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From: BGB <cr88192@gmail.com>
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Subject: Re: Instruction Tracing
Date: Sat, 10 Aug 2024 17:49:42 -0500
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On 8/10/2024 2:41 PM, John Dallman wrote:
> In article <2024Aug10.121802@mips.complang.tuwien.ac.at>,
> anton@mips.complang.tuwien.ac.at (Anton Ertl) wrote:
> 
>> IIRC IA-64 has no FP division.
> 
> You recall correctly. It has an "approximation to reciprocal" instruction,
> which gives you about 8 bits of precision, and then requires the compiler
> to generate Newton-Raphson sequences. Intel's manual, 2010 edition, says
> this is advantageous because users can generate only the precision they
> need. Writing Itanium assembler for customised precision? Not many people
> would have wanted to do that in 2001, let alone 2010.
> 
> In, I think, 1996, my employers had visitors from Intel trying to
> persuade us to adopt their C/C++ compiler for IA-32. They had been able
> to speed up one of our competitors' code by a factor of two, and hoped to
> do the same for us.
> 
> They failed. We already had that factor of two, which was "ordinary
> compiler optimisation." That competitor had some rather odd coding
> standards at the time, which meant most compilers failed if asked to
> optimise their code. Someone from Intel had stayed at their site for most
> of a year, reporting the bugs and getting them fixed until Intel's
> compiler could optimise the code.
> 
> While visiting us, Intel asked what may have been a significant question
> about the mixture of floating-point arithmetic instructions we used. We
> didn't have precise figures, but were sure that we used at least as many
> square roots as divides. IA-64 does square roots like divides, with a
> starter approximation and Newton-Raphson sequences. Slowly, because the
> N-R instructions all depend on the previous instruction, and can't be run
> in parallel.
> 

FWIW:
In my case it is similar (if not using the FDIV) instruction, where 
there are approximations for divide / reciprocal / square root.




Meanwhile, saw a video recently where someone had ported Doom to a 233 
MHz PowerPC (running Windows NT4) machine and, its performance was not 
good...

Not obvious is what combination of factors conspired to cause Doom to 
apparently run at single-digit framerates.

Video mentioned that it was drawing using GDI calls, but this by itself 
wouldn't explain the level of slowness seen in the video.

Like, presumably, this would require around 90% + of the clock cycles 
going into overhead, which seems a bit much.


Reference:
https://www.youtube.com/watch?v=LAkSJ-HqKw8



> John