Assembly optimizations

[Nathan Sidwell]

Klaus Winter wrote:
> Hi all,
> 
> I compiled the following code with gcc-3.3.1. Once without
> optimization and one time with optimization -O. With optimization the
> Decrement function takes a LOT longer than before whereas the
> Increment function stays constant. I haven't found anything in the
> manpage about it ... Any help is appreciated.
> 
> cheers,
> Klaus
> 
> #include <stdio.h>
> #include <time.h>
> #include <sys/time.h>
> 
> int g_Lock_Mini = 0;
> 
> inline int Increment( int* count)
> {
> 	__asm__ __volatile__( "movl $1, %%eax\n\t" 
> 			      "lock; xaddl %%eax, (%%ecx)\n\t"
> 			      "incl %%eax\n\t" : : "c" (count) );
> }
> inline int Decrement( int * count)
> {
> 	__asm__ __volatile__( "lock; decl (%%ecx)\n\t"
> 				  "movl (%%ecx), %%eax\n\t" : : "c" (count));
> }
these asms are broken. I think you want something like

asm volatile("lock;decl (%1)\n\tmovl (%1),%0" : "=r" (result) : "r" (count) : "memory")

gcc does not otherwise know what registers are used by the asm.

nathan

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