cmd/compile: derive relation between x+delta and x in prove

If x+delta cannot overflow/underflow, we can derive:
  x+delta < x if delta<0 (this CL included)
  x+delta > x if delta>0 (this CL not included due to
  a recursive stack overflow)

Remove 95 bounds checks during ./make.bat

Fixes #51622

Change-Id: I60d9bd84c5d7e81bbf808508afd09be596644f09
Reviewed-on: https://go-review.googlesource.com/c/go/+/406175
Reviewed-by: David Chase <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Run-TryBot: Wayne Zuo <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>

Author: wdvxdr1123

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

@@ -463,15 +463,23 @@ func (ft *factsTable) update(parent *Block, v, w *Value, d domain, r relation) {
 			if parent.Func.pass.debug > 1 {
 				parent.Func.Warnl(parent.Pos, "x+d %s w; x:%v %v delta:%v w:%v d:%v", r, x, parent.String(), delta, w.AuxInt, d)
 			}
+			underflow := true
+			if l, has := ft.limits[x.ID]; has && delta < 0 {
+				if (x.Type.Size() == 8 && l.min >= math.MinInt64-delta) ||
+					(x.Type.Size() == 4 && l.min >= math.MinInt32-delta) {
+					underflow = false
+				}
+			}
+			if delta < 0 && !underflow {
+				// If delta < 0 and x+delta cannot underflow then x > x+delta (that is, x > v)
+				ft.update(parent, x, v, signed, gt)
+			}
 			if !w.isGenericIntConst() {
 				// If we know that x+delta > w but w is not constant, we can derive:
-				//    if delta < 0 and x > MinInt - delta, then x > w (because x+delta cannot underflow)
+				//    if delta < 0 and x+delta cannot underflow, then x > w
 				// This is useful for loops with bounds "len(slice)-K" (delta = -K)
-				if l, has := ft.limits[x.ID]; has && delta < 0 {
-					if (x.Type.Size() == 8 && l.min >= math.MinInt64-delta) ||
-						(x.Type.Size() == 4 && l.min >= math.MinInt32-delta) {
-						ft.update(parent, x, w, signed, r)
-					}
+				if delta < 0 && !underflow {
+					ft.update(parent, x, w, signed, r)
 				}
 			} else {
 				// With w,delta constants, we want to derive: x+delta > w  ⇒  x > w-delta

test/prove.go

@@ -1,6 +1,8 @@
-// +build amd64
 // errorcheck -0 -d=ssa/prove/debug=1
 
+//go:build amd64
+// +build amd64
+
 // Copyright 2016 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.
@@ -793,7 +795,7 @@ func unrollUpExcl(a []int) int {
 func unrollUpIncl(a []int) int {
 	var i, x int
 	for i = 0; i <= len(a)-2; i += 2 { // ERROR "Induction variable: limits \[0,\?\], increment 2$"
-		x += a[i]
+		x += a[i] // ERROR "Proved IsInBounds$"
 		x += a[i+1]
 	}
 	if i == len(a)-1 {
@@ -833,7 +835,7 @@ func unrollDownInclStep(a []int) int {
 	var i, x int
 	for i = len(a); i >= 2; i -= 2 { // ERROR "Induction variable: limits \[2,\?\], increment 2$"
 		x += a[i-1] // ERROR "Proved IsInBounds$"
-		x += a[i-2]
+		x += a[i-2] // ERROR "Proved IsInBounds$"
 	}
 	if i == 1 {
 		x += a[i-1]
@@ -1044,6 +1046,13 @@ func and(p []byte) ([]byte, []byte) { // issue #52563
 	return blk, rem
 }
 
+func issue51622(b []byte) int {
+	if len(b) >= 3 && b[len(b)-3] == '#' { // ERROR "Proved IsInBounds$"
+		return len(b)
+	}
+	return 0
+}
+
 //go:noinline
 func useInt(a int) {
 }