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go/test/loopbce.go
Giovanni Bajo e0d37a33ab cmd/compile: teach prove to handle expressions like len(s)-delta 
When a loop has bound len(s)-delta, findIndVar detected it and
returned len(s) as (conservative) upper bound. This little lie
allowed loopbce to drop bound checks.

It is obviously more generic to teach prove about relations like
x+d<w for non-constant "w"; we already handled the case for
constant "w", so we just want to learn that if d<0, then x+d<w
proves that x<w.

To be able to remove the code from findIndVar, we also need
to teach prove that len() and cap() are always non-negative.

This CL allows to prove 633 more checks in cmd+std. Most
of them are cases where the code was already testing before
accessing a slice but the compiler didn't know it. For instance,
take strings.HasSuffix:

    func HasSuffix(s, suffix string) bool {
        return len(s) >= len(suffix) && s[len(s)-len(suffix):] == suffix
    }

When suffix is a literal string, the compiler now understands
that the explicit check is enough to not emit a slice check.

I also found a loopbce test that was incorrectly
written to detect an overflow but had a off-by-one (on the
conservative side), so it unexpectly passed with this CL; I
changed it to really trigger the overflow as intended.

Change-Id: Ib5abade337db46b8811425afebad4719b6e46c4a
Reviewed-on: https://go-review.googlesource.com/105635
Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
2018-04-29 09:38:32 +00:00

