[PATCH, Fix PR 31098] a*C == b*C is not foldded to a == b with --fast-math/floating point

[Paolo Bonzini]

>> 2007-10-07  Pranav Bhandarkar  <pranav.bhandarkar@celunite.com>
>>
>>    * fold-const.c (fold_comparison): fold (a OP c) CMP (b OP c) -> a
>>    CMP b, iff c is not zero
>>    * testsuite/pr31098.c: New
> 
>> +      /* (a OP c) CMP (b OP c) -> a CMP b, iff c is not zero */
>> +      if (fast_math_flags_set_p () && (TREE_CODE (arg0) == TREE_CODE (arg1))
> 
> Do we really need all "fast math" flags set to enable this opt?  E.g.
> surely we don't care about !flag_signed_zeros since you disable the code
> when "c" is zero.  Perhaps other sub-bits of "fast math" can also be
> ignored for this purpose.

flag_unsafe_math_optimizations == 0 should disable the optimization, 
since ((float)11184813.0 * 3) == ((float)11184814.0 * 3), and 
((float)16777215.0 + 2) == ((float)16777214.0 + 2).  Same for 
HONOR_INFINITIES, and for flag_trapping_math == 1.

It is not necessary to check HONOR_NANS and HONOR_SNANS, since neither 
is set for flag_trapping_math == 0.

Paolo