cmd/compile: fix prove pass when upper condition is <= maxint

When the terminating condition is <= X, we need to make sure that
X+step doesn't overflow.

Fixes #53600

Change-Id: I36e5384d05b4d7168e48db6094200fcae409bfe5
Reviewed-on: https://go-review.googlesource.com/c/go/+/415219
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>

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

@@ -159,6 +159,13 @@ func findIndVar(f *Func) []indVar {
 			step = -step
 		}
 
+		if flags&indVarMaxInc != 0 && max.Op == OpConst64 && max.AuxInt+step < max.AuxInt {
+			// For a <= comparison, we need to make sure that a value equal to
+			// max can be incremented without overflowing.
+			// (For a < comparison, the %step check below ensures no overflow.)
+			continue
+		}
+
 		// Up to now we extracted the induction variable (ind),
 		// the increment delta (inc), the temporary sum (nxt),
 		// the minimum value (min) and the maximum value (max).

test/fixedbugs/issue53600.go

@@ -0,0 +1,42 @@
+// run
+
+// Copyright 2022 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.
+
+package main
+
+import "math"
+
+func main() {
+	f()
+	g()
+	h()
+}
+func f() {
+	for i := int64(math.MaxInt64); i <= math.MaxInt64; i++ {
+		if i < 0 {
+			println("done")
+			return
+		}
+		println(i, i < 0)
+	}
+}
+func g() {
+	for i := int64(math.MaxInt64) - 1; i <= math.MaxInt64; i++ {
+		if i < 0 {
+			println("done")
+			return
+		}
+		println(i, i < 0)
+	}
+}
+func h() {
+	for i := int64(math.MaxInt64) - 2; i <= math.MaxInt64; i += 2 {
+		if i < 0 {
+			println("done")
+			return
+		}
+		println(i, i < 0)
+	}
+}

test/fixedbugs/issue53600.out

@@ -0,0 +1,8 @@
+9223372036854775807 false
+done
+9223372036854775806 false
+9223372036854775807 false
+done
+9223372036854775805 false
+9223372036854775807 false
+done