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[dev.ssa] cmd/compile: use 32-bit load to read writebarrier

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Avoid targeting a partial register with load;
ensure source of load (writebarrier) is aligned.

Better yet would be "CMPB $1,writebarrier" but that requires
wrestling with flagalloc (mem operand complicates moving
instruction around).

Didn't see a change in time for
   benchcmd -n 10 Build go build net/http

Verified that we clean the code up properly:
   0x20a8 <main.main+104>:	mov    0xc30a2(%rip),%eax
                            # 0xc5150 <runtime.writeBarrier>
   0x20ae <main.main+110>:	test   %al,%al

Change-Id: Id5fb8c260eaec27bd727cb0ae1476c60343b0986
Reviewed-on: https://go-review.googlesource.com/19998
Reviewed-by: Keith Randall <khr@golang.org>
This commit is contained in:
David Chase 2016-02-28 11:15:22 -05:00
parent 34f048c9d9
commit 8107b0012f
2 changed files with 12 additions and 7 deletions
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12
src/cmd/compile/internal/gc/ssa.go
View File

@ -2718,9 +2718,11 @@ func (s *state) insertWBmove(t *Type, left, right *ssa.Value, line int32) {
bEnd := s.f.NewBlock(ssa.BlockPlain) bEnd := s.f.NewBlock(ssa.BlockPlain)
aux := &ssa.ExternSymbol{Types[TBOOL], syslook("writeBarrier", 0).Sym} aux := &ssa.ExternSymbol{Types[TBOOL], syslook("writeBarrier", 0).Sym}
flagaddr := s.newValue1A(ssa.OpAddr, Ptrto(Types[TBOOL]), aux, s.sb) flagaddr := s.newValue1A(ssa.OpAddr, Ptrto(Types[TUINT32]), aux, s.sb)
// TODO: select the .enabled field. It is currently first, so not needed for now. // TODO: select the .enabled field. It is currently first, so not needed for now.
flag := s.newValue2(ssa.OpLoad, Types[TBOOL], flagaddr, s.mem()) // Load word, test byte, avoiding partial register write from load byte.
flag := s.newValue2(ssa.OpLoad, Types[TUINT32], flagaddr, s.mem())
flag = s.newValue1(ssa.OpTrunc64to8, Types[TBOOL], flag)
b := s.endBlock() b := s.endBlock()
b.Kind = ssa.BlockIf b.Kind = ssa.BlockIf
b.Likely = ssa.BranchUnlikely b.Likely = ssa.BranchUnlikely
@ -2761,9 +2763,11 @@ func (s *state) insertWBstore(t *Type, left, right *ssa.Value, line int32) {
bEnd := s.f.NewBlock(ssa.BlockPlain) bEnd := s.f.NewBlock(ssa.BlockPlain)
aux := &ssa.ExternSymbol{Types[TBOOL], syslook("writeBarrier", 0).Sym} aux := &ssa.ExternSymbol{Types[TBOOL], syslook("writeBarrier", 0).Sym}
flagaddr := s.newValue1A(ssa.OpAddr, Ptrto(Types[TBOOL]), aux, s.sb) flagaddr := s.newValue1A(ssa.OpAddr, Ptrto(Types[TUINT32]), aux, s.sb)
// TODO: select the .enabled field. It is currently first, so not needed for now. // TODO: select the .enabled field. It is currently first, so not needed for now.
flag := s.newValue2(ssa.OpLoad, Types[TBOOL], flagaddr, s.mem()) // Load word, test byte, avoiding partial register write from load byte.
flag := s.newValue2(ssa.OpLoad, Types[TUINT32], flagaddr, s.mem())
flag = s.newValue1(ssa.OpTrunc64to8, Types[TBOOL], flag)
b := s.endBlock() b := s.endBlock()
b.Kind = ssa.BlockIf b.Kind = ssa.BlockIf
b.Likely = ssa.BranchUnlikely b.Likely = ssa.BranchUnlikely

1
src/runtime/mgc.go
View File

@ -219,6 +219,7 @@ var writeBarrier struct {
enabled bool // compiler emits a check of this before calling write barrier enabled bool // compiler emits a check of this before calling write barrier
needed bool // whether we need a write barrier for current GC phase needed bool // whether we need a write barrier for current GC phase
cgo bool // whether we need a write barrier for a cgo check cgo bool // whether we need a write barrier for a cgo check
alignme uint64 // guarantee alignment so that compiler can use a 32 or 64-bit load
} }
// gcBlackenEnabled is 1 if mutator assists and background mark // gcBlackenEnabled is 1 if mutator assists and background mark