Philip Armstrong wrote:
On Thu, Jun 21, 2007 at 08:42:57PM +0100, Philip Armstrong wrote:
On Thu, Jun 21, 2007 at 03:29:17PM -0400, Mark T.B. Carroll wrote:
That's the old wiki. The new one gives the opposite advice! (As does the ghc manual):
http://www.haskell.org/ghc/docs/latest/html/users_guide/faster.html http://www.haskell.org/haskellwiki/Performance/Floating_Point
Incidentally, the latter page implies that ghc is being overly pessimistic when compilling FP code without -fexcess-precision:
"On x86 (and other platforms with GHC prior to version 6.4.2), use the -fexcess-precision flag to improve performance of floating-point intensive code (up to 2x speedups have been seen). This will keep more intermediates in registers instead of memory, at the expense of occasional differences in results due to unpredictable rounding."
IIRC, it is possible to issue an instruction to the x86 FP unit which makes all operations work on 64-bit Doubles, even though there are 80-bits available internally. Which then means there's no requirement to spill intermediate results to memory in order to get the rounding correct.
For some background on why GHC doesn't do this, see the comment "MORE FLOATING POINT MUSINGS..." in http://darcs.haskell.org/ghc/compiler/nativeGen/MachInstrs.hs The main problem is floats: even if you put the FPU into 64-bit mode, your float operations will be done at 64-bit precision. There are other technical problems that we found with doing this, the comment above elaborates. GHC passes -ffloat-store to GCC, unless you give the flag -fexcess-precision. The idea is to try to get reproducible floating-point results. The native code generator is unaffected by -fexcess-precision, but it produces rubbish floating-point code on x86 anyway.
Ideally, -fexcess-precision should just affect whether the FP unit uses 80 or 64 bit Doubles. It shouldn't make any performance difference, although obviously the generated results may be different.
As an aside, if you use the -optc-mfpmath=sse option, then you only get 64-bit Doubles anyway (on x86).
You probably want SSE2. If I ever get around to finishing it, the GHC native code generator will be able to generate SSE2 code on x86 someday, like it currently does for x86-64. For now, to get good FP performance on x86, you probably want -fvia-C -fexcess-precision -optc-mfpmath=sse2 Cheers, Simon