Re: [Haskell-cafe] Haskell version of ray tracer code is much slower than the original ML

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:

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Philip Armstrong writes: (snip)

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Why on earth would you use -fexcess-precision if you're using Floats? The excess precision only apples to Doubles held in registers on x86 IIRC. (If you spill a Double from a register to memory, then you lose the extra precision bits in the process).

Some googling suggests that point 2 on http://www.haskell.org/hawiki/FasterFloatingPointWithGhc might have been what I was thinking of.

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. 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). cheers, Phil -- http://www.kantaka.co.uk/ .oOo. public key: http://www.kantaka.co.uk/gpg.txt