Re: [patch match.pd c c++]: Ignore results of 'shorten_compare' and move missing patterns in match.pd

[Joseph Myers <joseph at codesourcery dot com>]

On Tue, 15 Sep 2015, Kai Tietz wrote:

> >> +/* Simplify '(type) X cmp CST' to 'X cmp (type-of-X) CST', if
> >> +   CST fits into the type of X.  */
> >> +(for cmp (simple_comparison)
> >> +  (simplify
> >> +    (cmp (convert@2 @0) INTEGER_CST@1)
> >
> > what about REAL_CSTs?
> 
> Well, for fast-math this might be something to be extended.  So I
> handled here just the common variant.

I don't think it needs fast-math, if the conversion (convert@2 @0) is a 
floating-point extension (all values in the source type exactly 
representable in the destination type[*]), although there are lots of 
cases to consider and it makes sense to address such issues separately.  
But, for example, for "float f;", it's always valid to convert "((double) 
f) > 0.5" to "f > 0.5f".  In fact, we already have such folding in 
match.pd, "Fold (double)float1 CMP (double)float2 into float1 CMP 
float2.", albeit with hard-coded references to float and double that seem 
less than ideal.  (If the constant isn't exactly representable in the 
source type then the constant needs to be rounded in the correct direction 
when shortening, but it's still always the case that there is a correct 
conversion to a comparison in the narrower type.)

[*] If you care about exact underflow with traps enabled, you also need 
subnormal source values to become normal destination values - removing a 
conversion from x86 extended to __float128 could potentially remove an 
exact underflow.

-- 
Joseph S. Myers
joseph@codesourcery.com