[BUG] android-ndk-r26c use -pg ld.lld: error: undefined symbol: _mcount

[mckaylinux]

Description

Hi All,

I want to use the -pg option in LLVM for performance profiling, but I'm encountering an error during linking that indicates _mcount is undefined. Can anyone help me out? Thank you.

android-ndk-r26c-linux/android-ndk-r26c/toolchains/llvm/prebuilt/linux-x86_64/bin/aarch64-linux-android34-clang -pg simd.c -o simdpg

ld.lld: error: undefined symbol: _mcount

Affected versions

r26

Canary version

No response

Host OS

Linux

Host OS version

Ubuntu20.04

Affected ABIs

arm64-v8a

Build system

ndk-build

Other build system

No response

minSdkVersion

android-ndk-r26c

Device API level

No response

[DanAlbert]

That symbol doesn't seem to come from bionic, so I'm guessing it's a compiler-rt thing?

[enh-google]

i'd ask @pirama-arumuga-nainar --- i don't know whether it's compiler-rt exactly, or there's an extra library you need for profiling? the latter rings a bell...

[DanAlbert]

There is (and it's part of the collection of libraries collectively called compiler-rt, I think), but Clang should be linking it automatically as long as those flags are correct.

[DanAlbert]

Oh, they might not be? https://github.com/DanAlbert/ndk-samples/blob/9cc55c332cfd9db9f8aaf9c7f0e042ada5b5d46f/pgo/app/src/main/cpp/CMakeLists.txt (sorry, I'd forgotten that I had to revert that sample and haven't put it back yet)

[mckaylinux]

@DanAlbert , Does that mean I need to use -fprofile-generate for profile data?

[DanAlbert]

That's how I was able to do it successfully. I don't know if that's the only way to do it, but the flags you used in the OP aren't familiar to me (they could be an alias for all I know).

[pirama-arumuga-nainar]

Yes, please use the clang-based techniques, which are the only ones we support.
https://clang.llvm.org/docs/SourceBasedCodeCoverage.html has instructions to use clang source-based coverage - which also doubles up as the profiling method (unlike gcov vs gprof). For Android specifically, there was an example in NDK samples which @DanAlbert is planning to resurrect (android/ndk-samples#956, android/ndk-samples#1070).

Alternatively, if you are looking for flame graphs and such to debug performance issues, use simpleperf.

(_mcount is provided by glibc. compiler-rt does not provide alternate implementations for non-glibc environments.)