[llvm-dev] My experience using -DLLVM_BUILD_INSTRUMENTED_COVERAGE to generate coverage

Tue Jun 27 14:54:27 PDT 2017

> On Jun 27, 2017, at 2:48 PM, Xinliang David Li <davidxl at google.com> wrote:
> 
> 
> 
> On Tue, Jun 27, 2017 at 2:41 PM, <vsk at apple.com <mailto:vsk at apple.com>> wrote:
> With llc, the size of the names section can vary widely depending on the value of -DLLVM_TARGETS_TO_BUILD.
> 
> 
> Sure.
>  
> Enabling coverage shouldn't increase the name section size much. I only see one place where this happens, and it's relatively cold:
> http://lab.llvm.org:8080/coverage/coverage-reports/llvm/coverage/Users/buildslave/jenkins/sharedspace/clang-stage2-coverage-R@2/llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp.html#L512 <http://lab.llvm.org:8080/coverage/coverage-reports/llvm/coverage/Users/buildslave/jenkins/sharedspace/clang-stage2-coverage-R@2/llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp.html#L512>
> 
> 
> 
> What is the data set used to create the coverage data there?  Looks like the lower Coverage function is called 6 times (aka only 6 modules have coverage data) and on average, each coverage data only references ~2 names.  This does not seem typical.

It's check-llvm, check-clang, plus the test suite. lowerCoverageData doesn't get called unless we need to emit empty counter mappings, for things like functions with empty bodies.

vedant

> 
>  
> 
>> On Jun 27, 2017, at 2:40 PM, Friedman, Eli <efriedma at codeaurora.org <mailto:efriedma at codeaurora.org>> wrote:
>> 
>> I get a bunch of unreadable binary.  Output piped to "less":
>> 
>> String dump of section '__llvm_prf_names':
>>   [     2]  #<CA>^Ex<DA><D4>is<E3>(^Z<EF>^O<DD>^P<A9><D5><F7><9B><CB><FB><A8>d<97>^U<96>O<9F><89><FB><85>AQ<B4>M<B5><B6>&)Wy~<FD>C^B\<B0>/$<A5><EA>s<E3><CE>L<97>E$^R<89>Dn<C8>L<84><FF>
>> <EF><C7>h<B7><FB><DC>ߊ,=<BC><BF>$ow~<B0>\<C4><FF>_X<FE><E0><C7><D1>&<C9>c<F4><E7>^_<AB><B0><F9>,`<BE>H^Oi1G<BF>{<E3><D7>({O<8A><E7>S<91>^^^O<B9>7<BB><CD><F7><F3>C^d<E7>}r("<F8>c^P P<83>
>> <D0><F3>`T^Z<ED><D2><FF>M<B2><F9>k^X^D/<8B><D5>8<A4><E1>N><A3><DD>9<C9><E7><DF><F1><F7>c^B38<9C><F7>^<BF><C2>ߕ^_<D6><F0>_<F2><BB>]<94><E7><C1><FD><E9><95>^A3<<9E>^\<B0>{\^O0<CC><C9>)<CA>r<84><D9>j^H<B3><DC><F9><F3><EF><B7><FE> <8C><E1>'L^Q<C9>"<F0><A7>"><E8><FF><DD>^^V^R<A4><D6><FC>d<EB>j*oHO<D5>^F<C2>p<D0>^U<82><EF>^Y!<90><9D>O5;:^TgL<F9>^Y<D3>z<D5>#=<81><E1><C3>6<C4>^\u%<85>7<EE>^Jaf^D0C<8B><8C>r^D^E<9D><B5>^W^L<A3>dx<FA><A3>1<FE><88>l^OO<AB>^Z<80>^\~^I<EE><DE>^O>]V~c<C9>^D<B7><88>[4|0^R<89><B5>ʳ<96><D7>Ӽ<E7><F9>^D<EB>^<BF><A5><9B>Mr^Ht<CC>A^PP<EF>
>> <8D>n<BA><89>q<91><DE>
> 
> This looks compressed to me.
> 
> 
> Yes.
> 
> David
>  
> vedant
> 
> 
>> 
>> -Eli
>> 
>> On 6/27/2017 2:32 PM, Xinliang David Li wrote:
>>> I had an old build of llc with FE instrumentation, the name section size is about 5MB.  Using coverage is likely to cause the name section to be larger as there are more references to dead/unused function names.
>>> 
>>> What do you see when 
>>> 
>>> readelf --string-dump=__llvm_prf_names llc
>>> 
>>> David
>>> 
>>> On Tue, Jun 27, 2017 at 2:23 PM, Xinliang David Li <davidxl at google.com <mailto:davidxl at google.com>> wrote:
>>> 
>>> 
>>> On Tue, Jun 27, 2017 at 2:09 PM, Friedman, Eli <efriedma at codeaurora.org <mailto:efriedma at codeaurora.org>> wrote:
>>> On 6/27/2017 1:47 PM, Xinliang David Li wrote:
>>>> 
>>>> 
>>>> On Mon, Jun 26, 2017 at 7:24 PM, Friedman, Eli <efriedma at codeaurora.org <mailto:efriedma at codeaurora.org>> wrote:
>>>> On 6/19/2017 7:29 PM, Vedant Kumar wrote:
>>>>> 
>>>>>>> We can reduce testing time by *not* instrumented basic tools like count, not, FileCheck etc. I filed: llvm.org/PR33501 <http://llvm.org/PR33501>.
>>>>>>> 
>>>>>>>> 3. The generated profile information takes up a lot of space: llc generates a 90MB profraw file.
>>>>>>> 
>>>>>>> I don't have any ideas about how to fix this. You can decrease the space overhead for raw profiles by altering LLVM_PROFILE_MERGE_POOL_SIZE from 4 to a lower value.
>>>>>> 
>>>>>> Disk space is cheap, but the I/O takes a long time.  I guess it's specifically bad for LLVM's "make check", maybe not so bad for other cases.
>>>>> 
>>>>> You can speed up "make check" a bit by using non-instrumented versions of count, not, FileCheck, etc.
>>>> 
>>>> I tried looking into this a bit more.  It looks like the profile data file generated by llc contains approximately 5MB of counters (__llvm_prf_cnts), 10MB of "data" (__llvm_prf_data), and 70MB of __llvm_prf_names.  __llvm_prf_data and __llvm_prf_names contain which can be read from the original binary, as far as I can tell.  The 80MB of data wouldn't be a big deal if it were just sitting on disk... but we also erase the whole file and rewrite it from scratch after we merge profile counters.
>>>> 
>>>> 
>>>> Can you check if name compression is turned on in your build?   
>>>> 
>>>> David
>>>> 
>>> 
>>> I think it is. At least, I didn't intentionally turn it off, and examining the file with objdump I don't see any uncompressed strings.  Not sure if there's any easy way to confirm that.
>>> 
>>> 
>>> Just a little surprised at the size of __llvm_prf_names section. The llc I built with IR PGO has a __llvm_prf_names section with size  ~1.4MB.    I expect FE instrumentation to produce larger name section size, but not so much bigger.
>>> 
>>> David
>>> 
>>> -Eli
>>> -- 
>>> Employee of Qualcomm Innovation Center, Inc.
>>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
>>> 
>>> 
>> 
>> -- 
>> Employee of Qualcomm Innovation Center, Inc.
>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
> 
> 

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