Fix use of COMPLETE_TYPE_P for -Wstrict-aliasing=1

[Richard Sandiford]

The handling of -Wstrict-aliasing=1 applied COMPLETE_TYPE_P to the
pointer type rather than the pointer target, so missed the warnings
for "struct incomplete" in the testcase.

I couldn't find any existing C tests for -Wstrict-aliasing=1,
so I added a few extra tests besides the ones fixed by the patch.
I'm sure there's lots more we could test -- this is just supposed
to be better than the status quo (i.e. nothing).

Tested on aarch64-linux-gnu.  Not sure this is a regression in any
meaningful sense, so I guess it should wait for GCC 10.  OK for stage 1?

Richard


2019-03-21  Richard Sandiford  <richard.sandiford@arm.com>

gcc/c-family/
	* c-warn.c (strict_aliasing_warning): Apply COMPLETE_TYPE_P to
	the pointer target rather than the pointer itself.

gcc/testsuite/
	* gcc.dg/alias-16.c: New test.

Index: gcc/c-family/c-warn.c
===================================================================
--- gcc/c-family/c-warn.c	2019-03-21 13:01:18.000000000 +0000
+++ gcc/c-family/c-warn.c	2019-03-21 13:01:18.453582721 +0000
@@ -746,7 +746,7 @@ strict_aliasing_warning (location_t loc,
 	 are not revealed at higher levels.  */
       alias_set_type set1 = get_alias_set (TREE_TYPE (otype));
       alias_set_type set2 = get_alias_set (TREE_TYPE (type));
-      if (!COMPLETE_TYPE_P (type)
+      if (!COMPLETE_TYPE_P (TREE_TYPE (type))
 	  || !alias_sets_must_conflict_p (set1, set2))
 	{
 	  warning_at (loc, OPT_Wstrict_aliasing,
Index: gcc/testsuite/gcc.dg/alias-16.c
===================================================================
--- /dev/null	2019-03-08 11:40:14.606883727 +0000
+++ gcc/testsuite/gcc.dg/alias-16.c	2019-03-21 13:01:18.453582721 +0000
@@ -0,0 +1,46 @@
+/* { dg-do compile } */
+/* { dg-options "-Wstrict-aliasing=1 -fstrict-aliasing" } */
+
+struct incomplete;
+struct s1 { int i; };
+struct s2 { double d; };
+
+void
+f (int *i, double *d, struct s1 *s1, struct s2 *s2, char *c)
+{
+  (char *) i;
+  (char *) d;
+  (char *) s1;
+  (char *) s2;
+  (char *) c;
+
+  (int *) i;
+  (int *) d; /* { dg-warning "dereferencing type-punned pointer might break strict-aliasing rules" } */
+  (int *) s1; /* { dg-warning "dereferencing type-punned pointer might break strict-aliasing rules" } */
+  (int *) s2; /* { dg-warning "dereferencing type-punned pointer might break strict-aliasing rules" } */
+  (int *) c;
+
+  (double *) i; /* { dg-warning "dereferencing type-punned pointer might break strict-aliasing rules" } */
+  (double *) d;
+  (double *) s1; /* { dg-warning "dereferencing type-punned pointer might break strict-aliasing rules" } */
+  (double *) s2; /* { dg-warning "dereferencing type-punned pointer might break strict-aliasing rules" } */
+  (double *) c;
+
+  (struct incomplete *) i; /* { dg-warning "dereferencing type-punned pointer might break strict-aliasing rules" } */
+  (struct incomplete *) d; /* { dg-warning "dereferencing type-punned pointer might break strict-aliasing rules" } */
+  (struct incomplete *) s1; /* { dg-warning "dereferencing type-punned pointer might break strict-aliasing rules" } */
+  (struct incomplete *) s2; /* { dg-warning "dereferencing type-punned pointer might break strict-aliasing rules" } */
+  (struct incomplete *) c; /* { dg-warning "dereferencing type-punned pointer might break strict-aliasing rules" } */
+
+  (struct s1 *) i; /* { dg-warning "dereferencing type-punned pointer might break strict-aliasing rules" } */
+  (struct s1 *) d; /* { dg-warning "dereferencing type-punned pointer might break strict-aliasing rules" } */
+  (struct s1 *) s1;
+  (struct s1 *) s2; /* { dg-warning "dereferencing type-punned pointer might break strict-aliasing rules" } */
+  (struct s1 *) c;
+
+  (struct s2 *) i; /* { dg-warning "dereferencing type-punned pointer might break strict-aliasing rules" } */
+  (struct s2 *) d; /* { dg-warning "dereferencing type-punned pointer might break strict-aliasing rules" } */
+  (struct s2 *) s1; /* { dg-warning "dereferencing type-punned pointer might break strict-aliasing rules" } */
+  (struct s2 *) s2;
+  (struct s2 *) c;
+}