Re: Floating point performance issue

[Vincent Lefevre <vincent+gcc@xxxxxxxxxx>]

On 2011-12-20 10:34:35 +0000, Jonathan Wakely wrote:
> On 20 December 2011 10:20, Ico wrote:
> >
> > Still, I'm not sure if sse is part of the problem and/or solution.
> 
> It's the solution.
> 
> > I have been reducing the program to see what the smallest code is
> > that still shows this behaviour. Latest version is below.
> >
> >
> > $ gcc -msse -mfpmath=sse -O3 -march=native test.c
> 
> What is "native" for your system, i686? (also, what does gcc
> -dumpmachine show?) i686 doesn't support SSE, you need at least
> pentium3.

I can reproduce the "problem" on an x86_64 machine, so it is not
due to the traditional FPU. I just think that the multiplication
by 0 is faster (because much easier than the generic case), as
I've said in my other message. But to have such an optimization,
I wouldn't complain. :)

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