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