[PATCH] tree-ssa-ccp: Fix up bit_value_binop on RSHIFT_EXPR [PR102134]

Richard Biener rguenther@suse.de
Tue Aug 31 07:52:21 GMT 2021


On Tue, 31 Aug 2021, Jakub Jelinek wrote:

> Hi!
> 
> As mentioned in the PR, this hunk is guarded with !wi::neg_p (r1val | r1mask, sgn)
> which means if sgn is UNSIGNED, it is always true, but r1val | r1mask in
> widest_int is still sign-extended.  That means wi::clz (arg) returns 0,
> wi::get_precision (arg) returns some very large number
> (WIDE_INT_MAX_PRECISION, on x86_64 576 bits) and width is 64, so we end up
> with lzcount of -512 where the code afterwards expects a non-negative
> lzcount.  For arg without the sign bit set the code works right, those
> numbers are zero extended and so wi::clz must return wi::get_precision (arg) - width
> plus number of leading zero bits within the width precision.
> The patch fixes it by handling the sign-extension specially, either it could
> be done through wi::neg_p (arg) check, but lzcount == 0 works identically.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

OK.

> 2021-08-31  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR tree-optimization/102134
> 	* tree-ssa-ccp.c (bit_value_binop) <case RSHIFT_EXPR>: If sgn is
> 	UNSIGNED and r1val | r1mask has MSB set, ensure lzcount doesn't
> 	become negative.
> 
> 	* gcc.c-torture/execute/pr102134.c: New test.
> 
> --- gcc/tree-ssa-ccp.c.jj	2021-08-30 08:36:11.302515439 +0200
> +++ gcc/tree-ssa-ccp.c	2021-08-30 22:49:21.957503630 +0200
> @@ -1695,7 +1695,8 @@ bit_value_binop (enum tree_code code, si
>  	      /* Logical right shift, or zero sign bit.  */
>  	      widest_int arg = r1val | r1mask;
>  	      int lzcount = wi::clz (arg);
> -	      lzcount -= wi::get_precision (arg) - width;
> +	      if (lzcount)
> +		lzcount -= wi::get_precision (arg) - width;
>  	      widest_int tmp = wi::mask <widest_int> (width, false);
>  	      tmp = wi::lrshift (tmp, lzcount);
>  	      tmp = wi::lrshift (tmp, wi::bit_and_not (r2val, r2mask));
> --- gcc/testsuite/gcc.c-torture/execute/pr102134.c.jj	2021-08-30 22:47:41.115920522 +0200
> +++ gcc/testsuite/gcc.c-torture/execute/pr102134.c	2021-08-30 22:47:20.811205820 +0200
> @@ -0,0 +1,23 @@
> +/* PR tree-optimization/102134 */
> +
> +typedef unsigned long long u64;
> +
> +u64 g;
> +
> +void
> +foo (u64 a, u64 b, u64 c, u64 *r)
> +{
> +  b *= b;
> +  u64 x = a && ((b >> (c & 63)) | ((b << (c & 63)) & g));
> +  *r = x + a;
> +}
> +
> +int
> +main ()
> +{
> +  u64 x;
> +  foo (1, 3000, 0, &x);
> +  if (x != 2)
> +    __builtin_abort ();
> +  return 0;
> +}
> 
> 	Jakub
> 
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)


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