Re: Does the gcc optimizer rearrange floating point expressions?

Jonathan R Shewchuk <jrs+@cs.cmu.edu> wrote:
>[does GCC reorganize floating point expresions like]
>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.


dmoore@reed.edu (Dave Moore) writes:
> 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. ...


Which version of gcc are you using? I do not see this effect with 2.5.8.


I believe that the answer to the original question is that the gcc
optimiser tries to preserve the exact behaviour of any floating-point
program, and that any effect such as you describe will be considered a bug
by the maintainers. This includes pedantic observation of the IEEE spec
on IEEE fp based machines. (Note; I am not a gcc maintainer, so I do not
speak from authority.)


The only problem you might encounter is if your program changes the
rounding mode at run-time; gcc assumes a particular rounding mode at
compile time when doing constant folding.


What is definitely true (look in combine.c) is that the gcc optimiser _is_
aware that floating-point numbers are not subject to the usual rules of
arithmetic.


David Robinson. (drtr@mail.ast.cam.ac.uk)
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