Date: Fri, 3 Sep 1999 18:37:41 -0700 (PDT) From: John Polstra <jdp@polstra.com> To: gmckinney@megabits.net Cc: alpha@freebsd.org Subject: Re: relative alpha speed Message-ID: <199909040137.SAA10144@vashon.polstra.com> In-Reply-To: <00ee01bef5fc$203599e0$1e00000a@gary2.megabits.net> References: <00ee01bef5fc$203599e0$1e00000a@gary2.megabits.net>
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In article <00ee01bef5fc$203599e0$1e00000a@gary2.megabits.net>,
Gary McKinney <gmckinney@megabits.net> wrote:
> There is also a thread in the Debian Linux group about performance of the
> floating point lib routines (the lib routines are patterned after the Intel
> FP unit and the Alpha is a different creature with different needs).
The test I used was a matrix multiplication function which had been
carefully hand-optimized to take advantage of the Alpha pipeline.
(Sorry, the program is not mine to redistribute. Hidetoshi Shimokawa
kindly got permission from the author to send me a copy.) It didn't
call any floating point library functions. It was blazingly fast when
compiled without "-mieee". But when built with "-mieee" it was much
slower on my 533 MHz 164LX than on my 400 MHz PII.
Hidetoshi told me:
This program exploits four pipelines(2 integers, 2 floats), and
L1(or L2, I forgot detail) cache efficiently. You can easily
check this by 'iprobe quad'. (Even if it is compiled with
`-mieee', it still issuing quad instructions at a cycle)
Further testing indicated that enabling precise traps was responsible
for the performance degradation. N.B., no traps were actually
generated -- I instrumented the kernel to make sure. But the changes
in the code generation to permit identifying which instruction caused
a trap slowed it down.
I'm not sure how much the code generation could improved. Right now
it emits "trapb" (trap barrier) instructions in the places it thinks
they're needed -- not after each FP instruction. It's not obvious
that it could do much better.
John
--
John Polstra jdp@polstra.com
John D. Polstra & Co., Inc. Seattle, Washington USA
"No matter how cynical I get, I just can't keep up." -- Nora Ephron
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