Re: [PATCH] Fix fold-const (X & C) ? C : 0 optimization (PR middle-end/87248)

[Richard Biener <rguenther at suse dot de>]

On Wed, 12 Sep 2018, Jakub Jelinek wrote:

> Hi!
> 
> The following testcase is miscompiled, because it optimizes
> (x & -128) ? -128 : 0 to (x & -128) when it shouldn't.  The problem is if
> the type of the COND_EXPR has smaller precision than the BIT_AND_EXPR in the
> test, we verify that integer_pow2p (arg1), which is true in this case (-128
> in signed char is a power of two), and then operand_equal_p between that
> value and the constant in BIT_AND_EXPR's second argument (operand_equal_p
> compares only the value, and -128 == -128).  The following patch verifies
> also that the BIT_AND_EXPR's second operand is a power of two.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk and
> release branches?

OK.

Richard.

> 
> 2018-09-12  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR middle-end/87248
> 	* fold-const.c (fold_ternary_loc) <case COND_EXPR>: Verify also that
> 	BIT_AND_EXPR's second operand is a power of two.  Formatting fix.
> 
> 	* c-c++-common/torture/pr87248.c: New test.
> 
> --- gcc/fold-const.c.jj	2018-09-06 09:41:59.000000000 +0200
> +++ gcc/fold-const.c	2018-09-08 00:12:28.332418784 +0200
> @@ -11607,10 +11607,16 @@ fold_ternary_loc (location_t loc, enum t
>  	  && integer_pow2p (arg1)
>  	  && TREE_CODE (TREE_OPERAND (arg0, 0)) == BIT_AND_EXPR
>  	  && operand_equal_p (TREE_OPERAND (TREE_OPERAND (arg0, 0), 1),
> -			      arg1, OEP_ONLY_CONST))
> +			      arg1, OEP_ONLY_CONST)
> +	  /* operand_equal_p compares just value, not precision, so e.g.
> +	     arg1 could be 8-bit -128 and be power of two, but BIT_AND_EXPR
> +	     second operand 32-bit -128, which is not a power of two (or vice
> +	     versa.  */
> +	  && integer_pow2p (TREE_OPERAND (TREE_OPERAND (arg0, 0), 1)))
>  	return pedantic_non_lvalue_loc (loc,
> -				    fold_convert_loc (loc, type,
> -						      TREE_OPERAND (arg0, 0)));
> +					fold_convert_loc (loc, type,
> +							  TREE_OPERAND (arg0,
> +									0)));
>  
>        /* Disable the transformations below for vectors, since
>  	 fold_binary_op_with_conditional_arg may undo them immediately,
> --- gcc/testsuite/c-c++-common/torture/pr87248.c.jj	2018-09-08 01:13:43.431334239 +0200
> +++ gcc/testsuite/c-c++-common/torture/pr87248.c	2018-09-08 01:14:55.446593520 +0200
> @@ -0,0 +1,36 @@
> +/* PR middle-end/87248 */
> +/* { dg-do run } */
> +
> +void
> +foo (signed char *p, int q)
> +{
> +  *p = q & (-__SCHAR_MAX__ - 1) ? (-__SCHAR_MAX__ - 1) : 0;
> +}
> +
> +int
> +bar (long long x)
> +{
> +  return x & (-__INT_MAX__ - 1) ? (-__INT_MAX__ - 1) : 0;
> +}
> +
> +int
> +main ()
> +{
> +#if __INT_MAX__ > 4 * __SCHAR_MAX__
> +  signed char a[4];
> +  foo (a, __SCHAR_MAX__ + 1U);
> +  foo (a + 1, 2 * (__SCHAR_MAX__ + 1U));
> +  foo (a + 2, -__INT_MAX__ - 1);
> +  foo (a + 3, (__SCHAR_MAX__ + 1U) / 2);
> +  if (a[0] != (-__SCHAR_MAX__ - 1) || a[1] != a[0] || a[2] != a[0] || a[3] != 0)
> +    __builtin_abort ();
> +#endif
> +#if __LONG_LONG_MAX__ > 4 * __INT_MAX__
> +  if (bar (__INT_MAX__ + 1LL) != (-__INT_MAX__ - 1)
> +      || bar (2 * (__INT_MAX__ + 1LL)) != (-__INT_MAX__ - 1)
> +      || bar (-__LONG_LONG_MAX__ - 1) != (-__INT_MAX__ - 1)
> +      || bar ((__INT_MAX__ + 1LL) / 2) != 0)
> +    __builtin_abort ();
> +#endif
> +  return 0;
> +}
> 
> 	Jakub
> 
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nuernberg)