On Fri, 5 May 2017, Richard Sandiford wrote:
> Richard Biener <rguent...@suse.de> writes:
> > On Fri, 5 May 2017, Georg-Johann Lay wrote:
> >> On 05.05.2017 13:04, Richard Biener wrote:
> >> > On Fri, 5 May 2017, Georg-Johann Lay wrote:
> >> >
> >> > > Applied this addendum to r247495 which removed flag_strict_overflow.
> >> > > There
> >> > > were remains of the flag in avr.md which broke the avr build.
> >> > >
> >> > > Committed as r247632.
> >> >
> >> > Whoops - sorry for not grepping besides .[ch] files...
> >> >
> >> > But... these patterns very much look like premature optimization
> >> > and/or bugs. combine is supposed to handle this via simplify_rtx.
> >>
> >> Well, for now the patch just restores avr BE to be able to be build.
> >
> > Sure.
> >
> >> > Also note that on RTL we generally assume overflow wraps as we lose
> >> > signedness of operands. Not sure what 'compare' in your patterns
> >> > will end up with.
> >> >
> >> > The only flag_wrapv checks in RTL otherwise are in simplify-rtx.c
> >> > for ABS which seems to be a singed RTL op.
> >>
> >> Which is a bug, IMO. Letting undefined overflow propagate to RTL
> >> renders some RTL as if it has undefined behaviour. Consequence is
> >> that testing the MSB must no more use signed comparisons on
> >> less-zero resp. greater-or-equal-to-zero.
> >>
> >> Cf. https://gcc.gnu.org/PR75964 for an example:
> >>
> >>
> >> typedef __UINT8_TYPE__ uint8_t;
> >>
> >> uint8_t abs8 (uint8_t x)
> >> {
> >> if (x & 0x80)
> >> x = -x;
> >>
> >> if (x & 0x80)
> >> x = 0x7f;
> >>
> >> return x;
> >> }
> >>
> >> The first comparison is performed by a signed test against 0 (which
> >> is reasonable and the best code in that case) but then we conclude
> >> that the second test is always false, which is BUG.
> >>
> >> IMO the culprit is to let slip undefined overflow to RTL.
> >
> > Yes. I thought in RTL overflow is always well-defined (but then
> > as I said your patterns are equally bogus).
>
> Yeah, me too. I don't see how the simplify-rtx.c code can be right.
>
> Is the following OK, if it passes testing?
Yes. Can you add the testcase?
Thanks,
Richard.
> Thanks,
> Richard
>
>
> 2017-05-05 Richard Sandiford <richard.sandif...@linaro.org>
>
> gcc/
> PR rtl-optimization/75964
> * simplify-rtx.c (simplify_const_relational_operation): Remove
> invalid handling of comparisons of integer ABS.
>
> Index: gcc/simplify-rtx.c
> ===================================================================
> --- gcc/simplify-rtx.c 2017-05-05 13:44:27.364724260 +0100
> +++ gcc/simplify-rtx.c 2017-05-05 13:44:36.580195277 +0100
> @@ -5316,34 +5316,14 @@ simplify_const_relational_operation (enu
> {
> case LT:
> /* Optimize abs(x) < 0.0. */
> - if (!HONOR_SNANS (mode)
> - && (!INTEGRAL_MODE_P (mode)
> - || (!flag_wrapv && !flag_trapv)))
> - {
> - if (INTEGRAL_MODE_P (mode)
> - && (issue_strict_overflow_warning
> - (WARN_STRICT_OVERFLOW_CONDITIONAL)))
> - warning (OPT_Wstrict_overflow,
> - ("assuming signed overflow does not occur when "
> - "assuming abs (x) < 0 is false"));
> - return const0_rtx;
> - }
> + if (!INTEGRAL_MODE_P (mode) && !HONOR_SNANS (mode))
> + return const0_rtx;
> break;
>
> case GE:
> /* Optimize abs(x) >= 0.0. */
> - if (!HONOR_NANS (mode)
> - && (!INTEGRAL_MODE_P (mode)
> - || (!flag_wrapv && !flag_trapv)))
> - {
> - if (INTEGRAL_MODE_P (mode)
> - && (issue_strict_overflow_warning
> - (WARN_STRICT_OVERFLOW_CONDITIONAL)))
> - warning (OPT_Wstrict_overflow,
> - ("assuming signed overflow does not occur when "
> - "assuming abs (x) >= 0 is true"));
> - return const_true_rtx;
> - }
> + if (!INTEGRAL_MODE_P (mode) && !HONOR_NANS (mode))
> + return const_true_rtx;
> break;
>
> case UNGE:
>
>
--
Richard Biener <rguent...@suse.de>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB
21284 (AG Nuernberg)
