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Diffstat (limited to 'clang/docs/SourceBasedCodeCoverage.rst')
| -rw-r--r-- | clang/docs/SourceBasedCodeCoverage.rst | 33 |
1 files changed, 30 insertions, 3 deletions
diff --git a/clang/docs/SourceBasedCodeCoverage.rst b/clang/docs/SourceBasedCodeCoverage.rst index 7e711819be34..1575e4faaa01 100644 --- a/clang/docs/SourceBasedCodeCoverage.rst +++ b/clang/docs/SourceBasedCodeCoverage.rst @@ -92,15 +92,42 @@ directory structure will be created. Additionally, the following special instrumented program crashes, or is killed by a signal, perfect coverage information can still be recovered. Continuous mode does not support value profiling for PGO, and is only supported on Darwin at the moment. Support for - Linux may be mostly complete but requires testing, and support for - Fuchsia/Windows may require more extensive changes: please get involved if - you are interested in porting this feature. + Linux may be mostly complete but requires testing, and support for Windows + may require more extensive changes: please get involved if you are interested + in porting this feature. .. code-block:: console # Step 2: Run the program. % LLVM_PROFILE_FILE="foo.profraw" ./foo +Note that continuous mode is also used on Fuchsia where it's the only supported +mode, but the implementation is different. The Darwin and Linux implementation +relies on padding and the ability to map a file over the existing memory +mapping which is generally only available on POSIX systems and isn't suitable +for other platforms. + +On Fuchsia, we rely on the the ability to relocate counters at runtime using a +level of indirection. On every counter access, we add a bias to the counter +address. This bias is stored in ``__llvm_profile_counter_bias`` symbol that's +provided by the profile runtime and is initially set to zero, meaning no +relocation. The runtime can map the profile into memory at abitrary location, +and set bias to the offset between the original and the new counter location, +at which point every subsequent counter access will be to the new location, +which allows updating profile directly akin to the continous mode. + +The advantage of this approach is that doesn't require any special OS support. +The disadvantage is the extra overhead due to additional instructions required +for each counter access (overhead both in terms of binary size and performance) +plus duplication of counters (i.e. one copy in the binary itself and another +copy that's mapped into memory). This implementation can be also enabled for +other platforms by passing the ``-runtime-counter-relocation`` option to the +backend during compilation. + +.. code-block:: console + + % clang++ -fprofile-instr-generate -fcoverage-mapping -mllvm -runtime-counter-relocation foo.cc -o foo + Creating coverage reports ========================= |