely to put likely branch first

Browse source 
When a neither of a conditional block's successors follows,
the block must end with a conditional branch followed by a
an unconditional branch.  If the (conditional) branch is
"unlikely", invert it and swap successors to make it
likely instead.

This doesn't matter to most benchmarks on amd64, but in one
instance on amd64 it caused a 30% improvement, and it is
otherwise harmless.  The problematic loop is

		for i := 0; i < w; i++ {
			if pw[i] != 0 {
				return true
			}
		}

compiled under GOEXPERIMENT=preemptibleloops
This the very worst-case benchmark for that experiment.

Also in this CL is a commoning up of heavily-repeated
boilerplate, which made it much easier to see that the
changes were applied correctly.  In the future this should
allow un-exporting of SSAGenState.Branches once the
boilerplate-replacement is done everywhere.

Change-Id: I0e5ded6eeb3ab1e3e0138e12d54c7e056bd99335
Reviewed-on: https://go-review.googlesource.com/104977
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
This commit is contained in:
David Chase 2018-04-05 16:14:42 -04:00
parent 2b23996939
commit ab48574bab
9 changed files with 102 additions and 134 deletions
Download patch file Download diff file Expand all files Collapse all files

23
src/cmd/compile/internal/x86/ssa.go
View file

@ -863,25 +863,20 @@ func ssaGenBlock(s *gc.SSAGenState, b, next *ssa.Block) {
ssa.Block386ULT, ssa.Block386UGT,
ssa.Block386ULE, ssa.Block386UGE:
jmp := blockJump[b.Kind]
var p *obj.Prog
switch next {
case b.Succs[0].Block():
p = s.Prog(jmp.invasm)
p.To.Type = obj.TYPE_BRANCH
s.Branches = append(s.Branches, gc.Branch{P: p, B: b.Succs[1].Block()})
s.Br(jmp.invasm, b.Succs[1].Block())
case b.Succs[1].Block():
p = s.Prog(jmp.asm)
p.To.Type = obj.TYPE_BRANCH
s.Branches = append(s.Branches, gc.Branch{P: p, B: b.Succs[0].Block()})
s.Br(jmp.asm, b.Succs[0].Block())
default:
p = s.Prog(jmp.asm)
p.To.Type = obj.TYPE_BRANCH
s.Branches = append(s.Branches, gc.Branch{P: p, B: b.Succs[0].Block()})
q := s.Prog(obj.AJMP)
q.To.Type = obj.TYPE_BRANCH
s.Branches = append(s.Branches, gc.Branch{P: q, B: b.Succs[1].Block()})
if b.Likely != ssa.BranchUnlikely {
s.Br(jmp.asm, b.Succs[0].Block())
s.Br(obj.AJMP, b.Succs[1].Block())
} else {
s.Br(jmp.invasm, b.Succs[1].Block())
s.Br(obj.AJMP, b.Succs[0].Block())
}
}
default:
b.Fatalf("branch not implemented: %s. Control: %s", b.LongString(), b.Control.LongString())
}