[gambit-list] Getting rid of the -fwrapv flag in gcc
Bradley Lucier
lucier at math.purdue.edu
Sun Apr 15 20:45:07 EDT 2007
Marc:
On Apr 15, 2007, at 6:39 PM, Bradley Lucier wrote:
> This should allow us to omit the -fwrapv flag for gcc. I don't
> have any performance figures for gsc compiled without -fwrapv,
OK, I have these now, and it seems to be a wash using -fwrapv, but it
is an advantage (about 4%) to use gcc-4.1.2 on a Powerbook instead of
Apple's current release of 4.0.1.
I timed my soon-to-be-released replacement for fft bignum
multiplication on a 1.67GHz G4 Powerbook. Bottom line for (fft-mul a
b) with (not (eq? a b)) and both = to (expt 3 1000000):
With -fwrapv: 702 ms
With fixed gambit.h and no -fwrapv: 708 ms
With fixed gambit.h and no -fwrapv and -fstrict-aliasing: 658 ms
The latter is valid because there are no aliasing violations in
fftmul-fast.scm. The code in _num.scm takes 733 ms, so the new code
is clearer and provably correct, but it's faster.
The number of cycles that the main loop of the direct (forward) fft
is scheduled by gcc-4.1.2:
With or without -fwrapv: 111
With fixed gambit.h and no -fwrapv and -fstrict-aliasing: 68(!)
This loop has 40 floating-point instructions. We're talking a
speedup of
> (/ 733 658.)
1.1139817629179332
just by tweaking the macros in gambit.h.
Brad
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