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Date: Tue, 29 Sep 2009 17:26:04 +0200
From: Georg Bauhaus <rm.tsoh.plus-bug.bauhaus@maps.futureapps.de>
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Subject: Re: Tasking for Mandelbrot program
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Date: 2009-09-29T17:26:05+02:00
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jonathan wrote:
> On Sep 27, 10:27 pm, Georg Bauhaus <rm.tsoh.plus-
> bug.bauh...@maps.futureapps.de> wrote:
>> Some more observations:
>>
>> - SSE2 code performs 8% faster when suitable compilation options
>>   are present, -mfpmath=sse -msse2 (this is currently the case).
>>   Then digits 15 should probably stay in the declaration of Real.
>>
> 
> Some more notes on this puzzle ...

Indeed...  I might well have lost track in the
labyrinth of settings, but seeing, as I do, a factor near 2
in Mandelbrot speed when switching from 15 to 16 digits
(or back) looks odd, especially when the combinations
alluded to below do not appear to be similarly far
from each other.

Next thing I'll do is work through the exponential
table of options and FPT definitions and CPUs
and compilers and OSs and ... carefully measuring
each cell. For now, here is a little test setup that does some
of this.
If you like, unpack in a fresh directory and type "make".
This will compile and run a few FPT related combinations
taking the core loop of Mandelbrot as an example.
(On Windows, type "make all-not-native" , after switching
three OS-related variables near the head of the Makefile.)

http://home.arcor.de/bauhaus/Ada/test1516fpt.zip

I will be short of time during the next few
days, and maybe off line.

FTR, as an experiment I have tried to
  pragma Import(Intrinsic, MULPD, "__builtin_ia32_mulpd")
i.e. GCC builtins for SIMD multiplication etc, like the leading
programs do.  Formally, this appears to be working, but
the compiler finally spit a bug box.