limit to 2 parallel links when using thinlto

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.

Reviewers: hans, ruiu

Subscribers: mgorny, mehdi_amini, Prazek

Differential Revision: https://reviews.llvm.org/D31990

llvm-svn: 301676

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -17,6 +17,9 @@ 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
@@ -32,16 +35,19 @@ endif()
 
 set(LLVM_PARALLEL_LINK_JOBS "" CACHE STRING
   "Define the maximum number of concurrent link jobs.")
-if(LLVM_PARALLEL_LINK_JOBS)
-  if(NOT CMAKE_MAKE_PROGRAM MATCHES "ninja")
-    message(WARNING "Job pooling is only available with Ninja generators.")
-  else()
+if(CMAKE_MAKE_PROGRAM MATCHES "ninja")
+  if(NOT LLVM_PARALLEL_LINK_JOBS AND uppercase_LLVM_ENABLE_LTO STREQUAL "THIN")
+    message(STATUS "ThinLTO provides its own parallel linking - limiting parallel link jobs to 2.")
+    set(LLVM_PARALLEL_LINK_JOBS "2")
+  endif()
+  if(LLVM_PARALLEL_LINK_JOBS)
     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
@@ -724,8 +730,6 @@ append_if(LLVM_BUILD_INSTRUMENTED_COVERAGE "-fprofile-instr-generate='${LLVM_PRO
   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()