Author: Christophe Theron
Date: 21:16:16 04/09/04
Go up one level in this thread
On April 09, 2004 at 18:04:10, Russell Reagan wrote:
>I'm posting this here because I know that some of the programmers here use MSVC
>.NET 2003, and in hopes that Eugene will read this :)
>
>Today I had a problem with a program I was writing, and I came across what I
>believe is an optimization bug. I wrote this little program to reproduce the
>bug. If I compile with optimizations set to "Disabled", there is no problem. If
>I compile with optimzations set to anything else (Maximize Speed, Minimize Size,
>Full Optimizations), the program crashes due to a loop never terminating. The
>source code is at the end of this message. The line where the problem occurs is:
>
>for (row = 11; row < 12; row--)
>
>The problem is the comparison: row < 12
Just a quick note:
for (row=11; row>=0; row--) {
...
}
is probably faster (at least I expect it to be not slower than your version).
I also find it more readable, but maybe that's just me.
row=11;
do {
...
} while (row--);
is maybe even slightly faster. But it's less readable.
But with modern compilers and cache magic, I'm not so sure. :)
Christophe
>
>When optimizations are turned off, this is the assembly output for that
>comparison:
>
>cmp DWORD PTR _row$[ebp], 12
>
>If optimizations are turned on, this is the assembly output:
>
>cmp ebx, OFFSET FLAT:_a+576
>
>I'm no assembly expert, but that looks like it is comparing the address of two
>pointers to me, and has nothing to do with comparing two values.
>
>Here is the example C source code. I compiled it under GCC with and without
>optimizations and it ran fine. Another person with MSVC .NET 2003 ran this code
>and he reported the same problem of crashing with optimizations turned on.
>
>Is this really an optimization bug, or is it possible that I am doing something
>wrong with the compiler settings? If this is indeed an optimization bug, how can
>I find out if Microsoft is already aware of it? If they are not aware of it, how
>can I report it to them?
>
>#include <stdio.h>
>
>unsigned int a[144];
>
>void init ()
>{
> unsigned int i;
> for (i = 0; i < 144; i++)
> a[i] = rand();
>}
>
>void dump ()
>{
> unsigned int row, col;
> for (row = 11; row < 12; row--) // <--- PROBLEM: row < 12
> {
> for (col = 0; col < 12; col++)
> printf("%u ", a[(row * 12) + col]);
> putchar('\n');
> }
>}
>
>int main ()
>{
> init();
> dump();
> return 0;
>}
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