Bug 52238

Confirmed.  The alignment doesn't seem to be ignored though, but the fields
position is not updated appropriately.

in stor-layout.c function place_field() it is said "We already align ms_struct fields, so don't re-align them".  This isn't completely true, as desired_align isn't handled in ms_struct case.

Following patch seems to solve this issue:

Index: stor-layout.c
===================================================================
--- stor-layout.c       (revision 184262)
+++ stor-layout.c       (working copy)
@@ -1141,15 +1141,14 @@
     }

   /* Does this field automatically have alignment it needs by virtue
-     of the fields that precede it and the record's own alignment?
-     We already align ms_struct fields, so don't re-align them.  */
-  if (known_align < desired_align
-      && !targetm.ms_bitfield_layout_p (rli->t))
+     of the fields that precede it and the record's own alignment?  */
+  if (known_align < desired_align)
     {
       /* No, we need to skip space before this field.
         Bump the cumulative size to multiple of field alignment.  */

-      if (DECL_SOURCE_LOCATION (field) != BUILTINS_LOCATION)
+      if (!targetm.ms_bitfield_layout_p (rli->t)
+          && DECL_SOURCE_LOCATION (field) != BUILTINS_LOCATION)
        warning (OPT_Wpadded, "padding struct to align %q+D", field);

       /* If the alignment is still within offset_align, just align
@@ -1302,7 +1301,7 @@
                 type size!) */
              HOST_WIDE_INT bitsize = tree_low_cst (DECL_SIZE (field), 1);

-             if (rli->remaining_in_alignment < bitsize)
+             if (rli->remaining_in_alignment < bitsize) /* $$$$ */
                {
                  HOST_WIDE_INT typesize = tree_low_cst (TYPE_SIZE (type), 1);

Ok, let's modify the test case - s/char a;/char a:6;/:

struct {
    char a:6;
    char b __attribute__ ((aligned (16)));
} s;

int main()
{
    return (__PTRDIFF_TYPE__)&s.b & 15;
}

The b member is still misaligned, moreover, its offset is equal to 0x11 after patching.

Hmm, right.  The previous field needs to be cleared for ms-bitfields, too.

Index: stor-layout.c
===================================================================
--- stor-layout.c       (revision 184287)
+++ stor-layout.c       (working copy)
@@ -1141,15 +1141,14 @@
     }

   /* Does this field automatically have alignment it needs by virtue
-     of the fields that precede it and the record's own alignment?
-     We already align ms_struct fields, so don't re-align them.  */
-  if (known_align < desired_align
-      && !targetm.ms_bitfield_layout_p (rli->t))
+     of the fields that precede it and the record's own alignment?  */
+  if (known_align < desired_align)
     {
       /* No, we need to skip space before this field.
         Bump the cumulative size to multiple of field alignment.  */

-      if (DECL_SOURCE_LOCATION (field) != BUILTINS_LOCATION)
+      if (!targetm.ms_bitfield_layout_p (rli->t)
+          && DECL_SOURCE_LOCATION (field) != BUILTINS_LOCATION)
        warning (OPT_Wpadded, "padding struct to align %q+D", field);

       /* If the alignment is still within offset_align, just align
@@ -1171,7 +1170,8 @@

       if (! TREE_CONSTANT (rli->offset))
        rli->offset_align = desired_align;
-
+      if (targetm.ms_bitfield_layout_p (rli->t))
+        rli->prev_field = NULL;
     }

   /* Handle compatibility with PCC.  Note that if the record has any

Second patch tested with VLC.

After rebuilding gcc 4.6.2 package from Debian, mingw-w64-dev package with new gcc, and VLC including all libraries (qt4, ffmpeg etc), I got DXVA2 decoding running perfectly.

Previously it would give uncorrectly decoded pictures because of different struct layout given to the GPU drivers.

So it works fine for me.

According to FATE (the Libav testsuite), the patch fixes the issue. Thanks.

It does not play a role here, nonetheless __SIZE_TYPE__ should be more correct than __PTRDIFF_TYPE__.

(In reply to comment #7)
> It does not play a role here, nonetheless __SIZE_TYPE__ should be more correct
> than __PTRDIFF_TYPE__.

I think here is the question for your reply missing in this bug-report.

Author: ktietz
Date: Mon Feb 20 22:05:08 2012
New Revision: 184409

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=184409
Log:
	PR target/52238
	* stor-layout.c (place_field): Handle desired_align for
	ms-bitfields, too.

	* gcc.dg/bf-ms-layout-3.c: New testcase.

Added:
    trunk/gcc/testsuite/gcc.dg/bf-ms-layout-3.c   (with props)
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/stor-layout.c
    trunk/gcc/testsuite/ChangeLog

Propchange: trunk/gcc/testsuite/gcc.dg/bf-ms-layout-3.c
            ('svn:eol-style' added)

Propchange: trunk/gcc/testsuite/gcc.dg/bf-ms-layout-3.c
            ('svn:mime-type' added)

Author: ktietz
Date: Mon Feb 20 22:09:48 2012
New Revision: 184410

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=184410
Log:
	PR target/52238
	* stor-layout.c (place_field): Handle desired_align for
	ms-bitfields, too.

	* gcc.dg/bf-ms-layout-3.c: New testcase.

Added:
    branches/gcc-4_6-branch/gcc/testsuite/gcc.dg/bf-ms-layout-3.c   (with props)
Modified:
    branches/gcc-4_6-branch/gcc/ChangeLog
    branches/gcc-4_6-branch/gcc/stor-layout.c
    branches/gcc-4_6-branch/gcc/testsuite/ChangeLog

Propchange: branches/gcc-4_6-branch/gcc/testsuite/gcc.dg/bf-ms-layout-3.c
            ('svn:eol-style' added)

Propchange: branches/gcc-4_6-branch/gcc/testsuite/gcc.dg/bf-ms-layout-3.c
            ('svn:mime-type' added)

I don't intend to back-port this patch to 4.5, or to 4.4.  Therefore close report

*** Bug 50057 has been marked as a duplicate of this bug. ***