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Re: Setting -frounding-math by default


On Sun, 8 Mar 2009, Sylvain Pion wrote:

> http://gcc.gnu.org/ml/gcc-patches/2003-09/msg00104.html
> introduced the -frounding-math option, and changed
> the default behavior of GCC to optimize "unsafely".

That is a misleading description.  The cautionary text added by that patch 
is still present:

    This option is experimental and does not currently guarantee to
    disable all GCC optimizations that are affected by rounding mode.

This is still true.  GCC did not before the patch, did not after the patch 
and does not now fully support disabling optimizations that are unsafe in 
the presence of rounding mode changes; a few affected optimizations are 
disabled, but noone has seriously attempted to cover them all.  GCC was 
"unsafe" before the patch and remains so whether or not you use the 
option.

> I am tired of waiting for a solution in GCC,

http://gcc.gnu.org/faq.html#support

There are plenty of bug reports for floating-point issues, and a lack of 
volunteers fixing them, so perhaps you should not expect filing more bugs 
to help, but "waiting for a solution" isn't listed there at all.

> Somehow, I also think that this -fno-rounding-math default is
> inconsistent with the general policy of defaults in GCC which
> is to aim at safety and correctness first.

The default is in accordance with standard requirements, and more more 
conservative than other compilers which tend to enable their -ffast-math 
equivalents by default rather than just -fno-rounding-math.

-- 
Joseph S. Myers
joseph@codesourcery.com


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