cmd/compile: allow InlMark operations to be speculatively executed

Although InlMark takes a memory argument it ultimately becomes a
NOP and therefore is safe to speculatively execute.

Fixes #74915

Change-Id: I64317dd433e300ac28de2bcf201845083ec2ac82
Reviewed-on: https://go-review.googlesource.com/c/go/+/693795
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/cmd/compile/internal/ssa/branchelim.go

@@ -436,8 +436,15 @@ func canSpeculativelyExecute(b *Block) bool {
 	// don't fuse memory ops, Phi ops, divides (can panic),
 	// or anything else with side-effects
 	for _, v := range b.Values {
-		if v.Op == OpPhi || isDivMod(v.Op) || isPtrArithmetic(v.Op) || v.Type.IsMemory() ||
-			v.MemoryArg() != nil || opcodeTable[v.Op].hasSideEffects {
+		if v.Op == OpPhi || isDivMod(v.Op) || isPtrArithmetic(v.Op) ||
+			v.Type.IsMemory() || opcodeTable[v.Op].hasSideEffects {
+			return false
+		}
+
+		// Allow inlining markers to be speculatively executed
+		// even though they have a memory argument.
+		// See issue #74915.
+		if v.Op != OpInlMark && v.MemoryArg() != nil {
 			return false
 		}
 	}

test/codegen/fuse.go

@@ -195,3 +195,24 @@ func ui4d(c <-chan uint8) {
 	for x := <-c; x < 126 || x >= 128; x = <-c {
 	}
 }
+
+// ------------------------------------ //
+// regressions                          //
+// ------------------------------------ //
+
+func gte4(x uint64) bool {
+	return x >= 4
+}
+
+func lt20(x uint64) bool {
+	return x < 20
+}
+
+func issue74915(c <-chan uint64) {
+	// Check that the optimization is not blocked by function inlining.
+
+	// amd64:"CMPQ\t.+, [$]16","ADDQ\t[$]-4,"
+	// s390x:"CLGIJ\t[$]4, R[0-9]+, [$]16","ADD\t[$]-4,"
+	for x := <-c; gte4(x) && lt20(x); x = <-c {
+	}
+}