cmd/compile: casts from slices to array pointers are known to be non-nil

The cast is proceeded by a bounds check. If the bounds check passes
then we know the pointer in the slice is non-nil.

... except casts to pointers of 0-sized arrays. They are strange, as
the bounds check can pass for a nil input.

Change-Id: Ic01cf4a82d59fbe3071d4b271c94efca9cafaec1
Reviewed-on: https://go-review.googlesource.com/c/go/+/479335
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Keith Randall <khr@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/test/nilptr3.go b/test/nilptr3.go
index 0e818ebf66..5f08a5b20c 100644
--- a/test/nilptr3.go
+++ b/test/nilptr3.go
@@ -248,3 +248,10 @@ func f10(p **int) int {
 	/* */
 	*p // ERROR "removed nil check"
 }
+
+func f11(x []byte) {
+	p := (*[0]byte)(x)
+	_ = *p // ERROR "generated nil check"
+	q := (*[4]byte)(x)
+	_ = *q // ERROR "removed nil check"
+}