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Re: [PATCH] Fix A < 0 ? <sign bit of A> : 0 optimization (PR middle-end/58564)


On Mon, 30 Sep 2013, Jakub Jelinek wrote:

> Hi!
> 
> Apparently sign_bit_p looks through both sign and zero extensions.
> That is just fine for the single bit test optimization, where we know the
> constant is a power of two, and it is (A & power_of_two) == 0 (or != 0)
> test.  sign_bit_p is also used to check for minimum value of some integral
> type, because it is called with the same argument twice, it must be
> INTEGER_CST if non-NULL is returned and thus there is no zero extensions.
> But in the last spot where sign_bit_p is used, sign extensions would be
> fine, ((int) x) < 0 for say signed char or signed short x iff x < 0,
> but if there is a zero extension, it is just a possible missed optimization
> on the comparison.
> 
> Fixed thusly, bootstrapped/regtested on x86_64-linux and i686-linux, ok for
> trunk/4.8?

Ok.

Thanks,
Richard.

> 2013-09-30  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR middle-end/58564
> 	* fold-const.c (fold_ternary_loc): For A < 0 : <sign bit of A> : 0
> 	optimization, punt if sign_bit_p looked through any zero extension.
> 
> 	* gcc.c-torture/execute/pr58564.c: New test.
> 
> --- gcc/fold-const.c.jj	2013-09-27 15:42:37.000000000 +0200
> +++ gcc/fold-const.c	2013-09-30 11:19:06.333978484 +0200
> @@ -14196,14 +14196,29 @@ fold_ternary_loc (location_t loc, enum t
>  	  && integer_zerop (op2)
>  	  && (tem = sign_bit_p (TREE_OPERAND (arg0, 0), arg1)))
>  	{
> +	  /* sign_bit_p looks through both zero and sign extensions,
> +	     but for this optimization only sign extensions are
> +	     usable.  */
> +	  tree tem2 = TREE_OPERAND (arg0, 0);
> +	  while (tem != tem2)
> +	    {
> +	      if (TREE_CODE (tem2) != NOP_EXPR
> +		  || TYPE_UNSIGNED (TREE_TYPE (TREE_OPERAND (tem2, 0))))
> +		{
> +		  tem = NULL_TREE;
> +		  break;
> +		}
> +	      tem2 = TREE_OPERAND (tem2, 0);
> +	    }
>  	  /* sign_bit_p only checks ARG1 bits within A's precision.
>  	     If <sign bit of A> has wider type than A, bits outside
>  	     of A's precision in <sign bit of A> need to be checked.
>  	     If they are all 0, this optimization needs to be done
>  	     in unsigned A's type, if they are all 1 in signed A's type,
>  	     otherwise this can't be done.  */
> -	  if (TYPE_PRECISION (TREE_TYPE (tem))
> -	      < TYPE_PRECISION (TREE_TYPE (arg1))
> +	  if (tem
> +	      && TYPE_PRECISION (TREE_TYPE (tem))
> +		 < TYPE_PRECISION (TREE_TYPE (arg1))
>  	      && TYPE_PRECISION (TREE_TYPE (tem))
>  		 < TYPE_PRECISION (type))
>  	    {
> --- gcc/testsuite/gcc.c-torture/execute/pr58564.c.jj	2013-09-30 11:09:38.691122488 +0200
> +++ gcc/testsuite/gcc.c-torture/execute/pr58564.c	2013-09-30 11:09:14.000000000 +0200
> @@ -0,0 +1,14 @@
> +/* PR middle-end/58564 */
> +
> +extern void abort (void);
> +int a, b;
> +short *c, **d = &c;
> +
> +int
> +main ()
> +{
> +  b = (0, 0 > ((&c == d) & (1 && (a ^ 1)))) | 0U;
> +  if (b != 0)
> +    abort ();
> +  return 0;
> +}
> 
> 	Jakub
> 
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE / SUSE Labs
SUSE LINUX Products GmbH - Nuernberg - AG Nuernberg - HRB 16746
GF: Jeff Hawn, Jennifer Guild, Felix Imend


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