Re: [PATCH] Fix ancient wrong-code with ?: (PR middle-end/81814)

[Marek Polacek <polacek at redhat dot com>]

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