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

This PR adds options to let CMake calculate the ninja job pools
depending on free memory and available cores.
You can provide memory requirements for each compile and link job which
is checked against CMake AVAILABLE_PHYSICAL_MEMORY and
NUMBER_OF_LOGICAL_CORES. [This information are available since CMake
3.0](https://cmake.org/cmake/help/v3.0/command/cmake_host_system_information.html).

This is very helpful in CI environments with multiple jobs per
environment or a VM with multiple users.
Its different to LLVM_PARALLEL_LINK_JOBS / LLVM_PARALLEL_COMPILE_JOBS
(or ninja -j 1) because it tries to use the resources more efficient
without being terminated. Only downside currently is that compile and
link jobs can run at the same time so there is an offset for link job
memory suggested which is added to the documentation.

The definitions aren't added as cache because if I understand it
correctly this would break it because values could be outdated.

Author: jusito

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -36,6 +36,21 @@ string(TOUPPER "${LLVM_ENABLE_LTO}" uppercase_LLVM_ENABLE_LTO)
 # The following only works with the Ninja generator in CMake >= 3.0.
 set(LLVM_PARALLEL_COMPILE_JOBS "" CACHE STRING
   "Define the maximum number of concurrent compilation jobs (Ninja only).")
+if(LLVM_RAM_PER_COMPILE_JOB OR LLVM_RAM_PER_LINK_JOB)
+  cmake_host_system_information(RESULT available_physical_memory QUERY AVAILABLE_PHYSICAL_MEMORY)
+  cmake_host_system_information(RESULT number_of_logical_cores QUERY NUMBER_OF_LOGICAL_CORES)
+endif()
+if(LLVM_RAM_PER_COMPILE_JOB)
+  math(EXPR jobs_with_sufficient_memory "${available_physical_memory} / ${LLVM_RAM_PER_COMPILE_JOB}" OUTPUT_FORMAT DECIMAL)
+  if (jobs_with_sufficient_memory LESS 1)
+    set(jobs_with_sufficient_memory 1)
+  endif()
+  if (jobs_with_sufficient_memory LESS number_of_logical_cores)
+    set(LLVM_PARALLEL_COMPILE_JOBS "${jobs_with_sufficient_memory}")
+  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 jobs_with_sufficient_memory "${available_physical_memory} / ${LLVM_RAM_PER_LINK_JOB}" OUTPUT_FORMAT DECIMAL)
+  if (jobs_with_sufficient_memory LESS 1)
+    set(jobs_with_sufficient_memory 1)
+  endif()
+  if (jobs_with_sufficient_memory LESS number_of_logical_cores)
+    set(LLVM_PARALLEL_LINK_JOBS "${jobs_with_sufficient_memory}")
+  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.")

llvm/docs/CMake.rst

@@ -755,6 +755,19 @@ 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.