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// +build amd64
// errorcheck -0 -d=ssa/prove/debug=1
package main
func f0a(a []int) int {
x := 0
for i := range a { // ERROR "Induction variable: limits \[0,\?\), increment 1$"
x += a[i] // ERROR "Proved IsInBounds$"
}
return x
}
func f0b(a []int) int {
x := 0
for i := range a { // ERROR "Induction variable: limits \[0,\?\), increment 1$"
b := a[i:] // ERROR "Proved IsSliceInBounds$"
x += b[0]
}
return x
}
func f0c(a []int) int {
x := 0
for i := range a { // ERROR "Induction variable: limits \[0,\?\), increment 1$"
b := a[:i+1] // ERROR "Proved IsSliceInBounds$"
x += b[0]
}
return x
}
func f1(a []int) int {
x := 0
for _, i := range a { // ERROR "Induction variable: limits \[0,\?\), increment 1$"
x += i
}
return x
}
func f2(a []int) int {
x := 0
for i := 1; i < len(a); i++ { // ERROR "Induction variable: limits \[1,\?\), increment 1$"
x += a[i] // ERROR "Proved IsInBounds$"
}
return x
}
func f4(a [10]int) int {
x := 0
for i := 0; i < len(a); i += 2 { // ERROR "Induction variable: limits \[0,10\), increment 2$"
x += a[i] // ERROR "Proved IsInBounds$"
}
return x
}
func f5(a [10]int) int {
x := 0
for i := -10; i < len(a); i += 2 { // ERROR "Induction variable: limits \[-10,10\), increment 2$"
x += a[i]
}
return x
}
func f6(a []int) {
for i := range a { // ERROR "Induction variable: limits \[0,\?\), increment 1$"
b := a[0:i] // ERROR "Proved IsSliceInBounds$"
f6(b)
}
}
func g0a(a string) int {
x := 0
for i := 0; i < len(a); i++ { // ERROR "Induction variable: limits \[0,\?\), increment 1$"
x += int(a[i]) // ERROR "Proved IsInBounds$"
}
return x
}
func g0b(a string) int {
x := 0
for i := 0; len(a) > i; i++ { // ERROR "Induction variable: limits \[0,\?\), increment 1$"
x += int(a[i]) // ERROR "Proved IsInBounds$"
}
return x
}
func g0c(a string) int {
x := 0
for i := len(a); i > 0; i-- { // ERROR "Induction variable: limits \(0,\?\], increment -1$"
x += int(a[i-1]) // ERROR "Proved IsInBounds$"
}
return x
}
func g0d(a string) int {
x := 0
for i := len(a); 0 < i; i-- { // ERROR "Induction variable: limits \(0,\?\], increment -1$"
x += int(a[i-1]) // ERROR "Proved IsInBounds$"
}
return x
}
func g0e(a string) int {
x := 0
for i := len(a) - 1; i >= 0; i-- { // ERROR "Induction variable: limits \[0,\?\], increment -1$"
x += int(a[i]) // ERROR "Proved IsInBounds$"
}
return x
}
func g0f(a string) int {
x := 0
for i := len(a) - 1; 0 <= i; i-- { // ERROR "Induction variable: limits \[0,\?\], increment -1$"
x += int(a[i]) // ERROR "Proved IsInBounds$"
}
return x
}
func g1() int {
a := "evenlength"
x := 0
for i := 0; i < len(a); i += 2 { // ERROR "Induction variable: limits \[0,10\), increment 2$"
x += int(a[i]) // ERROR "Proved IsInBounds$"
}
return x
}
func g2() int {
a := "evenlength"
x := 0
for i := 0; i < len(a); i += 2 { // ERROR "Induction variable: limits \[0,10\), increment 2$"
j := i
if a[i] == 'e' { // ERROR "Proved IsInBounds$"
j = j + 1
}
x += int(a[j])
}
return x
}
func g3a() {
a := "this string has length 25"
for i := 0; i < len(a); i += 5 { // ERROR "Induction variable: limits \[0,25\), increment 5$"
useString(a[i:]) // ERROR "Proved IsSliceInBounds$"
useString(a[:i+3])
}
}
func g3b(a string) {
for i := 0; i < len(a); i++ { // ERROR "Induction variable: limits \[0,\?\), increment 1$"
useString(a[i+1:]) // ERROR "Proved IsSliceInBounds$"
}
}
func g3c(a string) {
for i := 0; i < len(a); i++ { // ERROR "Induction variable: limits \[0,\?\), increment 1$"
useString(a[:i+1]) // ERROR "Proved IsSliceInBounds$"
}
}
func h1(a []byte) {
c := a[:128]
for i := range c { // ERROR "Induction variable: limits \[0,128\), increment 1$"
c[i] = byte(i) // ERROR "Proved IsInBounds$"
}
}
func h2(a []byte) {
for i := range a[:128] { // ERROR "Induction variable: limits \[0,128\), increment 1$"
a[i] = byte(i)
}
}
func k0(a [100]int) [100]int {
for i := 10; i < 90; i++ { // ERROR "Induction variable: limits \[10,90\), increment 1$"
a[i-11] = i
