https://github.com/tcreech-intel created https://github.com/llvm/llvm-project/pull/88438
1. Add `-fdebug-info-for-profiling -funique-internal-linkage-names`, which improve the usefulness of debug info for profiling. 2. Recommend the use of `br_inst_retired.near_taken:uppp`, which provides the most precise results on supporting hardware. Mention `branches:u` as a more portable backup. Both should portray execution counts better than the default event (`cycles`) and have a better chance of working as an unprivileged user due to the `:u` modifier. >From fe3404cbdf78b434f16f8351dc242175b4543112 Mon Sep 17 00:00:00 2001 From: Tim Creech <timothy.m.cre...@intel.com> Date: Thu, 11 Apr 2024 16:03:52 -0400 Subject: [PATCH] Improve documented sampling profiler steps to best known methods 1. Add `-fdebug-info-for-profiling -funique-internal-linkage-names`, which improve the usefulness of debug info for profiling. 2. Recommend the use of `br_inst_retired.near_taken:uppp`, which provides the most precise results on supporting hardware. Mention `branches:u` as a more portable backup. Both should portray execution counts better than the default event (`cycles`) and have a better chance of working as an unprivileged user due to the `:u` modifier. --- clang/docs/UsersManual.rst | 16 ++++++++++------ 1 file changed, 10 insertions(+), 6 deletions(-) diff --git a/clang/docs/UsersManual.rst b/clang/docs/UsersManual.rst index c464bc3a69adc5..818841285cfae5 100644 --- a/clang/docs/UsersManual.rst +++ b/clang/docs/UsersManual.rst @@ -2443,13 +2443,15 @@ usual build cycle when using sample profilers for optimization: usual build flags that you always build your application with. The only requirement is that DWARF debug info including source line information is generated. This DWARF information is important for the profiler to be able - to map instructions back to source line locations. + to map instructions back to source line locations. The usefulness of this + DWARF information can be improved with the ``-fdebug-info-for-profiling`` + and ``-funique-internal-linkage-names`` options. - On Linux, ``-g`` or just ``-gline-tables-only`` is sufficient: + On Linux: .. code-block:: console - $ clang++ -O2 -gline-tables-only code.cc -o code + $ clang++ -O2 -gline-tables-only -fdebug-info-for-profiling -funique-internal-linkage-names code.cc -o code While MSVC-style targets default to CodeView debug information, DWARF debug information is required to generate source-level LLVM profiles. Use @@ -2457,13 +2459,13 @@ usual build cycle when using sample profilers for optimization: .. code-block:: console - $ clang-cl -O2 -gdwarf -gline-tables-only coff-profile.cpp -fuse-ld=lld -link -debug:dwarf + $ clang-cl -O2 -gdwarf -gline-tables-only /clang:-fdebug-info-for-profiling /clang:-funique-internal-linkage-names code.cc -o code -fuse-ld=lld -link -debug:dwarf 2. Run the executable under a sampling profiler. The specific profiler you use does not really matter, as long as its output can be converted into the format that the LLVM optimizer understands. - Two such profilers are the the Linux Perf profiler + Two such profilers are the Linux Perf profiler (https://perf.wiki.kernel.org/) and Intel's Sampling Enabling Product (SEP), available as part of `Intel VTune <https://software.intel.com/content/www/us/en/develop/tools/oneapi/components/vtune-profiler.html>`_. @@ -2477,7 +2479,9 @@ usual build cycle when using sample profilers for optimization: .. code-block:: console - $ perf record -b ./code + $ perf record -b -e BR_INST_RETIRED.NEAR_TAKEN:uppp ./code + + If the event above is unavailable, ``branches:u`` is probably next-best. Note the use of the ``-b`` flag. This tells Perf to use the Last Branch Record (LBR) to record call chains. While this is not strictly required, _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits