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, 20 insertions, 1 deletions

diff --git a/src/runtime/checkptr.go b/src/runtime/checkptr.go
index 59891a06a5..d42950844b 100644
--- a/src/runtime/checkptr.go
+++ b/src/runtime/checkptr.go
@@ -16,11 +16,30 @@ func checkptrAlignment(p unsafe.Pointer, elem *_type, n uintptr) {
 	}
 
 	// Check that (*[n]elem)(p) doesn't straddle multiple heap objects.
-	if size := n * elem.size; size > 1 && checkptrBase(p) != checkptrBase(add(p, size-1)) {
+	// TODO(mdempsky): Fix #46938 so we don't need to worry about overflow here.
+	if checkptrStraddles(p, n*elem.size) {
 		throw("checkptr: converted pointer straddles multiple allocations")
 	}
 }
 
+// checkptrStraddles reports whether the first size-bytes of memory
+// addressed by ptr is known to straddle more than one Go allocation.
+func checkptrStraddles(ptr unsafe.Pointer, size uintptr) bool {
+	if size <= 1 {
+		return false
+	}
+
+	end := add(ptr, size-1)
+	if uintptr(end) < uintptr(ptr) {
+		return true
+	}
+
+	// TODO(mdempsky): Detect when [ptr, end] contains Go allocations,
+	// but neither ptr nor end point into one themselves.
+
+	return checkptrBase(ptr) != checkptrBase(end)
+}
+
 func checkptrArithmetic(p unsafe.Pointer, originals []unsafe.Pointer) {
 	if 0 < uintptr(p) && uintptr(p) < minLegalPointer {
 		throw("checkptr: pointer arithmetic computed bad pointer value")