[llvm] [cmake] Option to create Ninja job pools depending on available resources (PR #65274)

[via llvm-commits]

https://github.com/jusito updated https://github.com/llvm/llvm-project/pull/65274:

>From 02a8528f95de267541a1d6067144f05fd1539ff6 Mon Sep 17 00:00:00 2001
From: Lucas Briese <lucas.briese at iem.fraunhofer.de>
Date: Mon, 4 Sep 2023 17:49:42 +0200
Subject: [PATCH] [cmake] Option to create Ninja job pools depending on
 available resources.

Dynamically couple compile time to available resources in memory constrained environments.
Tweaked alternative to LLVM_PARALLEL_{COMPILE,LINK}_JOBS=1.
---
 llvm/cmake/modules/HandleLLVMOptions.cmake | 26 ++++++++++++++++++++++
 llvm/docs/CMake.rst                        | 13 +++++++++++
 2 files changed, 39 insertions(+)

diff --git a/llvm/cmake/modules/HandleLLVMOptions.cmake b/llvm/cmake/modules/HandleLLVMOptions.cmake
index 29b242d80051f0d..92b73a7bc164b90 100644
--- a/llvm/cmake/modules/HandleLLVMOptions.cmake
+++ b/llvm/cmake/modules/HandleLLVMOptions.cmake
@@ -36,6 +36,21 @@ string(TOUPPER "${LLVM_ENABLE_LTO}" uppercase_LLVM_ENABLE_LTO)
 # The following only works with the Ninja generator in CMake >= 3.0.
 set(LLVM_PARALLEL_COMPILE_JOBS "" CACHE STRING
   "Define the maximum number of concurrent compilation jobs (Ninja only).")
+if(LLVM_RAM_PER_COMPILE_JOB OR LLVM_RAM_PER_LINK_JOB)
+  cmake_host_system_information(RESULT available_physical_memory QUERY AVAILABLE_PHYSICAL_MEMORY)
+  cmake_host_system_information(RESULT number_of_logical_cores QUERY NUMBER_OF_LOGICAL_CORES)
+endif()
+if(LLVM_RAM_PER_COMPILE_JOB)
+  math(EXPR memory_available_jobs "${available_physical_memory} / ${LLVM_RAM_PER_COMPILE_JOB}" OUTPUT_FORMAT DECIMAL)
+  if (memory_available_jobs LESS 1)
+    set(memory_available_jobs 1)
+  endif()
+  if (memory_available_jobs LESS number_of_logical_cores)
+    set(LLVM_PARALLEL_COMPILE_JOBS "${memory_available_jobs}")
+  else()
+    set(LLVM_PARALLEL_COMPILE_JOBS "${number_of_logical_cores}")
+  endif()
+endif()
 if(LLVM_PARALLEL_COMPILE_JOBS)
   if(NOT CMAKE_GENERATOR MATCHES "Ninja")
     message(WARNING "Job pooling is only available with Ninja generators.")
@@ -47,6 +62,17 @@ endif()
 
 set(LLVM_PARALLEL_LINK_JOBS "" CACHE STRING
   "Define the maximum number of concurrent link jobs (Ninja only).")
+if(LLVM_RAM_PER_LINK_JOB)
+  math(EXPR memory_available_jobs "${available_physical_memory} / ${LLVM_RAM_PER_LINK_JOB}" OUTPUT_FORMAT DECIMAL)
+  if (memory_available_jobs LESS 1)
+    set(memory_available_jobs 1)
+  endif()
+  if (memory_available_jobs LESS number_of_logical_cores)
+    set(LLVM_PARALLEL_LINK_JOBS "${memory_available_jobs}")
+  else()
+    set(LLVM_PARALLEL_LINK_JOBS "${number_of_logical_cores}")
+  endif()
+endif()
 if(CMAKE_GENERATOR 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.")
diff --git a/llvm/docs/CMake.rst b/llvm/docs/CMake.rst
index 67f740447ed7bfb..6e61806a2b85c9a 100644
--- a/llvm/docs/CMake.rst
+++ b/llvm/docs/CMake.rst
@@ -736,6 +736,19 @@ 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_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 
+  will be between one and amount of logical cores.
+
+**LLVM_RAM_PER_LINK_JOB**:STRING
+  Calculates the amount of Ninja link jobs according to available resources.
+  Value has to be in MB, overwrites LLVM_PARALLEL_LINK_JOBS. Link jobs will 
+  be between one and amount of logical cores. Link jobs will not run 
+  exclusively therefore you should add an offset of one or two compile jobs 
+  to be sure its not terminated in your memory restricted environment. On ELF
+  platforms also consider ``LLVM_USE_SPLIT_DWARF`` in Debug build.
+
 **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.