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>

1 files changed, 24 insertions, 1 deletions

diff --git a/test/loopbce.go b/test/loopbce.go
index 6ef183dea8..c93bfc8f00 100644
--- a/test/loopbce.go
+++ b/test/loopbce.go
@@ -100,6 +100,22 @@ func g0d(a string) int {
 	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
@@ -265,7 +281,14 @@ func nobce2(a string) {
 		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$"
-		// tests an overflow of StringLen-MinInt64
+		useString(a[i:]) // ERROR "Proved IsSliceInBounds$"
+	}
+	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:])
 	}
 }