Re: GCC viciously beaten by ICC in trig test!

[Scott Robert Ladd <coyote at coyotegulch dot com>]

Roger Sayle wrote:
> Consider the following:
>
> #include <math.h>
>
> double doit(double a) { return sin(a) * sin(a); }
>
>
> Compiling with gcc -O2 -ffast-math on Linux generates x86 code that's
>  significantly slower than Intel's compiler output.  However,
> commenting out the "#include <math.h>" corrects the situation and GCC
> can then generate *exactly* the same sequence as icc.

Wonderful! This is *exactly* the insight I was looking for...

> The issue is that glibc's headers provide inline implementations for
> sin and cos, and thereby override all of GCC's internal builtin
> processing.

I should have thought of this; I've had some past problems with glibc
headers causing problems in my code.

> Once this is done, there's nothing tree-ssa, the middle-end or the
> i386 can do to improve the code.  If GCC is to have a hope of using
> "sincos" or SSE2 specific instruction sequences, the "best
> intentions" of glibc's headers (will) have to be neutralized first.
> Perhaps fixincludes :>

Sometimes, I wonder if GCC should ship its own Standard C library, just
as it ships a Standard C++ template library. However, I suspect the
suggesting such a move might be a bit controversial... ;)

--
Scott Robert Ladd
Coyote Gulch Productions (http://www.coyotegulch.com)
Software Invention for High-Performance Computing