cmd/compile: compute OR's maximum limits from argument's limits

Change-Id: I6902c405cab7bd573f6a721a6ca7c783713ea39a
Reviewed-on: https://go-review.googlesource.com/c/go/+/604456
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -1698,10 +1698,12 @@ func (ft *factsTable) flowLimit(v *Value) bool {
 		b := ft.limits[v.Args[1].ID]
 		return ft.unsignedMax(v, minU(a.umax, b.umax))
 	case OpOr64, OpOr32, OpOr16, OpOr8:
-		// OR can only make the value bigger.
+		// OR can only make the value bigger and can't flip bits proved to be zero in both inputs.
 		a := ft.limits[v.Args[0].ID]
 		b := ft.limits[v.Args[1].ID]
-		return ft.unsignedMin(v, maxU(a.umin, b.umin))
+		return ft.unsignedMinMax(v,
+			maxU(a.umin, b.umin),
+			1<<bits.Len64(a.umax|b.umax)-1)
 	case OpXor64, OpXor32, OpXor16, OpXor8:
 		// XOR can't flip bits that are proved to be zero in both inputs.
 		a := ft.limits[v.Args[0].ID]

test/prove.go

@@ -1408,6 +1408,24 @@ func xor64(a, b uint64, ensureBothBranchesCouldHappen bool) int {
 	return int(z)
 }
 
+func or64(a, b uint64, ensureBothBranchesCouldHappen bool) int {
+	a &= 0xff
+	b &= 0xfff
+
+	z := a | b
+
+	if ensureBothBranchesCouldHappen {
+		if z > 0xfff { // ERROR "Disproved Less64U$"
+			return 42
+		}
+	} else {
+		if z <= 0xfff { // ERROR "Proved Leq64U$"
+			return 1337
+		}
+	}
+	return int(z)
+}
+
 //go:noinline
 func useInt(a int) {
 }