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Subject: Re: Why is assembly more effecient than C?

Author: Robert Hyatt

Date: 05:04:28 09/29/98

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On September 29, 1998 at 00:57:32, Dave Gomboc wrote:

>On September 28, 1998 at 14:12:25, Robert Hyatt wrote:
>
>>On September 28, 1998 at 10:06:42, Jon Dart wrote:
>>
>>>
>>>On September 28, 1998 at 09:17:09, Robert Hyatt wrote:
>>>
>>>>On September 28, 1998 at 03:01:19, Danniel Corbit wrote:
>>>>
>>>>>On September 27, 1998 at 18:18:25, Robert Hyatt wrote:
>>>>>[snip]
>>>>>>not exactly.  IE I can't imagine that a C compiler + optimizer can beat
>>>>>>hand-tuned asm code, even if I write both the C and the asm code.  The
>>>>>>guys that write the optimizers are good, but they aren't as good as
>>>>>>someone that has been programming asm code for 30 years...
>>>>>>
>>>>>>The main reason everyone doesn't use ASM code is portability, *not*
>>>>>>speed.
>>>>>Risc C compilers can almost always outdo hand written code except for very small
>>>>>snippets.  For CISC I agree with you, especially Intel x86, since there are so
>>>>>many good Intel assembly programmers.  For thousands or millions of lines of C,
>>>>>an equivalent ASM is very hard to produce for Risc machines.
>>>>
>>>
>>>The Intel processors now do many of the tricks that RISC processors have
>>>traditionally done. It used to be that you could just get the processor manual,
>>>add up the instruction times, and figure out how fast your code would run.
>>>Now that's not true anymore. So writing optimal assembly language is
>>>non-trivial, even for the Intel machines. (However, I would add that few
>>>compilers do a really great job of register allocation - which is quite a bit
>>>harder on Intel than other architectures - so that is one area where a human can
>>>improve on the compiler).
>>>
>>>--Jon
>>
>>
>>There are other things too.  IE how many chess programmers do an x=x*2.5 in
>>their evaluation function?  None?  Better check out cray blitz.  And the reason
>>is buried in the Cray architecture and how floating point stuff is done in
>>parallel with integer stuff, so that I can do x=x*2 and y=y*2.0 in the same
>>time it would take to do just x=x*2...  but the compilers don't know whether
>>you can take a float and use it as an int, or vice-versa, while *I* do because
>>I know how the number will be used later, and where the important part of the
>>number is (whole or fraction or both).  The compiler *always* has to be
>>conservative and once it has a float, it has to stick with a float to avoid
>>losing those fractional bits, even when they will be zero (but it can't know
>>that of course.)
>>
>>That's the point.  I *know* *all* about the program and the values it is
>>computing.  The compiler doesn't...
>
>I think that if somebody had enough time to write it all in assembly, they might
>do a better job than a top-notch compiler.  That's a pretty big if though, it
>might take a lot longer than a human lifespan.  Anyway, I think more often the
>compiler will be better.  If I may make an analogy :), let's use computer chess.
>

It depends on how you define "better"...  Note that Crafty is pure C now
because of the portability issue and the time issue.  I'd love to write the
fastest code I could in asm, but the target platform changes about once every
2-3 years, while we had many man-years (Harry and I) in the assembly code for
Cray Blitz.




>The best programs are better than virtually all humans, they have limited
>knowledge but they apply it unfailingly, every place that it is possible.  There
>are a few humans who can still sit down against some PC software and crush it,
>but for the vast majority of us, the software is better than we are.
>
>The same holds for assembling: when it comes to peephole optimizations over a
>million lines of C code, the assembler is a far better grinder than any human
>could possibly be.  Humans beat it in cases like you mention above, where they
>simply understand something about the code that the assembler does not.
>
>So be practical, write in a high-level language, and if you really need to,
>hand-tune like crazy where there are bottlenecks. :-)
>
>Dave Gomboc



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