Tim Prince wrote:
> Alexander Monakov wrote:
>>> we are currently investigating some numerical algorithms and the claim
>>> appeared that a C statement like
>>> x = c - (c - a);
>>> would be easily transformed into
>>> x = a;
>>> by the compiler. Now investigating this with a vanilla GCC 4.1.2 failed
>>> to support the claim. Compiling the below program with -O3 -ffast-math
>>> keeps the computation of x. The output shows x is different from a. The
>>> question is, is there some compiler switch or the like to get GCC to
>>> make the above transformation? I searched the docs but had the
>>> impression that all relevant flags should be included in the above two
>>> (especially ffast-math).
>>
>> This transformation is indeed included into -ffast-math. I checked with
>>
>> $ gcc --version
>> gcc (GCC) 4.1.1 20070105 (Red Hat 4.1.1-52)
>>
>> and it does eliminate the calculation. What does generated assembly
>> code look
>> like in your case? Note you may as well check on this code:
>>
>> double f(double a, double c)
>> {
>>
> Normally, when such expressions are written in source code, there is a
> reasonable expectation that algebraic simplification will not be
> performed across parentheses.
There may well be such an expectation, but gcc only prevents re-association
across explicit parenthesis in FORTRAN, and it's only been doing that for
a few weeks.
Andrew.
