[llvm-exegesis] Add a flag to disable libpfm even if present.

[courbet]

Summary

Fixes PR37053.

[uabelho]

I don't know the build system very well but this looks good to me.

Perhaps document LLVM_ENABLE_LIBPFM in docs/CMake.rst?

[courbet]

document LLVM_ENABLE_LIBPFM in docs/CMake.rst

[courbet]

Good point, done.

[lebedev.ri]

cmake/config-ix.cmake

93

One more piece is missing, LLVM_ENABLE_LIBPFM needs to be checked where linking to pfm happens.

[courbet]

cmake/config-ix.cmake

93

Thanks Roman,

This is already done by checking on HAVE_LIBPFM, see tools/llvm-exegesis/CMakeLists.txt:

if(HAVE_LIBPFM)
  target_link_libraries(llvm-exegesis PRIVATE pfm)
endif()

[lebedev.ri]

cmake/config-ix.cmake

93

Exactly, and that is insufficient.
It should be

if(LLVM_ENABLE_LIBPFM AND HAVE_LIBPFM)
  target_link_libraries(llvm-exegesis PRIVATE pfm)
endif()

I'm not sure if LLVM can be built as part of some parent CMake project, so *this* may be not-an-issue,
but will just give a bad example for future changes. The problem is, what will happen if one
defines HAVE_LIBPFM (e.g. by using it in some parent cmake project), but disables LLVM_ENABLE_LIBPFM?

Similar problem: https://github.com/google/googletest/pull/975

[courbet]

Double check for LLVM_ENABLE_LIBPFM and HAVE_LIBPFM when linking.

cmake/config-ix.cmake

93

I see, that makes sense, thanks for the explanation.

[lebedev.ri]

I don't really know LLVM's buildsystem, but this looks about right to me.
Sidenote:

cmake/config-ix.cmake

94

Sidenote: normally, this should be in a cmake/Modules/FindPFM.cmake, and this should be just

if (LLVM_ENABLE_LIBPFM)
  find_package(PFM REQUIRED)
endif()

[courbet]

cmake/config-ix.cmake

94

Good idea. I've split this off to a separate module in r329783.

CMakeLists.txt

set(LLVM_TARGET_ARCH "host"
   CACHE STRING "Set target to use for LLVM JIT or use \"host\" for automatic detection.")
 
 option(LLVM_ENABLE_TERMINFO "Use terminfo database if available." ON)
 
 set(LLVM_ENABLE_LIBXML2 "ON" CACHE STRING "Use libxml2 if available. Can be ON, OFF, or FORCE_ON")
 
 option(LLVM_ENABLE_LIBEDIT "Use libedit if available." ON)
 
+option(LLVM_ENABLE_LIBPFM "Use libpfm for performance counters if available." ON)
+
 option(LLVM_ENABLE_THREADS "Use threads if available." ON)
 
 option(LLVM_ENABLE_ZLIB "Use zlib for compression/decompression if available." ON)
 
 if( LLVM_TARGETS_TO_BUILD STREQUAL "all" )
   set( LLVM_TARGETS_TO_BUILD ${LLVM_ALL_TARGETS} )
 endif()
 

cmake/config-ix.cmake

if (PTHREAD_IN_LIBC)
       check_library_exists(c pthread_mutex_lock "" HAVE_PTHREAD_MUTEX_LOCK)
     endif()
   endif()
   check_library_exists(dl dlopen "" HAVE_LIBDL)
   check_library_exists(rt clock_gettime "" HAVE_LIBRT)
 endif()
 
 # Check for libpfm.
+if (LLVM_ENABLE_LIBPFM)
   check_library_exists(pfm pfm_initialize "" HAVE_LIBPFM_INITIALIZE)
   if(HAVE_LIBPFM_INITIALIZE)
     check_include_file(perfmon/perf_event.h HAVE_PERFMON_PERF_EVENT_H)
     check_include_file(perfmon/pfmlib.h HAVE_PERFMON_PFMLIB_H)
     check_include_file(perfmon/pfmlib_perf_event.h HAVE_PERFMON_PFMLIB_PERF_EVENT_H)
     if(HAVE_PERFMON_PERF_EVENT_H AND HAVE_PERFMON_PFMLIB_H AND HAVE_PERFMON_PFMLIB_PERF_EVENT_H)
       set(HAVE_LIBPFM 1)
     endif()
   endif()
+endif()
 
 if(HAVE_LIBPTHREAD)
   # We want to find pthreads library and at the moment we do want to
   # have it reported as '-l<lib>' instead of '-pthread'.
   # TODO: switch to -pthread once the rest of the build system can deal with it.
   set(CMAKE_THREAD_PREFER_PTHREAD TRUE)
   set(THREADS_HAVE_PTHREAD_ARG Off)
   find_package(Threads REQUIRED)

docs/CMake.rst

 
 **LLVM_PROFDATA_FILE**:PATH
   Path to a profdata file to pass into clang's -fprofile-instr-use flag. This
   can only be specified if you're building with clang.
 
 **LLVM_USE_INTEL_JITEVENTS**:BOOL
   Enable building support for Intel JIT Events API. Defaults to OFF.
 
+**LLVM_ENABLE_LIBPFM**:BOOL
+  Enable building with libpfm to support hardware counter measurements in LLVM
+  tools.
+  Defaults to ON.
+
 **LLVM_ENABLE_ZLIB**:BOOL
   Enable building with zlib to support compression/uncompression in LLVM tools.
   Defaults to ON.
 
 **LLVM_ENABLE_DIA_SDK**:BOOL
   Enable building with MSVC DIA SDK for PDB debugging support. Available
   only with MSVC. Defaults to ON.