Re: [Mingw-users] Really slow executables generated by Msys/Mingw gcc

[Greg C. <chi...@co...>]

On 2006-3-17 14:38 UTC, Douglas Vechinski wrote:
> 
> Can a subroutine call add that much overhead.  Yes I understand that
> there is some overhead, but certainly it seems to me that the
> calculation of a trig function is alot more time consuming than such
> overhead.

Educated guesses may lead us astray, and measurements may prove
things that seem surprising.

> This example with the trig functions was just a small one I put together
> so other people could test.  If I were to use a Gaussian elimination
> code to solve a large matrix  i would get comparable results that Cases
> C and D are much slower than any of the others.  This would not involve
> any complex trig function just straight forward adding and
> multiplaction.

In that case, significant speed differences could still arise
from different settings in the floating-point control word,
or perhaps from some compilers using SSE instructions.

> As I said, I don't know the implications of using -ffast-math and don't
> necessarly want to use it as "the solution".

With '-ffast-math', gcc uses one particular set of options,
which are given in the manual. Without '-ffast-math', gcc
behaves in a different way. And msvc with no explicit
options behaves in a third way, which might be comparable
to gcc with '-ffast-math' and '-mfpmath=sse'. If msvc uses
the '387' instructions at all, it first reduces precision
to 64 bits by default IIRC; MinGW's default is 80 bits.