diff --git a/src/cmd/gc/dcl.c b/src/cmd/gc/dcl.c
index 9f7244fc7ca..d5d3a9bf4df 100644
--- a/src/cmd/gc/dcl.c
+++ b/src/cmd/gc/dcl.c
@@ -1483,3 +1483,54 @@ loop:
 	c = listnext(&citer);
 	goto loop;
 }
+
+/*
+ * look for
+ *	unsafe.Sizeof
+ *	unsafe.Offsetof
+ * rewrite with a constant
+ */
+Node*
+unsafenmagic(Node *l, Node *r)
+{
+	Node *n;
+	Sym *s;
+	long v;
+	Val val;
+
+	if(l == N || r == N)
+		goto no;
+	if(l->op != ONAME)
+		goto no;
+	s = l->sym;
+	if(s == S)
+		goto no;
+	if(strcmp(s->opackage, "unsafe") != 0)
+		goto no;
+
+	if(strcmp(s->name, "Sizeof") == 0) {
+		walktype(r, Erv);
+		if(r->type == T)
+			goto no;
+		v = r->type->width;
+		goto yes;
+	}
+	if(strcmp(s->name, "Offsetof") == 0) {
+		if(r->op != ODOT && r->op != ODOTPTR)
+			goto no;
+		walktype(r, Erv);
+		v = n->xoffset;
+		goto yes;
+	}
+
+no:
+	return N;
+
+yes:
+	val.ctype = CTINT;
+	val.u.xval = mal(sizeof(*n->val.u.xval));
+	mpmovecfix(val.u.xval, v);
+	n = nod(OLITERAL, N, N);
+	n->val = val;
+	return n;
+}
diff --git a/src/cmd/gc/go.h b/src/cmd/gc/go.h
index 7b861d38c26..436ddd9a9b9 100644
--- a/src/cmd/gc/go.h
+++ b/src/cmd/gc/go.h
@@ -763,6 +763,7 @@ void	constiter(Node*, Type*, Node*);
 
 void	funclit0(Type*);
 Node*	funclit1(Type*, Node*);
+Node*	unsafenmagic(Node*, Node*);
 
 /*
  *	export.c
diff --git a/src/cmd/gc/go.y b/src/cmd/gc/go.y
index 5ed7ed09c9f..29a08912d4a 100644
--- a/src/cmd/gc/go.y
+++ b/src/cmd/gc/go.y
@@ -804,7 +804,9 @@ pexpr:
 	}
 |	pexpr '(' oexpr_list ')'
 	{
-		$$ = nod(OCALL, $1, $3);
+		$$ = unsafenmagic($1, $3);
+		if($$ == N)
+			$$ = nod(OCALL, $1, $3);
 	}
 |	LLEN '(' expr ')'
 	{
diff --git a/src/cmd/gc/unsafe.go b/src/cmd/gc/unsafe.go
index 2b2187b3e3b..47703f6e0ff 100644
--- a/src/cmd/gc/unsafe.go
+++ b/src/cmd/gc/unsafe.go
@@ -6,3 +6,5 @@
 package PACKAGE
 
 type	Pointer	*any;
+func	Offsetof(any) int;
+func	Sizeof(any) int;