limit to 2 parallel links when using thinlto

[inglorion]

Summary

When using ThinLTO, the linker performs its own parallelism. This
change limits the number of parallel link jobs that Ninja will issue
to keep the total number of threads reasonable when linking with
ThinLTO.

[inglorion]

Alternatively, we could limit the parallelism of the link jobs, but limiting the number of link jobs is better, because it avoids having to wait a very long time for the last link job. For the same reason, the number of concurrent link jobs is 2 rather than 1.

cmake/modules/HandleLLVMOptions.cmake

20

This code is unchanged, but has been moved up to make uppercase_LLVM_ENABLE_LTO available when setting the number of link jobs.

[inglorion]

This will emit a warning when ThinLTO is enabled and the build system is not Ninja. I'll cook up a version that inhibits that warning.

[mehdi_amini]

LGTM if you can avoid the warning when using something else than Ninja.

[inglorion]

• No warning when not using Ninja.
• Simplified the change.
• Fixed buggy behavior on Linux.

[inglorion]

@mehdi_amini, still lgty?

[mehdi_amini]

Sure, still LGTM!

cmake/modules/HandleLLVMOptions.cmake

38

I don't know if there is a more canonical way of checking for ninja?

40

I'd add a note to be displayed in the console log.

[inglorion]

cmake/modules/HandleLLVMOptions.cmake

38

This is the same check that was there before (was on line 36).

40

Something like

message(STATUS "ThinLTO provides its own parallel linking - limiting parallel link jobs to 2.")

?

[mehdi_amini]

cmake/modules/HandleLLVMOptions.cmake

38

Oh right.

40

Yes something like this looks good to me. Thanks!

[inglorion]

added message and fixed case

cmake/modules/HandleLLVMOptions.cmake

 include(CheckCXXCompilerFlag)
 
 if(CMAKE_LINKER MATCHES "lld-link.exe" OR (WIN32 AND LLVM_USE_LINKER STREQUAL "lld"))
   set(LINKER_IS_LLD_LINK TRUE)
 else()
   set(LINKER_IS_LLD_LINK FALSE)
 endif()
 
+set(LLVM_ENABLE_LTO OFF CACHE STRING "Build LLVM with LTO. May be specified as Thin or Full to use a particular kind of LTO")
+string(TOUPPER "${LLVM_ENABLE_LTO}" uppercase_LLVM_ENABLE_LTO)
+
 # Ninja Job Pool support
 # The following only works with the Ninja generator in CMake >= 3.0.
 set(LLVM_PARALLEL_COMPILE_JOBS "" CACHE STRING
   "Define the maximum number of concurrent compilation jobs.")
 if(LLVM_PARALLEL_COMPILE_JOBS)
   if(NOT CMAKE_MAKE_PROGRAM MATCHES "ninja")
     message(WARNING "Job pooling is only available with Ninja generators.")
   else()
     set_property(GLOBAL APPEND PROPERTY JOB_POOLS compile_job_pool=${LLVM_PARALLEL_COMPILE_JOBS})
     set(CMAKE_JOB_POOL_COMPILE compile_job_pool)
   endif()
 endif()
 
 set(LLVM_PARALLEL_LINK_JOBS "" CACHE STRING
   "Define the maximum number of concurrent link jobs.")
+if(CMAKE_MAKE_PROGRAM MATCHES "ninja")
+  if(NOT LLVM_PARALLEL_LINK_JOBS AND uppercase_LLVM_ENABLE_LTO STREQUAL "thin")
+    set(LLVM_PARALLEL_LINK_JOBS "2")
+  endif()
   if(LLVM_PARALLEL_LINK_JOBS)
-  if(NOT CMAKE_MAKE_PROGRAM MATCHES "ninja")
-    message(WARNING "Job pooling is only available with Ninja generators.")
-  else()
     set_property(GLOBAL APPEND PROPERTY JOB_POOLS link_job_pool=${LLVM_PARALLEL_LINK_JOBS})
     set(CMAKE_JOB_POOL_LINK link_job_pool)
   endif()
+elseif(LLVM_PARALLEL_LINK_JOBS)
+  message(WARNING "Job pooling is only available with Ninja generators.")
 endif()
 
-
 if (LINKER_IS_LLD_LINK)
   # Pass /MANIFEST:NO so that CMake doesn't run mt.exe on our binaries.  Adding
   # manifests with mt.exe breaks LLD's symbol tables and takes as much time as
   # the link. See PR24476.
   append("/MANIFEST:NO"
     CMAKE_EXE_LINKER_FLAGS CMAKE_MODULE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
 endif()
 

 option(LLVM_BUILD_INSTRUMENTED_COVERAGE "Build LLVM and tools with Code Coverage instrumentation (experimental)" Off)
 mark_as_advanced(LLVM_BUILD_INSTRUMENTED_COVERAGE)
 append_if(LLVM_BUILD_INSTRUMENTED_COVERAGE "-fprofile-instr-generate='${LLVM_PROFILE_FILE_PATTERN}' -fcoverage-mapping"
   CMAKE_CXX_FLAGS
   CMAKE_C_FLAGS
   CMAKE_EXE_LINKER_FLAGS
   CMAKE_SHARED_LINKER_FLAGS)
 
-set(LLVM_ENABLE_LTO OFF CACHE STRING "Build LLVM with LTO. May be specified as Thin or Full to use a particular kind of LTO")
-string(TOUPPER "${LLVM_ENABLE_LTO}" uppercase_LLVM_ENABLE_LTO)
 if(LLVM_ENABLE_LTO AND LLVM_ON_WIN32 AND NOT LINKER_IS_LLD_LINK)
   message(FATAL_ERROR "When compiling for Windows, LLVM_ENABLE_LTO requires using lld as the linker (point CMAKE_LINKER at lld-link.exe)")
 endif()
 if(uppercase_LLVM_ENABLE_LTO STREQUAL "THIN")
   append("-flto=thin" CMAKE_CXX_FLAGS CMAKE_C_FLAGS)
   if(NOT LINKER_IS_LLD_LINK)
     append("-flto=thin" CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
   endif()