Re: [PATCH 1/1] Fix bit-position comparison

[Richard Biener <rguenther@suse.de>]

On Wed, 27 Jul 2022, juzhe.zhong@rivai.ai wrote:

> From: zhongjuzhe <juzhe.zhong@rivai.ai>
> 
> gcc/ChangeLog:
> 
>         * expr.cc (expand_assignment): Change GET_MODE_PRECISION to GET_MODE_BITSIZE
> 
> ---
>  gcc/expr.cc | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/gcc/expr.cc b/gcc/expr.cc
> index 80bb1b8a4c5..ac2b3c07df6 100644
> --- a/gcc/expr.cc
> +++ b/gcc/expr.cc
> @@ -5574,7 +5574,7 @@ expand_assignment (tree to, tree from, bool nontemporal)
>  	 code contains an out-of-bounds access to a small array.  */
>        if (!MEM_P (to_rtx)
>  	  && GET_MODE (to_rtx) != BLKmode
> -	  && known_ge (bitpos, GET_MODE_PRECISION (GET_MODE (to_rtx))))
> +	  && known_ge (bitpos, GET_MODE_BITSIZE (GET_MODE (to_rtx))))

I think this has the chance to go wrong with regard to endianess.
Consider to_rtx with 32bit mode size but 12bit mode precision.  bitpos
is relative to the start of to_rtx as if it were in memory and bitsize
determines the contiguous region affected.  But since we are actually
storing into a non-memory endianess comes into play.

So no, I don't think the patch is correct, it would be way more
complicated to get the desired improvement.

Richard.

>  	{
>  	  expand_normal (from);
>  	  result = NULL;
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
HRB 36809 (AG Nuernberg)