Xilinx PPC unexplainable cycle count

Hello,

This is what I am experiencing:
- I need to compare to code versions regarding execution speed
- comparison is done counting cycles, using the time base registers
- the only difference in code is the following:
    #if POST_PROC_CS_UPD
      // do nothing
    #else
      update_checksum();
    #endif

When compiled with POST_PROC_CS_UPD = 1 the execution time
of this portion is shorter, however other code that should not be
affected and is executed before the above sequence is dramatically
slower.

Any ideas ?

Regards,
Ernest

eascheiber@yahoo.com wrote:
> This is what I am experiencing:
> - I need to compare to code versions regarding execution speed
> - comparison is done counting cycles, using the time base registers
> - the only difference in code is the following:
>     #if POST_PROC_CS_UPD
>       // do nothing
>     #else
>       update_checksum();
>     #endif
>
> When compiled with POST_PROC_CS_UPD = 1 the execution time
> of this portion is shorter, however other code that should not be
> affected and is executed before the above sequence is dramatically
> slower.


Could be a cache effect.  To see if this is true, turn off both caches
and see if execution time is equal.

What is dramatic (be more specific)?

Alan Nishioka