[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 #15 from joseph at codesourcery dot com  2008-10-16 16:39 -------
Subject: Re:  Optimization generates incorrect code
 with -frounding-math option (#pragma STDC FENV_ACCESS not implemented)

On Thu, 16 Oct 2008, vincent at vinc17 dot org wrote:

> The compiler should generate correct code by default, and options like

Sure, it generates correct code for the supported features, and 
<http://gcc.gnu.org/c99status.html>, which is linked from the manual, 
documents the state of support for the features: standard pragmas are 
documented as Missing and IEC 60559 support as Broken, with the 
explanation:

     * IEC 60559 is IEEE 754 floating point. This works if and only if
       the hardware is perfectly compliant, but GCC does not define
       __STDC_IEC_559__ or implement the associated standard pragmas; nor
       do some options such as -frounding-math to enable the pragmas
       globally work in all cases (for example, required exceptions may
       not be generated) and contracting expressions (e.g., using fused
       multiply-add) is not restricted to source-language expressions as
       required by C99.

So you are using features documented not to be fully implemented and 
whether they will work is unpredictable.


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