On Thu, Aug 17, 2017 at 04:17:54PM +0200, Richard Biener wrote:
> On Thu, 17 Aug 2017, Marek Polacek wrote:
>
> > This PR is about wrong-code and has gone undetected for over 10 years (!).
> > The issue is that e.g. the following
> >
> > (signed char) x == 0 ? (unsigned long long) x : 0
> >
> > was wrongly folded to 0, because fold_cond_expr_with_comparison will fold
> > A != 0 ? A : 0 to 0. But for x = 0x01000000 this is wrong: (signed char)
> > is 0,
> > but (unsigned long long) x is not. The culprit is
> > operand_equal_for_comparison_p
> > which contains shorten_compare-like code which says that the above is safe
> > to
> > fold. The code harks back to 1992 so I thought it worth to just get rid of
> > it.
> >
> > But I did some measurements and it turns out that substituting
> > operand_equal_p
> > for operand_equal_for_comparison_p prevents folding ~60000 times in
> > bootstrap.
> > So I feel uneasy about removing the function completely. Instead, I propose
> > to
> > remove just the part that is causing trouble. (Maybe I should also delete
> > the
> > first call to operand_equal_p in operand_equal_for_comparison_p.)
> >
> > Bootstrapped/regtested on x86_64-linux, ok for trunk? What about 7?
>
> Ok for trunk. Do you have numbers for this patch variant as well?
Thanks. Yeah, I've gathered some, too. This patch prevents calling
fold_cond_expr_with_comparison that would end up with non-NULL_TREE result
8322 times (all Ada files), this is the
11325 if (COMPARISON_CLASS_P (arg0)
11326 && operand_equal_for_comparison_p (TREE_OPERAND (arg0, 0), arg1)
11327 && !HONOR_SIGNED_ZEROS (element_mode (arg1)))
case; plus 648 times in the
11334 if (COMPARISON_CLASS_P (arg0)
11335 && operand_equal_for_comparison_p (TREE_OPERAND (arg0, 0), op2)
11336 && !HONOR_SIGNED_ZEROS (element_mode (op2)))
case (and a lot of that is coming from libgfortran/generated/*.c and reload.c).
> It seems that with some refactoring the remaining transforms should
> be easily expressible as match.pd patterns now.
That'd be great.
Marek
