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
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