cmd/compile: use len(s)<=cap(s) to remove more bounds checks

When we discover a relation x <= len(s), also discover the relation
x <= cap(s).  That way, in situations like:

a := s[x:]  // tests 0 <= x <= len(s)
b := s[:x]  // tests 0 <= x <= cap(s)

the second check can be eliminated.

Fixes #16813

Change-Id: Ifc037920b6955e43bac1a1eaf6bac63a89cfbd44
Reviewed-on: https://go-review.googlesource.com/33633
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alexandru Moșoi <alexandru@mosoi.ro>
Reviewed-by: David Chase <drchase@google.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/test/prove.go b/test/prove.go
index 9ef8949e1c..5f4de604c6 100644
--- a/test/prove.go
+++ b/test/prove.go
@@ -451,6 +451,18 @@ func f14(p, q *int, a []int) {
 	useInt(a[i2+j]) // ERROR "Proved boolean IsInBounds$"
 }
 
+func f15(s []int, x int) {
+	useSlice(s[x:])
+	useSlice(s[:x]) // ERROR "Proved IsSliceInBounds$"
+}
+
+func f16(s []int) []int {
+	if len(s) >= 10 {
+		return s[:10] // ERROR "Proved non-negative bounds IsSliceInBounds$"
+	}
+	return nil
+}
+
 //go:noinline
 func useInt(a int) {
 }