cmd/compile,runtime: change unsafe.Slice((*T)(nil), 0) to return []T(nil)

This CL removes the unconditional OCHECKNIL check added in
walkUnsafeSlice by instead passing it as a pointer to
runtime.unsafeslice, and hiding the check behind a `len == 0` check.

While here, this CL also implements checkptr functionality for
unsafe.Slice and disallows use of unsafe.Slice with //go:notinheap
types.

Updates #46742.

Change-Id: I743a445ac124304a4d7322a7fe089c4a21b9a655
Reviewed-on: https://go-review.googlesource.com/c/go/+/331070
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/src/runtime/testdata/testprog/checkptr.go b/src/runtime/testdata/testprog/checkptr.go
index e0a2794f4c..f76b64ad96 100644
--- a/src/runtime/testdata/testprog/checkptr.go
+++ b/src/runtime/testdata/testprog/checkptr.go
@@ -13,6 +13,8 @@ func init() {
 	register("CheckPtrArithmetic2", CheckPtrArithmetic2)
 	register("CheckPtrSize", CheckPtrSize)
 	register("CheckPtrSmall", CheckPtrSmall)
+	register("CheckPtrSliceOK", CheckPtrSliceOK)
+	register("CheckPtrSliceFail", CheckPtrSliceFail)
 }
 
 func CheckPtrAlignmentNoPtr() {
@@ -49,3 +51,14 @@ func CheckPtrSize() {
 func CheckPtrSmall() {
 	sink2 = unsafe.Pointer(uintptr(1))
 }
+
+func CheckPtrSliceOK() {
+	p := new([4]int64)
+	sink2 = unsafe.Slice(&p[1], 3)
+}
+
+func CheckPtrSliceFail() {
+	p := new(int64)
+	sink2 = p
+	sink2 = unsafe.Slice(p, 100)
+}