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

[Friedman, Eli via llvm-dev]

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 .

>
>> 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?

> 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. :)

>
> 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.

>> 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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