[LLVMdev] Measure execution time of each basic block

[sathvik]

If you are on X86, you can use the rdtsc/rdtscp llvm intrinsics -
http://permalink.gmane.org/gmane.comp.compilers.llvm.cvs/185208

with RDTSCP you can double fence your basic block so you can be sure that
the OOO scheduling does not schedule instructions that you don't intend to
measure, however note that there is a huge overhead as a result of this.
RDTSC has a much lower overhead since it only reads your TSC reg but if you
are measuring a small basic block it won't be accurate.

thanks,
sathvik


On Wed, Sep 17, 2014 at 1:35 PM, Vanderson Martins do Rosario <
vandersonmr2 at gmail.com> wrote:

> Jon,
> I need to create a database of basics blocks and their execution time. The
> only thing I'm concerned is if a block A is more expensive than a block B.
> Do you think that even with the overhead I would be able to get the A > B
> information?
> Like: overhead + time(A) > overhead + time(B) => A > B.
> If so, I'm not too much concerned about the accuracy.
>
> Not sure if I was clear,
>
> Thanks again
>
> Vanderson M. Rosario
>
> 2014-09-17 17:26 GMT-03:00 Jonathan Roelofs <jonathan at codesourcery.com>:
>
>> I think if you do this, you're quickly going to realize that there's
>> quite a lot of overhead in getting the time stamps needed to record basic
>> block duration, so you're not going to get accurate results except for
>> really big basic blocks.
>>
>>
>> Cheers,
>>
>> Jon
>>
>>
>> On 9/17/14 1:53 PM, Vanderson Martins do Rosario wrote:
>>
>>> Guys,
>>>
>>> Someone have any idea how could I measure the execution time of each
>>> LLVM basic
>>> block of a program?
>>>
>>> I tried to use some profiling tools like gcov and perf, but as far as I
>>> know
>>> they can only give me the frequency that each basic block is executed.
>>>
>>> I was thinking about writing a pass to add PAPI instructions in each
>>> LLVM basic
>>> block. Do you think is a good idea?
>>>
>>> Thanks
>>>
>>> Vanderson M. Rosario
>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>
>>>
>> --
>> Jon Roelofs
>> jonathan at codesourcery.com
>> CodeSourcery / Mentor Embedded
>>
>
>
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