[llvm-dev] My experience using -DLLVM_BUILD_INSTRUMENTED_COVERAGE to generate coverage
Friedman, Eli via llvm-dev
llvm-dev at lists.llvm.org
Fri Jun 16 18:08:08 PDT 2017
I've started looking at the state of code coverage recently; we figured
LLVM itself would be a good test to figure out how mature it is, so I
gave it a shot. 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.
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
3. The generated profile information takes up a lot of space: llc
generates a 90MB profraw file.
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?
5. Using prepare-code-coverage-artifact.py generates "warning: 229
functions have mismatched data". I'm not sure what's causing this... I
guess it has something to do with merging the profile data for multiple
binaries? The error message is not very helpful.
5. The HTML output highlights the semicolon after a break or return
statement in some switch statements in red. (For example,
LowerADDC_ADDE_SUBC_SUBE in ARMISelLowering.cpp.) Not really important,
6. On the bright side, when it works, the generated coverage information
is precise and easy to read.
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