Re: Does the gcc optimizer rearrange floating point expressions?

Jonathan R Shewchuk <jrs+@cs.cmu.edu> wrote:
>Although I havent started coding yet, I'm worried that optimizing C
>compilers, particularly gcc (which is my main compiler), may foolishly
>rearrange expressions under the mistaken belief that floating point
>numbers are subject to the usual rules of arithmetic. For instance, given
>the expression (x + y) - y, which I use to determine the amount of
>floating point roundoff error in (x + y), a non-floating point-aware
>optimizer might mistakenly reduce it to x.


    From an examination of the assembler output of gcc with and without
the -O flag, it appears that gcc does indeed optimize away the
expression. However, setting the variables to be "volatile" seems to
correct the problem. Incidentally, aren't the values in <float.h> what
you are looking for? i.e., DBL_EPSILON, the smallest number such that
1.0 + x != 1.0? I may just misunderstand your question tho.


...Dave
--
                                    Dave Moore - Student at Large - Reed College
                                  dmoore@reed.edu - http://www.reed.edu/~dmoore/
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