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

Author: Robert Hyatt

Date: 11:12:25 09/28/98

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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...



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