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

[Vedant Kumar via llvm-dev]

> On Jun 19, 2017, at 7:29 PM, Vedant Kumar <vsk at apple.com> wrote:
> 
> 
>> On Jun 19, 2017, at 4:32 PM, Friedman, Eli <efriedma at codeaurora.org <mailto:efriedma at codeaurora.org>> wrote:
>> 
>> On 6/18/2017 3:51 PM, Vedant Kumar wrote:
>>>> My experience:
>>>> 
>>>> 1. You have to specify -DLLVM_USE_LINKER=gold (or maybe lld works; I didn't try).  If you link with binutils ld, the program will generate broken profile information.  Apparently, the linked binary is missing the __llvm_prf_names section.  This took me half a day to figure out.  This issue isn't documented anywhere, and the only error message I got was "Assertion `!Key.empty()' failed." from llvm-cov.
>>> 
>>> I expect llvm-cov to print out "Failed to load coverage: <reason>" in this situation. There was some work done to tighten up error reporting in ProfileData and its clients in r270020. If your host toolchain does have these changes, please file a bug, and I'll have it fixed.
>> 
>> Host toolchain is trunk clang... but using system binutils (which is 2.24 on my Ubuntu 14.04 system... and apparently that's too old per David Li's response).  Anyway, filed https://bugs.llvm.org/show_bug.cgi?id=33517 <https://bugs.llvm.org/show_bug.cgi?id=33517> .
> 
> I've updated the clang docs re: 'Source based code coverage' to reflect this issue. I've also tightened up our error reporting a bit so we fail earlier with something better than an assertion message (r305765, r305767).
> 
>>>> 2. The generated binaries are big and slow.  Comparing to a build without coverage, llc becomes 8x larger overall (text section becomes roughly 2x larger).  And check-llvm-codegen-arm goes from 3 seconds to 250 seconds.
>>> 
>>> The binary size increase comes from coverage mapping data, counter increment instrumentation, and profiling metadata.
>>> 
>>> The coverage mapping section is highly compressible, but exploiting the compressibility has proven to be tricky. I filed: llvm.org/PR33499 <http://llvm.org/PR33499>.
>> 
>> If I'm cross-compiling for a target where the space matters, can I rid of the data for the copy on the device using "strip -R __llvm_covmap" or something like that, then use llvm-cov on the original?
> 
> I haven't tried this but I expect it to work. Instrumented programs don't reference the __llvm_covmap section.
> 
>>> Coverage makes use of frontend-based instrumentation, which is much less efficient than the IR-based kind. If we can find a way to map counters inserted by IR PGO to AST nodes, we could improve the situation. I filed: llvm.org/PR33500 <http://llvm.org/PR33500>.
>> 
>> This would be nice... but I assume it's hard. :)
> 
> It seems like it is. At a high level, you'd need some way to associate the counters placed by IR PGO instrumentation to the counters that clang expects to see while walking an AST. I don't have a concrete design for this in mind.
> 
>>> 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.

Ah, sorry for mentioning this twice.

On another note, I'm looking into the "N mismatched functions" warnings issue, and suspect that it happens when there are conflicting definitions of the same function in different binaries. The issue doesn't seem to occur when using profiles from just one binary to generate a report for that binary. I'll dig into this a bit more and update PR33502.

vedant

> 
> vedant
> 
>>>> 4. When prepare-code-coverage-artifact.py invokes llvm-profdata for the profiles generated by "make check", it takes 50GB of memory to process about 1.5GB of profiles.  Is it supposed to use that much?
>>> 
>>> By default, llvm-profdata uses hardware_concurrency() to determine the number of threads to use to merge profiles. You can change the default by passing -j/--num-threads to llvm-profdata. I'm open to changing the 'prep' script to use -j4 or something like that.
>>> 
>> 
>> Oh, so it's using a couple gigabytes per thread multiplied by 24 cores?  Okay, now I'm not so worried. :)
>> -Eli
>> -- 
>> 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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