Re: [PATCH] Fix (-A) - B -> (-B) - A optimization in fold_binary_loc (PR tree-optimization/83269)

[Richard Biener <rguenther at suse dot de>]

On Fri, 15 Dec 2017, Jakub Jelinek wrote:

> On Fri, Dec 15, 2017 at 09:38:52AM +0100, Richard Biener wrote:
> > On Thu, 14 Dec 2017, Jakub Jelinek wrote:
> > 
> > > Hi!
> > > 
> > > As the following testcase shows, the (-A) - B -> (-B) - A optimization can't
> > > be done the way it is if the negation of A is performed in type with
> > > wrapping behavior while the subtraction is done in signed type (with the
> > > same precision), as if A is (unsigned) INT_MIN, then (int) -(unsigned) INT_MIN
> > > is INT_MIN and INT_MIN - B is different from (-B) - INT_MIN.
> > > The reason we can see this is because we check that arg0 is NEGATE_EXPR, but
> > > arg0 is STRIP_NOPS from op0.  If the NEGATE_EXPR is already done in signed
> > > type, then it would be already UB if A was INT_MIN and so we can safely do
> > > it.
> > > 
> > > Whether we perform the subtraction in the unsigned type or just don't
> > > optimize I think doesn't matter that much, at least the only spot during
> > > x86_64-linux and i686-linux bootstraps/regtests this new condition triggered
> > > was the new testcase, nothing else.  So if you instead prefer to punt, I can
> > > tweak the patch, move the negated condition to the if above it.
> > > 
> > > Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
> > 
> > I think a better fix would be to just check TREE_CODE (op0) == NEGATE_EXPR
> > and use op0, like we do for op1 (probably fixed that earlier).  I'd rather
> > not complicate the fold-const.c code more at this point.
> 
> That would regress the case when type is unsigned.  If you don't want to
> complicate fold-const.c, my preference would be to add the extra && !, it
> isn't that much.

Ok, that works for me.

> Of course, a question is why this optimization hasn't been moved to match.pd
> when others had been.

Mostly laziness and the "fear" of match.pd negate_expr_p not being
powerful enough (it isn't recursive as the fold-const.c one and it
doesn't have all the complicated cases).

Thanks,
Richard.

> 2017-12-15  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR tree-optimization/83269
> 	* fold-const.c (fold_binary_loc): Perform (-A) - B -> (-B) - A
> 	subtraction in arg0's type if type is signed and arg0 is unsigned.
> 	Formatting fix.
> 
> 	* gcc.c-torture/execute/pr83269.c: New test.
> 
> --- gcc/fold-const.c.jj	2017-12-08 00:50:27.000000000 +0100
> +++ gcc/fold-const.c	2017-12-14 17:42:31.221398170 +0100
> @@ -9098,8 +9098,8 @@ expr_not_equal_to (tree t, const wide_in
>     return NULL_TREE.  */
>  
>  tree
> -fold_binary_loc (location_t loc,
> -	     enum tree_code code, tree type, tree op0, tree op1)
> +fold_binary_loc (location_t loc, enum tree_code code, tree type,
> +		 tree op0, tree op1)
>  {
>    enum tree_code_class kind = TREE_CODE_CLASS (code);
>    tree arg0, arg1, tem;
> @@ -9769,11 +9769,18 @@ fold_binary_loc (location_t loc,
>  
>        /* (-A) - B -> (-B) - A  where B is easily negated and we can swap.  */
>        if (TREE_CODE (arg0) == NEGATE_EXPR
> -	  && negate_expr_p (op1))
> -	return fold_build2_loc (loc, MINUS_EXPR, type,
> -				negate_expr (op1),
> -				fold_convert_loc (loc, type,
> -						  TREE_OPERAND (arg0, 0)));
> +	  && negate_expr_p (op1)
> +	  /* If arg0 is e.g. unsigned int and type is int, then this could
> +	     introduce UB, because if A is INT_MIN at runtime, the original
> +	     expression can be well defined while the latter is not.
> +	     See PR83269.  */
> +	  && !(ANY_INTEGRAL_TYPE_P (type)
> +	       && TYPE_OVERFLOW_UNDEFINED (type)
> +	       && ANY_INTEGRAL_TYPE_P (TREE_TYPE (arg0))
> +	       && !TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (arg0))))
> +	return fold_build2_loc (loc, MINUS_EXPR, type, negate_expr (op1),
> +			        fold_convert_loc (loc, type,
> +						  TREE_OPERAND (arg0, 0)));
>  
>        /* Fold __complex__ ( x, 0 ) - __complex__ ( 0, y ) to
>  	 __complex__ ( x, -y ).  This is not the same for SNaNs or if
> --- gcc/testsuite/gcc.c-torture/execute/pr83269.c.jj	2017-12-14 17:43:24.534710997 +0100
> +++ gcc/testsuite/gcc.c-torture/execute/pr83269.c	2017-12-14 17:43:10.000000000 +0100
> @@ -0,0 +1,14 @@
> +/* PR tree-optimization/83269 */
> +
> +int
> +main ()
> +{
> +#if __SIZEOF_INT__ == 4 && __SIZEOF_LONG_LONG__ > 4 && __CHAR_BIT__ == 8
> +  volatile unsigned char a = 1;
> +  long long b = 0x80000000L;
> +  int c = -((int)(-b) - (-0x7fffffff * a));
> +  if (c != 1)
> +    __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)