cmd/compile: learn transitive proofs for safe unsigned subs

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

Change-Id: I436ff21a3e2362b3924de25a458534eb9947e013
Reviewed-on: https://go-review.googlesource.com/c/go/+/685821
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>

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

@@ -2164,6 +2164,10 @@ func signedAddOverflowsOrUnderflows(a, b int64, t *types.Type) bool {
 	}
 }
 
+func unsignedSubUnderflows(a, b uint64) bool {
+	return a < b
+}
+
 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
@@ -2226,6 +2230,17 @@ func addLocalFacts(ft *factsTable, b *Block) {
 				}
 				ft.update(b, v, v.Args[0], signed, r)
 			}
+		case OpSub64, OpSub32, OpSub16, OpSub8:
+			x := ft.limits[v.Args[0].ID]
+			y := ft.limits[v.Args[1].ID]
+			if !unsignedSubUnderflows(x.umin, y.umax) {
+				r := lt
+				if !y.nonzero() {
+					r |= eq
+				}
+				ft.update(b, v, v.Args[0], unsigned, r)
+			}
+			// FIXME: we could also do signed facts but the overflow checks are much trickier and I don't need it yet.
 		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

@@ -2241,6 +2241,67 @@ func transitiveProofsThroughOverflowingSignedAddNegative(x, y, z int64) {
 	}
 }
 
+func transitiveProofsThroughNonOverflowingUnsignedSub(x, y, z uint64) {
+	x |= 0xfff
+	y &= 0xfff
+
+	a := x - y
+	if a < z {
+		return
+	}
+
+	if x < z { // ERROR "Disproved Less64U$"
+		return
+	}
+	if y < z {
+		return
+	}
+	if a == x {
+		return
+	}
+	if a == y {
+		return
+	}
+
+	y |= 1
+	a = x - y
+	if a == x { // ERROR "Disproved Eq64$"
+		return
+	}
+	if a == y {
+		return
+	}
+}
+
+func transitiveProofsThroughOverflowingUnsignedSub(x, y, z uint64) {
+	a := x - y
+	if a < z {
+		return
+	}
+
+	if x < z {
+		return
+	}
+	if y < z {
+		return
+	}
+	if a == x {
+		return
+	}
+	if a == y {
+		return
+	}
+
+	y |= 1
+	a = x - y
+	if a == x {
+		return
+	}
+	if a == y {
+		return
+	}
+}
+
 //go:noinline
 func useInt(a int) {
 }