Author: Matt Taylor
Date: 22:11:39 12/20/02
Go up one level in this thread
On December 20, 2002 at 13:58:20, Robert Hyatt wrote:
>On December 20, 2002 at 11:21:40, Vincent Diepeveen wrote:
>
>>On December 19, 2002 at 13:16:02, Frank Sgarra wrote:
>>
>>After that diep gave away a pawn with the intel c++ compiler
>>and not with any other compiler (also giving difference in score
>>for mainlines), i started checking out that intel compiler
>>more.
>>
>>I found several bugs in intel c++.
>>
>>One of them i posted here at CCC some time ago for version 6.0 of
>>intel c++.
>>
>>it was something in factorization code to see whether there
>>is a coefficient somewhere. something like:
>>
>>int lineaircoefficient(int a,int b) {
>> float a,b,c;
>>
>> (formula)
>> a = ..
>> b = ..
>> c = ..
>>
>> return((int)(a*b+c));
>>}
>>
>>There was a big bug in the:
>> return((int)(a*b+c));
>
>You posted that, I tried it and it worked _perfectly_ as I reported at the
>time. I'd much more likely suspect some programming mistakes in your code
>rather than in the compiler...
>
>
>>
>>It is not my work to detect bugs in the intel c++ compiler, which
>>is a fulltime work i bet.
>>
>>Just compile any non-specint software and see what happens with it.
>
>I have done that...
>
>
>
>
>>
>>A good example is to compile some linux stuff with gcc.
>>
>>Perhaps the gcc compiler itself would be a good example. Please compile
>>it with intel c++ and you'll see yourself zillions of examples.
>>
>>Then you first find out whether it's a bug in the software secondly
>>you try to find out whether it's a compiler bug.
>>
>>I am not big compiler expert but i even understood what a compiler
>>expert wrote at a homepage somewhere.
>>
>>Most compilers are single pass compilers.
>>the intel c++ compiler is not.
>
>
>Most compilers are _not_ "single pass". In fact, that is impossible to
>do, because of forward references. Something has to make the second pass
>to fill in the missing blanks.
>
>Not to mention optimizations that get done on the second or third pass.
>GCC certainly isn't "one pass".
Yeah, pretty much. I was thinking about replying with, "???," but I decided it
wasn't worth the time. I'm trying to figure out how it would be -possible- to
write a single-pass compiler for C. It's not even possible to do Intel assembly
in a single pass.
>>advantage: they can use more optimizations than other compilers especially
>> for specint that is very useful.
>>disadvantage: it is a more buggy approach.
>
>That is nonsense, spoken from someone that knows _zero_ about compiling
>techniques. Pick up _any_ good compiler book. Read it. Come back to the
>discussion with some understanding of what goes on...
Seems to me that making a problem simpler (and multiple stages) would make it a
-LESS- buggy approach as the coder need only concentrate on the particular task
he had to do. Since the data structures don't vary widely (obviously you use an
AST for the tree and then whatever form of three-operand IL or whatever you like
in between), I couldn't see how misconceptions about data structures would cause
bugs either.
>>Additionally i own AMD machines which are faster than the intel machines
>>i own. Intel is of course not doing effort to be fast on AMD machines.
>>
>>In contradiction. It's trivial that if version 5.0 is not doing bad for
>>AMD K7 processors, that when version 6 is doing a lot better for P4,
>>and a lot worse for K7, that version 7 and 8 aren't going to perform
>>better.
>>
>>Even a default compile with a compiler from years ago (msvc6 sp4 procpack)
>>is already a lot faster on any K7 than intel c++ 6.0 is.
>
>I seriously doubt that. The K7 is not _that_ different.
Spoken truely from ignorance. Optimizations that benefit the P7 benefit the K7
too. The designs are different, but like Dr. Hyatt says, they aren't -that-
different. Just about every P7 optimization I can think of benefits the K7 too.
The only place they really differ is in loop unrolling. The K7 benefits heavily
from unrolled loops. It's not as dramatic on the P7.
I have seen the output from the Intel C compiler. It estimates rates that a
branch will be taken at and does -specific- optimizations to maximize branch
prediction success rates. Does VC do this? No. With processor pack? No.
To put it more bluntly: I can hand-optimize better than VC 6. VC 7 and the Intel
C compiler both beat me.
-Matt
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