cmd/compile: handle non-negatives in prove

Handle this case:
if 0 <= i && i < len(a) {
        use a[i]
}

Shaves about 5k from pkg/tools/linux_amd64/*.

Change-Id: I6675ff49aa306b0d241b074c5738e448204cd981
Reviewed-on: https://go-review.googlesource.com/21431
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 22 insertions, 3 deletions

diff --git a/test/prove.go b/test/prove.go
index 4fc1d674d8..a78adf03dc 100644
--- a/test/prove.go
+++ b/test/prove.go
@@ -29,11 +29,30 @@ func f1(a []int) int {
 }
 
 func f1b(a []int, i int, j uint) int {
-	if i >= 0 && i < len(a) { // TODO: handle this case
-		return a[i]
+	if i >= 0 && i < len(a) {
+		return a[i] // ERROR "Proved non-negative bounds IsInBounds$"
+	}
+	if i >= 10 && i < len(a) {
+		return a[i] // ERROR "Proved non-negative bounds IsInBounds$"
+	}
+	if i >= 10 && i < len(a) {
+		return a[i] // ERROR "Proved non-negative bounds IsInBounds$"
+	}
+	if i >= 10 && i < len(a) { // todo: handle this case
+		return a[i-10]
 	}
 	if j < uint(len(a)) {
-		return a[j] // ERROR "Proved IsInBounds"
+		return a[j] // ERROR "Proved IsInBounds$"
+	}
+	return 0
+}
+
+func f1c(a []int, i int64) int {
+	c := uint64(math.MaxInt64 + 10) // overflows int
+	d := int64(c)
+	if i >= d && i < int64(len(a)) {
+		// d overflows, should not be handled.
+		return a[i]
 	}
 	return 0
 }