patch to generate align 0 for packed attributes

[Chandra Chavva]

Hi Jeff,


On Thu, 27 Jul 2000, Jeffrey A Law wrote:

>   In message < Pine.SOL.3.91.1000726120707.23599I-100000@emperor.cygnus.com >you 
> write:
>   > Hi,
>   > 
>   > If a member of a struct has packed attribute and initialized, the compiler 
>   > does not correct alignment for such variables. 
>   > 
>   > Here is the testcase which fails for mips architecture,
> My first question, I'm guessing that the belief is that the MIPS assembler
> is automatically aligning data to their natural boundary?
> 
  In the example i mentioned, the compiled generates 

	.byte 1
	.half 2

  In the mips assembler, the .byte directive is aligned to 0 and the
.half directive is aligned to 1 generating the above to

   address    value
   0000	      01	.byte 1
   0001	      00 0200   .half 2

In the assembler, the directives are aligned only if .align is not zero. 

> If so, how does an .align 0 stop that?  That seems like awful strange behavior
> from an assembler.

  Yes it does. This is the check done in the assembler which does that:

s_cons (log_size)
     int log_size;
{
  symbolS *label;

  label = insn_labels != NULL ? insn_labels->label : NULL;
  mips_emit_delays (false);
  if (log_size > 0 && auto_align)
    mips_align (log_size, 0, label);
  mips_clear_insn_labels ();
  cons (1 << log_size);
}

auto_align is a static variable is set to 1 if .align has some value else 
it is set to 0.

> 
> Second, do all MIPS assemblers do this?  If not, then we need to test for
> it using autoconf.
> 
> Third, you didn't include any documentation on the new target macros.
> 
  Yes i realized this after i sent out the patch.

> Fourth, I would be very leery of this code since it appears to be laying
> out bitfields within structures.  If you do it wrong, you'll break ABI
> compatibility.

  Yes my patch is wrong as it align's to 0 for every element of a struct. 
Here is the revised patch.

Please let me know it is okay to apply.

Thanks
Chandra


	* varasm.c (output_constructor): Add .align 0 for packed vars.

Index: varasm.c
===================================================================
RCS file: /cvs/gcc/egcs/gcc/varasm.c,v
retrieving revision 1.125
diff -p -r1.125 varasm.c
*** varasm.c    2000/06/22 09:42:10     1.125
--- varasm.c    2000/07/27 19:21:03
*************** output_constructor (exp, size)
*** 4471,4476 ****
--- 4471,4485 ----
              assemble_zeros (bitpos - total_bytes);
              total_bytes = bitpos;
            }
+           else if (field != 0 && DECL_PACKED (field))
+             {
+                /* Some assemblers automaticallly align a datum 
according to
+                   its size if no align directive is specified.  The datum,
+                   however, may be declared with 'packed' attribute, so we
+                   have to disable such a feature.  */
+ 
+                ASM_OUTPUT_ALIGN (asm_out_file, 0);
+             }
  
          /* Determine size this element should occupy.  */
          if (field)