intel compiler flag clues

I'm wondering if anyone has favorite intel GCC compile flag clues  
they would like to share when one compiles a VM.

tossing -Os at GCC 4.0 for the mac intel port seems so primitive....


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John M. McIntosh <[hidden email]> 1-800-477-2659
Corporate Smalltalk Consulting Ltd.  http://www.smalltalkconsulting.com
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John M McIntosh wrote:

> I'm wondering if anyone has favorite intel GCC compile flag clues
> they would like to share when one compiles a VM.
>
> tossing -Os at GCC 4.0 for the mac intel port seems so primitive....


Yes -- we've found that adding -mcpu=your_cpu can make a huge difference.

Use -fomit-frame-pointer to free up another register (should help a lot
on x86
CPUs).

Using profile-directed-optimisation can also make a big difference, but
you'd
need to have a set of tests that represent your typical usage to run in
squeak
to generate the profiling data.  You build first with
-fprofile-generate, run your
tests, and then rebuild with -fprofile-use.  Since squeak can be told to
run a
.st file at startup, it's easy to script this whole process.

Use -fwhole-program -funit-at-a-time to build all the sources files in
one go, to
allow for inter-module-analysis to take place.  This however requires
changes
to the build system, so I haven't tried it.

Allowing GCC to do the type-based-alias-analysis (TBAA) can also give a very
nice speedup, but when I tried it (vm-3.6.something), the VM code contained
lots of pointer aliasing.  I seem to remember that the build used
-fno-sctrict-aliasing at the time.  I don't know if the situation's
changed since then.

Using the latest officially-released GCC is a good starting point.  And
of course
measuring before, during, and after is vital to convince yourself that any
speedups are "for real".

Let us know how it goes,
Andrew.