Re: [PATCH] Fix iv_number_of_iterations (PR rtl-optimization/55838)

[Richard Biener <rguenther at suse dot de>]

On Thu, 3 Jan 2013, Jakub Jelinek wrote:

> Hi!
> 
> When one (or both) IVs have extend_mode wider than mode, but step doesn't
> fit into mode (the IV is
> (subreg:MODE (plus:EXTEND_MODE base (mult:EXTEND_MODE i step)) lowpart)
> ), such as for EXTEND_MODE SImode, MODE QImode and step e.g. 129, 128, 256
> or 517, iv_number_of_iterations can create invalid rtl.  I think it is safe
> to just use the lowpart subreg of the step.  The second hunk isn't enough,
> we use iv0.step resp. iv1.step directly in several other places in the
> routine, and the first hunk IMHO isn't enough either, if for the above
> extend_mode SI and mode QI iv1.step is 128, the first hunk will make -128 out of
> it, but then we negate it and get step 128 out of it again, not valid QImode
> CONST_INT, and use it e.g. as argument to UMOD.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

Truncating step is ok I think and so is the patch unless Zdenek
has any comments in the next few days.

Thanks,
Richard.

> Or shall we just punt (either as done by H.J. in the PR, or in
> iv_number_of_iterations if the steps aren't valid mode CONST_INTs)?
> 
> 2013-01-03  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR rtl-optimization/55838
> 	* loop-iv.c (iv_number_of_iterations): Call lowpart_subreg on
> 	iv0.step, iv1.step and step.
> 
> 	* gcc.dg/pr55838.c: New test.
> 
> --- gcc/loop-iv.c.jj	2012-10-22 08:42:25.000000000 +0200
> +++ gcc/loop-iv.c	2013-01-02 09:17:42.215591646 +0100
> @@ -2406,6 +2406,9 @@ iv_number_of_iterations (struct loop *lo
>        iv1.step = const0_rtx;
>      }
>  
> +  iv0.step = lowpart_subreg (mode, iv0.step, comp_mode);
> +  iv1.step = lowpart_subreg (mode, iv1.step, comp_mode);
> +
>    /* This is either infinite loop or the one that ends immediately, depending
>       on initial values.  Unswitching should remove this kind of conditions.  */
>    if (iv0.step == const0_rtx && iv1.step == const0_rtx)
> @@ -2516,6 +2519,7 @@ iv_number_of_iterations (struct loop *lo
>  	step = simplify_gen_unary (NEG, comp_mode, iv1.step, comp_mode);
>        else
>  	step = iv0.step;
> +      step = lowpart_subreg (mode, step, comp_mode);
>        delta = simplify_gen_binary (MINUS, comp_mode, iv1.base, iv0.base);
>        delta = lowpart_subreg (mode, delta, comp_mode);
>        delta = simplify_gen_binary (UMOD, mode, delta, step);
> --- gcc/testsuite/gcc.dg/pr55838.c.jj	2012-11-17 15:43:17.572007394 +0100
> +++ gcc/testsuite/gcc.dg/pr55838.c	2013-01-02 21:09:55.726580313 +0100
> @@ -0,0 +1,13 @@
> +/* PR rtl-optimization/55838 */
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -funroll-loops" } */
> +
> +int a;
> +unsigned char c;
> +
> +void
> +f (void)
> +{
> +  while (c++ < 2)
> +    c = a += 129;
> +}
> 
> 	Jakub
> 
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE / SUSE Labs
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