Re: [Valgrind-users] Counting instructions for specific part of my code using Callgrind

[Florian K. <fl...@ei...>]

On 28.08.2015 09:03, Josef Weidendorfer wrote:

> This suggests we should document the difference between
> START/STOP_INSTRUMENTAITON
> and TOGGLE_COLLECT better. 

Hmm, yes :) Any chance you can write something up in time for 3.11 ?

> 
>>      for (int i = 1; i <= 1000; ++i) {
>>        CALLGRIND_START_INSTRUMENTATION;
>>        n += i;
>>        CALLGRIND_STOP_INSTRUMENTATION;
>>      }
> 
> With macros in C, the compiler can reorder stuff, so you may end up with
> something like
> 
>>      for (int i = 1; i <= 1000; ++i) {
>>        n += i;
>>        CALLGRIND_START_INSTRUMENTATION;
>>        CALLGRIND_STOP_INSTRUMENTATION;
>>      }
> 

Huh? CALLGRIND_START_INSTRUMENTATION expands into a statement sequence
that includes
   __asm__ volatile("whatever" ::: "memory");

and GCC docs say:
<quote>
 If your assembler instructions access memory in an unpredictable
fashion, add 'memory' to the list of clobbered registers.
</quote>

Wouldn't that prevent the CALLGRIND_START_INSTRUMENTATION from being
moved around? Reason being that the memory clobber might change the
value of "i" in a way not visible to the compiler?

   Florian