[CMake] Add tablegen job pool support (#84762)

Add the ability to set the number of tablegen jobs that can run in
parallel
similar to the LLVM_PARALLEL_[COMPILE|LINK]_JOBS options that already
exist.

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -36,7 +36,7 @@ string(TOUPPER "${LLVM_ENABLE_LTO}" uppercase_LLVM_ENABLE_LTO)
 # 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 (Ninja only).")
-if(LLVM_RAM_PER_COMPILE_JOB OR LLVM_RAM_PER_LINK_JOB)
+if(LLVM_RAM_PER_COMPILE_JOB OR LLVM_RAM_PER_LINK_JOB OR LLVM_RAM_PER_TABLEGEN_JOB)
   cmake_host_system_information(RESULT available_physical_memory QUERY AVAILABLE_PHYSICAL_MEMORY)
   cmake_host_system_information(RESULT number_of_logical_cores QUERY NUMBER_OF_LOGICAL_CORES)
 endif()
@@ -86,6 +86,28 @@ elseif(LLVM_PARALLEL_LINK_JOBS)
   message(WARNING "Job pooling is only available with Ninja generators.")
 endif()
 
+set(LLVM_PARALLEL_TABLEGEN_JOBS "" CACHE STRING
+  "Define the maximum number of concurrent tablegen jobs (Ninja only).")
+if(LLVM_RAM_PER_TABLEGEN_JOB)
+  math(EXPR jobs_with_sufficient_memory "${available_physical_memory} / ${LLVM_RAM_PER_TABLEGEN_JOB}" OUTPUT_FORMAT DECIMAL)
+  if (jobs_with_sufficient_memory LESS 1)
+    set(jobs_with_sufficient_memory 1)
+  endif()
+  if (jobs_with_sufficient_memory LESS number_of_logical_cores)
+    set(LLVM_PARALLEL_TABLEGEN_JOBS "${jobs_with_sufficient_memory}")
+  else()
+    set(LLVM_PARALLEL_TABLEGEN_JOBS "${number_of_logical_cores}")
+  endif()
+endif()
+if(LLVM_PARALLEL_TABLEGEN_JOBS)
+  if(NOT CMAKE_GENERATOR MATCHES "Ninja")
+    message(WARNING "Job pooling is only available with Ninja generators.")
+  else()
+    set_property(GLOBAL APPEND PROPERTY JOB_POOLS tablegen_job_pool=${LLVM_PARALLEL_TABLEGEN_JOBS})
+    # Job pool for tablegen is set on the add_custom_command
+  endif()
+endif()
+
 if( LLVM_ENABLE_ASSERTIONS )
   # MSVC doesn't like _DEBUG on release builds. See PR 4379.
   if( NOT MSVC )

llvm/cmake/modules/TableGen.cmake

@@ -125,6 +125,12 @@ function(tablegen project ofn)
   set(tablegen_exe ${${project}_TABLEGEN_EXE})
   set(tablegen_depends ${${project}_TABLEGEN_TARGET} ${tablegen_exe})
 
+  if(LLVM_PARALLEL_TABLEGEN_JOBS)
+    set(LLVM_TABLEGEN_JOB_POOL JOB_POOL tablegen_job_pool)
+  else()
+    set(LLVM_TABLEGEN_JOB_POOL "")
+  endif()
+
   add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${ofn}
     COMMAND ${tablegen_exe} ${ARG_UNPARSED_ARGUMENTS} -I ${CMAKE_CURRENT_SOURCE_DIR}
     ${tblgen_includes}
@@ -139,6 +145,7 @@ function(tablegen project ofn)
       ${local_tds} ${global_tds}
     ${LLVM_TARGET_DEFINITIONS_ABSOLUTE}
     ${LLVM_TARGET_DEPENDS}
+    ${LLVM_TABLEGEN_JOB_POOL}
     COMMENT "Building ${ofn}..."
     )
 

llvm/docs/CMake.rst

@@ -762,6 +762,9 @@ enabled sub-projects. Nearly all of these variable names begin with
 **LLVM_PARALLEL_LINK_JOBS**:STRING
   Define the maximum number of concurrent link jobs.
 
+**LLVM_PARALLEL_TABLEGEN_JOBS**:STRING
+  Define the maximum number of concurrent tablegen jobs.
+
 **LLVM_RAM_PER_COMPILE_JOB**:STRING
   Calculates the amount of Ninja compile jobs according to available resources.
   Value has to be in MB, overwrites LLVM_PARALLEL_COMPILE_JOBS. Compile jobs 
@@ -775,6 +778,11 @@ enabled sub-projects. Nearly all of these variable names begin with
   to be sure its not terminated in your memory restricted environment. On ELF
   platforms also consider ``LLVM_USE_SPLIT_DWARF`` in Debug build.
 
+**LLVM_RAM_PER_TABLEGEN_JOB**:STRING
+  Calculates the amount of Ninja tablegen jobs according to available resources.
+  Value has to be in MB, overwrites LLVM_PARALLEL_TABLEGEN_JOBS. Tablegen jobs
+  will be between one and amount of logical cores.
+
 **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/docs/GettingStarted.rst

@@ -90,11 +90,11 @@ Getting the Source Code and Building LLVM
        is installed on your system. This can dramatically speed up link times
        if the default linker is slow.
 
-     * ``-DLLVM_PARALLEL_{COMPILE,LINK}_JOBS=N`` --- Limit the number of
-       compile/link jobs running in parallel at the same time. This is
+     * ``-DLLVM_PARALLEL_{COMPILE,LINK,TABLEGEN}_JOBS=N`` --- Limit the number of
+       compile/link/tablegen jobs running in parallel at the same time. This is
        especially important for linking since linking can use lots of memory. If
        you run into memory issues building LLVM, try setting this to limit the
-       maximum number of compile/link jobs running at the same time.
+       maximum number of compile/link/tablegen jobs running at the same time.
 
    * ``cmake --build build [--target <target>]`` or the build system specified
      above directly.