cmd/gc: fix uintptr(nil) issues.

A constant node of type uintptr with a nil literal could
happen in two cases: []int(nil)[1:] and
uintptr(unsafe.Pointer(nil)).

Fixes #4614.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7059043

src/cmd/gc/const.c

@@ -162,6 +162,16 @@ convlit1(Node **np, Type *t, int explicit)
 		case TFUNC:
 		case TUNSAFEPTR:
 			break;
+
+		case TUINTPTR:
+			// A nil literal may be converted to uintptr
+			// if it is an unsafe.Pointer
+			if(n->type->etype == TUNSAFEPTR) {
+				n->val.u.xval = mal(sizeof(*n->val.u.xval));
+				mpmovecfix(n->val.u.xval, 0);
+				n->val.ctype = CTINT;
+			} else
+				goto bad;
 		}
 		break;
 

src/cmd/gc/gen.c

@@ -810,7 +810,11 @@ cgen_slice(Node *n, Node *res)
 		checkref(n->left);
 	}
 
-	src = *n->left;
+	if(isnil(n->left)) {
+		tempname(&src, n->left->type);
+		cgen(n->left, &src);
+	} else
+		src = *n->left;
 	src.xoffset += Array_array;
 	src.type = types[TUINTPTR];
 

test/fixedbugs/issue4614.go

@@ -0,0 +1,20 @@
+// compile
+
+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 4614: slicing of nil slices confuses the compiler
+// with a uintptr(nil) node.
+
+package p
+
+import "unsafe"
+
+var n int
+
+var _ = []int(nil)[1:]
+var _ = []int(nil)[n:]
+
+var _ = uintptr(unsafe.Pointer(nil))
+var _ = unsafe.Pointer(uintptr(0))