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[Bug rtl-optimization/82692] [8 Regression] Ordered comparisons used for unordered built-ins


https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82692

--- Comment #14 from Uroš Bizjak <ubizjak at gmail dot com> ---
(In reply to Uroš Bizjak from comment #13)
> (In reply to Segher Boessenkool from comment #12)
> > But why only do this for FLOAT_MODE_P?  Either the logic here isn't
> > correct, or cc_modes_compatible isn't the correct hook (we'll need
> > a new hook then?), or determining ordered/unordered by CC mode does
> > not work (does not fit into how RTL works).
> 
> Non-FP compares can select different mode depending on their operands (e.g.
> CCmode to CCZmode when one operand is zero) without secondary effects. But
> when reversing the condition from UNGE -> LT, SELECT_CC_MODE will return
> trapping mode (LT), whereas original, non-reversed mode (UNGE) was
> non-trapping. Please see how targets depend mode of their FP compares on the
> condition code.
> 
> The solution here is to keep the original comparison mode for FP compares
> (as was proposed in the first version of the patch): when qNaN is
> encountered at this point, an exception has to be generated, no matter how
> the condition was changed.

... an exception should not be generated ... in the above case, when UNGE is
reversed to LT.

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