[CMake][LIT] Add option to run lit testsuites in parallel (#82899)

Currently `add_lit_target` sets the `USES_TERMINAL` CMake option. When
using Ninja, this forces all lit testsuite targets into the
single-threaded `console` pool.

This PR adds a new option `LLVM_PARALLEL_LIT` which drops the
`USES_TERMINAL` flag, allowing Ninja to run them in parallel.

The default setting (`LLVM_PARALLEL_LIT=OFF`) retains the existing
behavior of serial testsuite execution.

llvm/CMakeLists.txt

@@ -718,6 +718,8 @@ if(LLVM_INDIVIDUAL_TEST_COVERAGE)
 endif()
 set(LLVM_LIT_ARGS "${LIT_ARGS_DEFAULT}" CACHE STRING "Default options for lit")
 
+option(LLVM_PARALLEL_LIT "Enable multiple lit suites to run in parallel" OFF)
+
 # On Win32 hosts, provide an option to specify the path to the GnuWin32 tools.
 if( WIN32 AND NOT CYGWIN )
   set(LLVM_LIT_TOOLS_DIR "" CACHE PATH "Path to GnuWin32 tools")

llvm/cmake/modules/AddLLVM.cmake

@@ -1947,11 +1947,18 @@ function(add_lit_target target comment)
     list(APPEND LIT_COMMAND --param ${param})
   endforeach()
   if (ARG_UNPARSED_ARGUMENTS)
-    add_custom_target(${target}
-      COMMAND ${LIT_COMMAND} ${ARG_UNPARSED_ARGUMENTS}
-      COMMENT "${comment}"
-      USES_TERMINAL
-      )
+    if (LLVM_PARALLEL_LIT)
+     add_custom_target(${target}
+       COMMAND ${LIT_COMMAND} ${ARG_UNPARSED_ARGUMENTS}
+       COMMENT "${comment}"
+       )
+    else()
+     add_custom_target(${target}
+       COMMAND ${LIT_COMMAND} ${ARG_UNPARSED_ARGUMENTS}
+       COMMENT "${comment}"
+       USES_TERMINAL
+       )
+    endif()
   else()
     add_custom_target(${target}
       COMMAND ${CMAKE_COMMAND} -E echo "${target} does nothing, no tools built.")

llvm/docs/CMake.rst

@@ -762,6 +762,12 @@ 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_LIT**:BOOL
+  Defaults to ``OFF``. If set to ``OFF``, lit testsuites will be configured
+  with CMake's ``USES_TERMINAL`` flag to give direct access to the terminal. If
+  set to ``ON``, that flag will be removed allowing Ninja to schedule multiple
+  lit testsuites in parallel.
+
 **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