[testcase] Failure with new bitfield patch (was Re: [RFA] Fix type of bitfields)

[Jakub Jelinek <jakub at redhat dot com>]

On Mon, Jan 28, 2002 at 09:43:05PM +0000, Neil Booth wrote:
> This patch is for the C / ObjC front ends only (C++ is another
> issue altogether).  Joseph wouldn't approve it since it touches code
> outside the C front end, so I'd appreciate it if a global write privs
> person would review it.
> 
> The standards require a bitfield to have a type with precision its
> width, which we don't currently do at present.  This leads to various
> issues, including integer promotion bugs and missing warnings about
> overflow.  These are fixed with the patch below, which will enable
> me to close PRs 2511, 3325, 3326 and 3347 (and possibly others).
> 
> Bootstrapped x86 Linux without regressions; testcases for all corrected
> bugs included.  Also, testcases for all bitfield-related diagnostics.

Unfortunately, this patch breaks ABI seriously (e.g. tons of hardware
drivers will stop working).

Here is a testcase, ok to commit?

ATM struct A will be 3 bytes wide while it used to be 2 bytes wide.
The problem is that with your patch at place_field time field b
has unsigned char type (while it used to have unsigned short type),
and place_field uses this to decide whether it needs to advance bitpos
to avoid spanning type's boundary:

      /* A bit field may not span the unit of alignment of its type.
         Advance to next boundary if necessary.  */
      /* ??? This code should match the code above for the
         PCC_BITFIELD_TYPE_MATTERS case.  */
      if ((offset * BITS_PER_UNIT + bit_offset) / type_align
          != ((offset * BITS_PER_UNIT + bit_offset + field_size - 1)
              / type_align))
        rli->bitpos = round_up (rli->bitpos, type_align);

Now, cannot the field types be changed in finish_struct after they all have
been placed instead of changing them early?

2002-01-02  Jakub Jelinek  <jakub@redhat.com>

	* gcc.c-torture/execute/bitfld-2.c: New test.

--- gcc/testsuite/gcc.c-torture/execute/bitfld-2.c.jj	Thu Aug 30 22:30:55 2001
+++ gcc/testsuite/gcc.c-torture/execute/bitfld-2.c	Fri Feb  1 16:31:17 2002
@@ -0,0 +1,28 @@
+/* Test whether bit field boundaries aren't advanced if bit field type
+   has alignment large enough.  */
+extern void abort (void);
+extern void exit (int);
+
+struct A {
+  unsigned short a : 5;
+  unsigned short b : 5;
+  unsigned short c : 6;
+};
+
+struct B {
+  unsigned short a : 5;
+  unsigned short b : 3;
+  unsigned short c : 8;
+};
+
+int main ()
+{
+  /* If short is not at least 16 bits wide, don't test anything.  */
+  if ((unsigned short) 65521 != 65521)
+    exit (0);
+
+  if (sizeof (struct A) != sizeof (struct B))
+    abort ();
+
+  exit (0);
+}

	Jakub