cmd/compile: in prove, fix fence-post implications for unsigned domain

Fence-post implications of the form "x-1 >= w && x > min ⇒ x > w"
were not correctly handling unsigned domain, by always checking signed
limits.

This bug was uncovered once we taught prove that len(x) is always
>= 0 in the signed domain.

In the code being miscompiled (s[len(s)-1]), prove checks
whether len(s)-1 >= len(s) in the unsigned domain; if it proves
that this is always false, it can remove the bound check.

Notice that len(s)-1 >= len(s) can be true for len(s) = 0 because
of the wrap-around, so this is something prove should not be
able to deduce.

But because of the bug, the gate condition for the fence-post
implication was len(s) > MinInt64 instead of len(s) > 0; that
condition would be good in the signed domain but not in the
unsigned domain. And since in CL105635 we taught prove that
len(s) >= 0, the condition incorrectly triggered
(len(s) >= 0 > MinInt64) and things were going downfall.

Fixes #27251
Fixes #27289

Change-Id: I3dbcb1955ac5a66a0dcbee500f41e8d219409be5
Reviewed-on: https://go-review.googlesource.com/132495
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -425,13 +425,13 @@ func (ft *factsTable) update(parent *Block, v, w *Value, d domain, r relation) {
 			//
 			// Useful for i > 0; s[i-1].
 			lim, ok := ft.limits[x.ID]
-			if ok && lim.min > opMin[v.Op] {
+			if ok && ((d == signed && lim.min > opMin[v.Op]) || (d == unsigned && lim.umin > 0)) {
 				ft.update(parent, x, w, d, gt)
 			}
 		} else if x, delta := isConstDelta(w); x != nil && delta == 1 {
 			// v >= x+1 && x < max  ⇒  v > x
 			lim, ok := ft.limits[x.ID]
-			if ok && lim.max < opMax[w.Op] {
+			if ok && ((d == signed && lim.max < opMax[w.Op]) || (d == unsigned && lim.umax < opUMax[w.Op])) {
 				ft.update(parent, v, x, d, gt)
 			}
 		}
@@ -527,6 +527,11 @@ var opMax = map[Op]int64{
 	OpAdd32: math.MaxInt32, OpSub32: math.MaxInt32,
 }
 
+var opUMax = map[Op]uint64{
+	OpAdd64: math.MaxUint64, OpSub64: math.MaxUint64,
+	OpAdd32: math.MaxUint32, OpSub32: math.MaxUint32,
+}
+
 // isNonNegative reports whether v is known to be non-negative.
 func (ft *factsTable) isNonNegative(v *Value) bool {
 	if isNonNegative(v) {

test/fixedbugs/issue27289.go

@@ -0,0 +1,24 @@
+// run
+
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Make sure we don't prove that the bounds check failure branch is unreachable.
+
+package main
+
+//go:noinline
+func f(a []int) {
+	_ = a[len(a)-1]
+}
+
+func main() {
+	defer func() {
+		if err := recover(); err != nil {
+			return
+		}
+		panic("f should panic")
+	}()
+	f(nil)
+}

test/prove.go

@@ -542,7 +542,7 @@ func fence2(x, y int) {
 	}
 }
 
-func fence3(b []int, x, y int64) {
+func fence3(b, c []int, x, y int64) {
 	if x-1 >= y {
 		if x <= y { // Can't prove because x may have wrapped.
 			return
@@ -555,6 +555,8 @@ func fence3(b []int, x, y int64) {
 		}
 	}
 
+	c[len(c)-1] = 0 // Can't prove because len(c) might be 0
+
 	if n := len(b); n > 0 {
 		b[n-1] = 0 // ERROR "Proved IsInBounds$"
 	}