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