cmd/compile: don't fuse branches with side effects

Count Values with side effects but no use as live, and don't fuse
branches that contain such Values. (This can happen e.g. when it
is followed by an infinite loop.) Otherwise this may lead to
miscompilation (side effect fired at wrong condition) or ICE (two
stores live simultaneously).

Fixes #36005.

Change-Id: If202eae4b37cb7f0311d6ca120ffa46609925157
Reviewed-on: https://go-review.googlesource.com/c/go/+/210179Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -145,7 +145,7 @@ func fuseBlockIf(b *Block) bool {
 // There may be false positives.
 func isEmpty(b *Block) bool {
 	for _, v := range b.Values {
-		if v.Uses > 0 || v.Type.IsVoid() {
+		if v.Uses > 0 || v.Op.IsCall() || v.Op.HasSideEffects() || v.Type.IsVoid() {
 			return false
 		}
 	}

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

@@ -63,7 +63,7 @@ func TestFuseEliminatesBothBranches(t *testing.T) {
 			t.Errorf("then was not eliminated, but should have")
 		}
 		if b == fun.blocks["else"] && b.Kind != BlockInvalid {
-			t.Errorf("then was not eliminated, but should have")
+			t.Errorf("else was not eliminated, but should have")
 		}
 	}
 }
@@ -97,7 +97,7 @@ func TestFuseHandlesPhis(t *testing.T) {
 			t.Errorf("then was not eliminated, but should have")
 		}
 		if b == fun.blocks["else"] && b.Kind != BlockInvalid {
-			t.Errorf("then was not eliminated, but should have")
+			t.Errorf("else was not eliminated, but should have")
 		}
 	}
 }
@@ -131,6 +131,40 @@ func TestFuseEliminatesEmptyBlocks(t *testing.T) {
 	}
 }
 
+func TestFuseSideEffects(t *testing.T) {
+	// Test that we don't fuse branches that have side effects but
+	// have no use (e.g. followed by infinite loop).
+	// See issue #36005.
+	c := testConfig(t)
+	fun := c.Fun("entry",
+		Bloc("entry",
+			Valu("mem", OpInitMem, types.TypeMem, 0, nil),
+			Valu("b", OpArg, c.config.Types.Bool, 0, nil),
+			If("b", "then", "else")),
+		Bloc("then",
+			Valu("call1", OpStaticCall, types.TypeMem, 0, nil, "mem"),
+			Goto("empty")),
+		Bloc("else",
+			Valu("call2", OpStaticCall, types.TypeMem, 0, nil, "mem"),
+			Goto("empty")),
+		Bloc("empty",
+			Goto("loop")),
+		Bloc("loop",
+			Goto("loop")))
+
+	CheckFunc(fun.f)
+	fuseAll(fun.f)
+
+	for _, b := range fun.f.Blocks {
+		if b == fun.blocks["then"] && b.Kind == BlockInvalid {
+			t.Errorf("then is eliminated, but should not")
+		}
+		if b == fun.blocks["else"] && b.Kind == BlockInvalid {
+			t.Errorf("else is eliminated, but should not")
+		}
+	}
+}
+
 func BenchmarkFuse(b *testing.B) {
 	for _, n := range [...]int{1, 10, 100, 1000, 10000} {
 		b.Run(strconv.Itoa(n), func(b *testing.B) {

src/cmd/compile/internal/ssa/gen/main.go

@@ -405,6 +405,7 @@ func genOp() {
 
 	fmt.Fprintln(w, "func (o Op) SymEffect() SymEffect { return opcodeTable[o].symEffect }")
 	fmt.Fprintln(w, "func (o Op) IsCall() bool { return opcodeTable[o].call }")
+	fmt.Fprintln(w, "func (o Op) HasSideEffects() bool { return opcodeTable[o].hasSideEffects }")
 	fmt.Fprintln(w, "func (o Op) UnsafePoint() bool { return opcodeTable[o].unsafePoint }")
 
 	// generate registers

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

@@ -31636,6 +31636,7 @@ func (o Op) String() string       { return opcodeTable[o].name }
 func (o Op) UsesScratch() bool    { return opcodeTable[o].usesScratch }
 func (o Op) SymEffect() SymEffect { return opcodeTable[o].symEffect }
 func (o Op) IsCall() bool         { return opcodeTable[o].call }
+func (o Op) HasSideEffects() bool { return opcodeTable[o].hasSideEffects }
 func (o Op) UnsafePoint() bool    { return opcodeTable[o].unsafePoint }
 
 var registers386 = [...]Register{