diff --git a/src/cmd/compile/internal/ssa/rewrite.go b/src/cmd/compile/internal/ssa/rewrite.go index 7db5c99061..b32823283b 100644 --- a/src/cmd/compile/internal/ssa/rewrite.go +++ b/src/cmd/compile/internal/ssa/rewrite.go @@ -286,7 +286,18 @@ func canMergeLoadClobber(target, load, x *Value) bool { // approximate x dying with: // 1) target is x's only use. // 2) target is not in a deeper loop than x. - if x.Uses != 1 { + switch { + case x.Uses == 2 && x.Op == OpPhi && len(x.Args) == 2 && (x.Args[0] == target || x.Args[1] == target) && target.Uses == 1: + // This is a simple detector to determine that x is probably + // not live after target. (It does not need to be perfect, + // regalloc will issue a reg-reg move to save it if we are wrong.) + // We have: + // x = Phi(?, target) + // target = Op(load, x) + // Because target has only one use as a Phi argument, we can schedule it + // very late. Hopefully, later than the other use of x. (The other use died + // between x and target, or exists on another branch entirely). + case x.Uses > 1: return false } loopnest := x.Block.Func.loopnest() diff --git a/test/codegen/memcombine.go b/test/codegen/memcombine.go index 9ede80132c..fa0e902ac2 100644 --- a/test/codegen/memcombine.go +++ b/test/codegen/memcombine.go @@ -396,6 +396,15 @@ func load_op_no_merge(p, q *int) { } } +func load_op_in_loop(a []int) int { + r := 0 + for _, x := range a { + // amd64:`ADDQ\t\([A-Z]+\)\([A-Z]+\*8\), [A-Z]+` + r += x + } + return r +} + // Make sure offsets are folded into loads and stores. func offsets_fold(_, a [20]byte) (b [20]byte) { // arm64:`MOVD\tcommand-line-arguments\.a\+[0-9]+\(FP\), R[0-9]+`,`MOVD\tR[0-9]+, command-line-arguments\.b\+[0-9]+\(FP\)`