[Bug middle-end/34678] Optimization generates incorrect code with -frounding-math option (#pragma STDC FENV_ACCESS not implemented)

["joseph at codesourcery dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #5 from joseph at codesourcery dot com  2008-01-06 15:12 -------
Subject: Re:  Optimization generates incorrect code
 with -frounding-math option (#pragma STDC FENV_ACCESS not implemented)

On Sat, 5 Jan 2008, rguenth at gcc dot gnu dot org wrote:

> I wouldn't read the language this way.  Because that will forcefully disable
> all redundancy removing optimizations (which is what happens in this testcase).
> What it currently guards is expression rewriting that changes the outcome if
> a rounding mode different than round-to-nearest is used.

My understanding has always been that -frounding-math should be usable for 
the code that calls rounding-mode-changing functions, rather than having 
no way to compile that code safely with GCC, as well as the code that does 
not call those functions but may execute with non-default rounding modes.  
The FENV_ACCESS pragma does not distinguish between the two.

> The finer-grained control the documentation mentions should not be globbed
> to -frounding-math IMHO.

The pragma would in effect set -frounding-math for particular regions of 
code; it isn't more fine-grained regarding whether the code sets the mode 
or merely runs under a different mode.

It is of course possible that -frounding-math should be split into 
multiple options (more fine-grained than the pragma) as the other related 
flags have been split over time.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34678