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Date: -- (:)
From: David Chase <chase@w...>
Subject: Re: Coyote Gulch test in Caml (was Re: [Caml-list] speed )
At 03:50 PM 1/18/2003 -0800, Shawn Wagner wrote:
>On Sat, Jan 18, 2003 at 05:49:47PM -0500, Oleg wrote:
>> On Saturday 04 January 2003 01:31 pm, Xavier Leroy wrote:
>> > Apparently, the ocamlopt-generated code
>> > offers less instruction-level parallelism than the g++-generated code
>> > for the float computations.  Still, I haven't really understood where
>> > the factor of 2 comes from.  
>
>> Just as wild guess: the code contains calls to "sin" and "cos" on the same 
>> value. Perhaps GCC manages to optimize those into one call to "sincos"
>
>It doesn't. I tried making a C++ version that does when I was fooling around
>with it. Didn't really help. The single greatest speed increase I got (Which
>did something like cut the runtime in half) was -ffast-math, which cuts out
>the trig function calls in favor of direct use of the proper x86
>instructions. But the inlined-sincos (__sincos()) in glibc causes a segfault
>on my athlon when I tried using it. :P

Just a silly question, but if you want sin and cos to go faster,
how much accuracy are you willing to trade away for improved
performance?  Just for example, by using the Pentium instructions,
you reduce the number of (accurate) significant bits in the result
from 53 (IEEE double) to 13 (for some inputs between zero and 2*PI).
(If you are using 64-bit mantissas, the worst case is only 4 bits of
accuracy.)

I've noticed over the years that people focus on speed over many other
things, usually because they can measure it.  Well, we can measure
accuracy of transcendental functions, too, so I thought I would
ask the question.  How much is enough for your application?  Of the
languages being benchmarked, which one has the most accurate
transcendental functions?  Is this less important than speed?

David Chase


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