On Thu, Aug 17, 2017 at 04:36:02PM +0200, Richard Biener wrote:
> On Thu, 17 Aug 2017, Marek Polacek wrote:
>
> > 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).
>
> So you should be able to extract a C testcase? I suspect sth like
>
> long foo (long x, int y)
> {
> return y > x ? y : x;
> }
>
> to no longer be folded to return MAX_EXPR (x, (long) y).
This is still folded to return MAX_EXPR <(long int) y, x>; so that's fine.
It's got to be something more complex that will not be handled now. I'll
look tomorrow for a testcase.
Marek
