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Re: Continue strict-volatile-bitfields fixes


Hi!

How do we move this issue forward?

On Mon, 23 Jan 2012 15:46:34 +0100, Bernd Schmidt <bernds@codesourcery.com> wrote:
> On 11/29/2011 05:35 PM, Mitchell, Mark wrote:
> >>> So, I still think this patch is the best way to go forward, and it
> >> does
> >>> fix incorrect code generation. Would appreciate an OK.
> >>
> >> Ping.
> > 
> > If you don't hear any objections within a week, please proceed.
> 
> That was committed a while ago. The part in stor-layout.c that stops us
> from promoting bitfields to normal fields apparently caused some
> testsuite regressions in sh tests, where some optimization dump scans
> show that we can't perform the optimizations if there are BIT_FIELD_REFs
> rather than a normal member access.
> 
> The testcases use things like
>   enum something field:8;
> and I think applying strict-volatile-bitfields to enums is probably
> meaningless. Should we adjust semantics so that the compiler is free to
> optimize enum bitfields? The patch would look something like the below.
> Thomas has tested this on arm and sh with our internal tree.
> 
> 
> Bernd
> 
> 
> Index: gcc/stor-layout.c
> ===================================================================
> --- gcc/stor-layout.c	(revision 355696)
> +++ gcc/stor-layout.c	(working copy)
> @@ -665,8 +665,7 @@
>  	     may make a volatile object later.  */
>  	  if (TYPE_SIZE (type) != 0
>  	      && TREE_CODE (TYPE_SIZE (type)) == INTEGER_CST
> -	      && GET_MODE_CLASS (TYPE_MODE (type)) == MODE_INT
> -	      && flag_strict_volatile_bitfields <= 0)
> +	      && GET_MODE_CLASS (TYPE_MODE (type)) == MODE_INT)
>  	    {
>  	      enum machine_mode xmode
>  		= mode_for_size_tree (DECL_SIZE (decl), MODE_INT, 1);
> @@ -674,7 +673,12 @@
> 
>  	      if (xmode != BLKmode
>  		  && !(xalign > BITS_PER_UNIT && DECL_PACKED (decl))
> -		  && (known_align == 0 || known_align >= xalign))
> +		  && (known_align == 0 || known_align >= xalign)
> +                  && (flag_strict_volatile_bitfields <= 0
> +                      /* Same size makes strict volatile bitfields
> meaningless.  */
> +                      || GET_MODE_SIZE (xmode) == GET_MODE_SIZE
> (TYPE_MODE (type))
> +                      /* Strict volatile bitfields shouldn't apply to
> enums.  */
> +                      || TREE_CODE (type) == ENUMERAL_TYPE))
>  		{
>  		  DECL_ALIGN (decl) = MAX (xalign, DECL_ALIGN (decl));
>  		  DECL_MODE (decl) = xmode;
> 


GrÃÃe,
 Thomas

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