Ninja: Add a separate job pool for PCH creation

Add a `JOB_POOL_PRECOMPILE_HEADER` target property to specify the pool
name, and its associated `CMAKE_JOB_POOL_PRECOMPILE_HEADER` variable.

Fixes: #20217

Auxiliary/vim/syntax/cmake.vim

@@ -221,6 +221,7 @@ syn keyword cmakeProperty contained
             \ JOB_POOLS
             \ JOB_POOL_COMPILE
             \ JOB_POOL_LINK
+            \ JOB_POOL_PRECOMPILE_HEADER
             \ KEEP_EXTENSION
             \ LABELS
             \ LANGUAGE
@@ -1066,6 +1067,7 @@ syn keyword cmakeVariable contained
             \ CMAKE_JOB_POOLS
             \ CMAKE_JOB_POOL_COMPILE
             \ CMAKE_JOB_POOL_LINK
+            \ CMAKE_JOB_POOL_PRECOMPILE_HEADER
             \ CMAKE_Java
             \ CMAKE_Java_ANDROID_TOOLCHAIN_MACHINE
             \ CMAKE_Java_ANDROID_TOOLCHAIN_PREFIX

Help/manual/cmake-properties.7.rst

@@ -253,6 +253,7 @@ Properties on Targets
    /prop_tgt/IOS_INSTALL_COMBINED
    /prop_tgt/JOB_POOL_COMPILE
    /prop_tgt/JOB_POOL_LINK
+   /prop_tgt/JOB_POOL_PRECOMPILE_HEADER
    /prop_tgt/LABELS
    /prop_tgt/LANG_CLANG_TIDY
    /prop_tgt/LANG_COMPILER_LAUNCHER

Help/manual/cmake-variables.7.rst

@@ -65,6 +65,7 @@ Variables that Provide Information
    /variable/CMAKE_IMPORT_LIBRARY_SUFFIX
    /variable/CMAKE_JOB_POOL_COMPILE
    /variable/CMAKE_JOB_POOL_LINK
+   /variable/CMAKE_JOB_POOL_PRECOMPILE_HEADER
    /variable/CMAKE_JOB_POOLS
    /variable/CMAKE_LANG_COMPILER_AR
    /variable/CMAKE_LANG_COMPILER_RANLIB

Help/prop_tgt/JOB_POOL_PRECOMPILE_HEADER.rst

@@ -0,0 +1,21 @@
+JOB_POOL_PRECOMPILE_HEADER
+--------------------------
+
+Ninja only: Pool used for generating pre-compiled headers.
+
+The number of parallel compile processes could be limited by defining
+pools with the global :prop_gbl:`JOB_POOLS`
+property and then specifying here the pool name.
+
+For instance:
+
+.. code-block:: cmake
+
+  set_property(TARGET myexe PROPERTY JOB_POOL_PRECOMPILE_HEADER two_jobs)
+
+This property is initialized by the value of
+:variable:`CMAKE_JOB_POOL_PRECOMPILE_HEADER`.
+
+If neither :prop_tgt:`JOB_POOL_PRECOMPILE_HEADER` nor
+:variable:`CMAKE_JOB_POOL_PRECOMPILE_HEADER` are set then
+:prop_tgt:`JOB_POOL_COMPILE` will be used for this task.

Help/variable/CMAKE_JOB_POOL_PRECOMPILE_HEADER.rst

@@ -0,0 +1,6 @@
+CMAKE_JOB_POOL_PRECOMPILE_HEADER
+--------------------------------
+
+This variable is used to initialize the :prop_tgt:`JOB_POOL_PRECOMPILE_HEADER`
+property on all the targets. See :prop_tgt:`JOB_POOL_PRECOMPILE_HEADER`
+for additional information.

Modules/Platform/Windows-PGI.cmake

@@ -9,9 +9,10 @@ endif()
 set(__WINDOWS_COMPILER_PGI 1)
 
 # PGI on Windows doesn't support parallel compile processes
-if(NOT DEFINED CMAKE_JOB_POOL_LINK OR NOT DEFINED CMAKE_JOB_POOL_COMPILE)
+if(NOT DEFINED CMAKE_JOB_POOL_LINK OR NOT DEFINED CMAKE_JOB_POOL_COMPILE OR NOT DEFINED CMAKE_JOB_POOL_PRECOMPILE_HEADER)
   set(CMAKE_JOB_POOL_LINK PGITaskPool)
   set(CMAKE_JOB_POOL_COMPILE PGITaskPool)
+  set(CMAKE_JOB_POOL_PRECOMPILE_HEADER PGITaskPool)
   get_property(_pgijp GLOBAL PROPERTY JOB_POOLS)
   if(NOT _pgijp MATCHES "PGITaskPool=")
       set_property(GLOBAL APPEND PROPERTY JOB_POOLS PGITaskPool=1)

Source/cmNinjaTargetGenerator.cxx

@@ -1196,6 +1196,13 @@ void cmNinjaTargetGenerator::WriteObjectBuildStatement(
   this->addPoolNinjaVariable("JOB_POOL_COMPILE", this->GetGeneratorTarget(),
                              vars);
 
+  if (!pchSource.empty() && !source->GetProperty("SKIP_PRECOMPILE_HEADERS")) {
+    if (source->GetFullPath() == pchSource) {
+      this->addPoolNinjaVariable("JOB_POOL_PRECOMPILE_HEADER",
+                                 this->GetGeneratorTarget(), vars);
+    }
+  }
+
   this->SetMsvcTargetPdbVariable(vars, config);
 
   objBuild.RspFile = objectFileName + ".rsp";

Source/cmTarget.cxx

@@ -513,6 +513,7 @@ cmTarget::cmTarget(std::string const& name, cmStateEnums::TargetType type,
       this->GetType() != cmStateEnums::UTILITY) {
     initProp("JOB_POOL_COMPILE");
     initProp("JOB_POOL_LINK");
+    initProp("JOB_POOL_PRECOMPILE_HEADER");
   }
 
   if (impl->TargetType <= cmStateEnums::UTILITY) {