Re: gcc 2.95 vs 3.21 performance

Denis Vlasenko
2003-02-04T06:54:46Z

On 4 February 2003 01:31, Richard B. Johnson wrote:
> On Mon, 3 Feb 2003, Martin J. Bligh wrote:
> > People keep extolling the virtues of gcc 3.2 to me, which I'm
> > reluctant to switch to, since it compiles so much slower. But
> > it supposedly generates better code, so I thought I'd compile
> > the kernel with both and compare the results. This is gcc 2.95
> > and 3.2.1 from debian unstable on a 16-way NUMA-Q. The kernbench
> > tests still use 2.95 for the compile-time stuff.
>
> [SNIPPED tests...]

What was the size of uncompressed kernel binaries?
This is a simple (and somewhat inaccurate) measure of compiler
improvement ;)

> Don't let this get out, but egcs-2.91.66 compiled FFT code
> works about 50 percent of the speed of whatever M$ uses for
> Visual C++ Version 6.0 I was awfully disheartened when I

Yes. M$ (and some other compilers) beat GCC badly.

> found that identical code executed twice as fast on M$ than
> it does on Linux. I tried to isolate what was causing the
> difference. So I replaced 'hypot()' with some 'C' code that
> does sqrt(x^2 + y^2) just to see if it was the 'C' library.
> It didn't help. When I find out what type (section) of code
> is running slower, I'll report. In the meantime, it's fast
> enough, but I don't like being beat by M$.

I'm afraid it's code generation engine. It is just worse than
M$ or Intel's one. It is not easily fixable,
GCC folks have tremendous task at hand.

I wonder whether some big companies supposedly supporting
Linux (e.g. Intel) can help GCC team (for example by giving
away some code and/or developer time).
--
vda