a[i-10] = i // ERROR "Proved IsInBounds$"
a[i-5] = i // ERROR "Proved IsInBounds$"
a[i] = i // ERROR "Proved IsInBounds$"
a[i+5] = i // ERROR "Proved IsInBounds$"
a[i+10] = i // ERROR "Proved IsInBounds$"
a[i+11] = i
}
return a
}
func k1(a [100]int) [100]int {
for i := 10; i < 90; i++ { // ERROR "Induction variable: limits \[10,90\), increment 1$"
useSlice(a[:i-11])
useSlice(a[:i-10]) // ERROR "Proved IsSliceInBounds$"
useSlice(a[:i-5]) // ERROR "Proved IsSliceInBounds$"
useSlice(a[:i]) // ERROR "Proved IsSliceInBounds$"
useSlice(a[:i+5]) // ERROR "Proved IsSliceInBounds$"
useSlice(a[:i+10]) // ERROR "Proved IsSliceInBounds$"
useSlice(a[:i+11]) // ERROR "Proved IsSliceInBounds$"
useSlice(a[:i+12])
}
return a
}
func k2(a [100]int) [100]int {
for i := 10; i < 90; i++ { // ERROR "Induction variable: limits \[10,90\), increment 1$"
useSlice(a[i-11:])
useSlice(a[i-10:]) // ERROR "Proved IsSliceInBounds$"
useSlice(a[i-5:]) // ERROR "Proved IsSliceInBounds$"
useSlice(a[i:]) // ERROR "Proved IsSliceInBounds$"
useSlice(a[i+5:]) // ERROR "Proved IsSliceInBounds$"
useSlice(a[i+10:]) // ERROR "Proved IsSliceInBounds$"
useSlice(a[i+11:]) // ERROR "Proved IsSliceInBounds$"
useSlice(a[i+12:])
}
return a
}
func k3(a [100]int) [100]int {
for i := -10; i < 90; i++ { // ERROR "Induction variable: limits \[-10,90\), increment 1$"
a[i+9] = i
a[i+10] = i // ERROR "Proved IsInBounds$"
a[i+11] = i
}
return a
}
func k3neg(a [100]int) [100]int {
for i := 89; i > -11; i-- { // ERROR "Induction variable: limits \(-11,89\], increment -1$"
a[i+9] = i
a[i+10] = i // ERROR "Proved IsInBounds$"
a[i+11] = i
}
return a
}
func k3neg2(a [100]int) [100]int {
for i := 89; i >= -10; i-- { // ERROR "Induction variable: limits \[-10,89\], increment -1$"
a[i+9] = i
a[i+10] = i // ERROR "Proved IsInBounds$"
a[i+11] = i
}
return a
}
func k4(a [100]int) [100]int {
min := (-1) << 63
for i := min; i < min+50; i++ { // ERROR "Induction variable: limits \[-9223372036854775808,-9223372036854775758\), increment 1$"
a[i-min] = i // ERROR "Proved IsInBounds$"
}
return a
}
func k5(a [100]int) [100]int {
max := (1 << 63) - 1
for i := max - 50; i < max; i++ { // ERROR "Induction variable: limits \[9223372036854775757,9223372036854775807\), increment 1"
a[i-max+50] = i // ERROR "Proved IsInBounds$"
a[i-(max-70)] = i // ERROR "Proved IsInBounds$"
}
return a
}
func nobce1() {
// tests overflow of max-min
a := int64(9223372036854774057)
b := int64(-1547)
z := int64(1337)
if a%z == b%z {
panic("invalid test: modulos should differ")
}
for i := b; i < a; i += z {
// No induction variable is possible because i will overflow a first iteration.
useString("foobar")
}
}
func nobce2(a string) {
for i := int64(0); i < int64(len(a)); i++ { // ERROR "Induction variable: limits \[0,\?\), increment 1$"
useString(a[i:]) // ERROR "Proved IsSliceInBounds$"
}
for i := int64(0); i < int64(len(a))-31337; i++ { // ERROR "Induction variable: limits \[0,\?\), increment 1$"
useString(a[i:]) // ERROR "Proved IsSliceInBounds$"
}
for i := int64(0); i < int64(len(a))+int64(-1<<63); i++ { // ERROR "Induction variable: limits \[0,\?\), increment 1$"
useString(a[i:]) // ERROR "Proved IsSliceInBounds$"
}
j := int64(len(a)) - 123
for i := int64(0); i < j+123+int64(-1<<63); i++ { // ERROR "Induction variable: limits \[0,\?\), increment 1$"
useString(a[i:]) // ERROR "Proved IsSliceInBounds$"
}
for i := int64(0); i < j+122+int64(-1<<63); i++ { // ERROR "Induction variable: limits \[0,\?\), increment 1$"
// len(a)-123+122+MinInt overflows when len(a) == 0, so a bound check is needed here
useString(a[i:])
}
}
func nobce3(a [100]int64) [100]int64 {
min := int64((-1) << 63)
max := int64((1 << 63) - 1)
for i := min; i < max; i++ { // ERROR "Induction variable: limits \[-9223372036854775808,9223372036854775807\), increment 1$"
a[i] = i
}
return a
}
//go:noinline
func useString(a string) {
}
//go:noinline
func useSlice(a []int) {
}
func main() {
}
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