> From: Andreas Schwab [mailto:schwab@linux-m68k.org]
> 
> This adds a full byte of padding between each bitfield.  If you want a
> single padding bit you should use :1, but you also need to update the
> test to check for 0x44434241 (0x88868482 is impossible, since that
> requires at least 8 bits per bitfield).

Actually if I understood C99 correctly it depends on the storage unit
allocated for the bitfield preceding the 0 length bitfield. Instead of
trying to cover all possible value read from this bitfield I rewrote the
test to check if bswap misinterpret the expression and replace it with
a load or load+bswap. This reduce the number of possible values to 2
and thus makes the test less fragile and easier to understand.

By the way, I couldn't understand how you reached the value
0x44434241. Can you explain me?

Here is the ChangeLog:

2014-05-29  Thomas Preud'homme  <thomas.preudhomme@arm.com>

	* gcc.c-torture/execute/bswap-2.c: Add alignment constraints to
	bitfield and test wrong results instead of correct results to make the
	test more portable.

And the patch:

diff --git a/gcc/testsuite/gcc.c-torture/execute/bswap-2.c b/gcc/testsuite/gcc.c-torture/execute/bswap-2.c
index 38f18fd..a47e01a 100644
--- a/gcc/testsuite/gcc.c-torture/execute/bswap-2.c
+++ b/gcc/testsuite/gcc.c-torture/execute/bswap-2.c
@@ -6,8 +6,11 @@ typedef __UINT32_TYPE__ unsigned;
 
 struct bitfield {
   unsigned char f0:7;
+  unsigned char   :1;
   unsigned char f1:7;
+  unsigned char   :1;
   unsigned char f2:7;
+  unsigned char   :1;
   unsigned char f3:7;
 };
 
@@ -74,11 +77,17 @@ main ()
     return 0;
   bfin.inval = (struct ok) { 0x83, 0x85, 0x87, 0x89 };
   out = partial_read_le32 (bfin);
-  if (out != 0x09070503 && out != 0x88868482 && out != 0x78306141)
+  /* Test what bswap would do if its check are not strict enough instead of
+     what is the expected result as there is too many possible results with
+     bitfields.  */
+  if (out == 0x89878583)
     __builtin_abort ();
   bfin.inval = (struct ok) { 0x83, 0x85, 0x87, 0x89 };
   out = partial_read_be32 (bfin);
-  if (out != 0x03050709 && out != 0x82848688 && out != 0x41613078)
+  /* Test what bswap would do if its check are not strict enough instead of
+     what is the expected result as there is too many possible results with
+     bitfields.  */
+  if (out == 0x83858789)
     __builtin_abort ();
   out = fake_read_le32 (cin, &cin[2]);
   if (out != 0x89018583)

Best regards,

Thomas