cmd/compile: learn transitive proofs for safe positive signed adds

I've split this into it's own CL to make git bisect more effective.

Change-Id: I3fbb42ec7d29169a29f7f55ef2c188317512f532
Reviewed-on: https://go-review.googlesource.com/c/go/+/685819
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

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

@@ -2148,6 +2148,22 @@ func unsignedAddOverflows(a, b uint64, t *types.Type) bool {
 	}
 }
 
+func signedAddOverflowsOrUnderflows(a, b int64, t *types.Type) bool {
+	r := a + b
+	switch t.Size() {
+	case 8:
+		return (a >= 0 && b >= 0 && r < 0) || (a < 0 && b < 0 && r >= 0)
+	case 4:
+		return r < math.MinInt32 || math.MaxInt32 < r
+	case 2:
+		return r < math.MinInt16 || math.MaxInt16 < r
+	case 1:
+		return r < math.MinInt8 || math.MaxInt8 < r
+	default:
+		panic("unreachable")
+	}
+}
+
 func addLocalFacts(ft *factsTable, b *Block) {
 	// Propagate constant ranges among values in this block.
 	// We do this before the second loop so that we have the
@@ -2182,6 +2198,20 @@ func addLocalFacts(ft *factsTable, b *Block) {
 				}
 				ft.update(b, v, v.Args[0], unsigned, r)
 			}
+			if x.min >= 0 && !signedAddOverflowsOrUnderflows(x.max, y.max, v.Type) {
+				r := gt
+				if !x.nonzero() {
+					r |= eq
+				}
+				ft.update(b, v, v.Args[1], signed, r)
+			}
+			if y.min >= 0 && !signedAddOverflowsOrUnderflows(x.max, y.max, v.Type) {
+				r := gt
+				if !y.nonzero() {
+					r |= eq
+				}
+				ft.update(b, v, v.Args[0], signed, r)
+			}
 		case OpAnd64, OpAnd32, OpAnd16, OpAnd8:
 			ft.update(b, v, v.Args[0], unsigned, lt|eq)
 			ft.update(b, v, v.Args[1], unsigned, lt|eq)

test/prove.go

@@ -2104,6 +2104,73 @@ func transitiveProofsThroughOverflowingUnsignedAdd(x, y, z uint64) {
 	}
 }
 
+func transitiveProofsThroughNonOverflowingSignedAddPositive(x, y, z int64) {
+	x &= 1<<62 - 1
+	y &= 1<<62 - 1
+
+	a := x + y
+	if a > z {
+		return
+	}
+
+	if x > z { // ERROR "Disproved Less64$"
+		return
+	}
+	if y > z { // ERROR "Disproved Less64$"
+		return
+	}
+	if a == x {
+		return
+	}
+	if a == y {
+		return
+	}
+
+	x |= 1
+	y |= 1
+	a = x + y
+	if a == x { // ERROR "Disproved Eq64$"
+		return
+	}
+	if a == y { // ERROR "Disproved Eq64$"
+		return
+	}
+}
+
+func transitiveProofsThroughOverflowingSignedAddPositive(x, y, z int64) {
+	if x < 0 || y < 0 {
+		return
+	}
+
+	a := x + y
+	if a > z {
+		return
+	}
+
+	if x > z {
+		return
+	}
+	if y > z {
+		return
+	}
+	if a == x {
+		return
+	}
+	if a == y {
+		return
+	}
+
+	x |= 1
+	y |= 1
+	a = x + y
+	if a == x { // ERROR "Disproved Eq64$"
+		return
+	}
+	if a == y { // ERROR "Disproved Eq64$"
+		return
+	}
+}
+
 //go:noinline
 func useInt(a int) {
 }

test/prove_constant_folding.go

@@ -14,7 +14,7 @@ func f0i(x int) int {
 	}
 
 	if (x + 20) == 20 {
-    return x + 5 // ERROR "Proved.+is constant 0$" "Proved.+is constant 5$"
+		return x + 5 // ERROR "Proved.+is constant 0$" "Proved.+is constant 5$" "x\+d >=? w"
 	}
 
 	return x / 2
@@ -26,7 +26,7 @@ func f0u(x uint) uint {
 	}
 
 	if (x + 20) == 20 {
-    return x + 5 // ERROR "Proved.+is constant 0$" "Proved.+is constant 5$"
+		return x + 5 // ERROR "Proved.+is constant 0$" "Proved.+is constant 5$" "x\+d >=? w"
 	}
 
 	return x / 2