[flang] [llvm] [CMake] Fix using precompiled headers with ccache (PR #131397)

[Kajetan Puchalski via llvm-commits]

https://github.com/mrkajetanp updated https://github.com/llvm/llvm-project/pull/131397

>From 2c910431c93a58c131450bc138057e3ee2d1ff07 Mon Sep 17 00:00:00 2001
From: Kajetan Puchalski <kajetan.puchalski at arm.com>
Date: Fri, 14 Mar 2025 21:35:08 +0000
Subject: [PATCH 1/2] [CMake] Fix using precompiled headers with ccache

Using precompiled headers with ccache requires special accommodations.
Add the required CCACHE option and clang compiler flag to CMake.

Signed-off-by: Kajetan Puchalski <kajetan.puchalski at arm.com>
---
 flang/CMakeLists.txt | 5 +++++
 llvm/CMakeLists.txt  | 4 ++--
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/flang/CMakeLists.txt b/flang/CMakeLists.txt
index 4b703b456cae2..3be84eeca21ba 100644
--- a/flang/CMakeLists.txt
+++ b/flang/CMakeLists.txt
@@ -446,6 +446,11 @@ if (APPLE)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -D_POSIX_C_SOURCE=200809")
 endif()
 
+# Clang requires this flag in order for precompiled headers to work with ccache.
+if (CMAKE_CXX_COMPILER_ID MATCHES Clang)
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Xclang -fno-pch-timestamp")
+endif()
+
 list(REMOVE_DUPLICATES CMAKE_CXX_FLAGS)
 
 # Determine HOST_LINK_VERSION on Darwin.
diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
index c5d3e23a47f0e..1b1def5e2537c 100644
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -277,7 +277,7 @@ if(LLVM_CCACHE_BUILD)
   if(CCACHE_PROGRAM)
     set(LLVM_CCACHE_MAXSIZE "" CACHE STRING "Size of ccache")
     set(LLVM_CCACHE_DIR "" CACHE STRING "Directory to keep ccached data")
-    set(LLVM_CCACHE_PARAMS "CCACHE_CPP2=yes CCACHE_HASHDIR=yes"
+    set(LLVM_CCACHE_PARAMS "CCACHE_CPP2=yes CCACHE_HASHDIR=yes CCACHE_SLOPPINESS=pch_defines,time_macros"
         CACHE STRING "Parameters to pass through to ccache")
 
     if(NOT CMAKE_SYSTEM_NAME MATCHES "Windows")
@@ -291,7 +291,7 @@ if(LLVM_CCACHE_BUILD)
       set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ${CCACHE_PROGRAM})
     else()
       if(LLVM_CCACHE_MAXSIZE OR LLVM_CCACHE_DIR OR
-         NOT LLVM_CCACHE_PARAMS MATCHES "CCACHE_CPP2=yes CCACHE_HASHDIR=yes")
+         NOT LLVM_CCACHE_PARAMS MATCHES "CCACHE_CPP2=yes CCACHE_HASHDIR=yes CCACHE_SLOPPINESS=pch_defines,time_macros")
         message(FATAL_ERROR "Ccache configuration through CMake is not supported on Windows. Please use environment variables.")
       endif()
       # RULE_LAUNCH_COMPILE should work with Ninja but currently has issues

>From a05645be1f224c5e93b9ae3c660df6f2c9535d4a Mon Sep 17 00:00:00 2001
From: Kajetan Puchalski <kajetan.puchalski at arm.com>
Date: Tue, 18 Mar 2025 15:01:00 +0000
Subject: [PATCH 2/2] [CMake] Configure ccache using command line options

Since ccache 4.8, it is possible to pass configuration options directly
to ccache on the command line as opposed to through environment
variables.
This is the intended way for CMake to configure ccache, as described on
the GitHub wiki for the project:
https://github.com/ccache/ccache/wiki/CMake

Rework the way ccache is configured by LLVM accordingly.

Signed-off-by: Kajetan Puchalski <kajetan.puchalski at arm.com>
---
 llvm/CMakeLists.txt | 31 +++++++++++--------------------
 1 file changed, 11 insertions(+), 20 deletions(-)

diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
index 1b1def5e2537c..dfc506b643570 100644
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -277,28 +277,19 @@ if(LLVM_CCACHE_BUILD)
   if(CCACHE_PROGRAM)
     set(LLVM_CCACHE_MAXSIZE "" CACHE STRING "Size of ccache")
     set(LLVM_CCACHE_DIR "" CACHE STRING "Directory to keep ccached data")
-    set(LLVM_CCACHE_PARAMS "CCACHE_CPP2=yes CCACHE_HASHDIR=yes CCACHE_SLOPPINESS=pch_defines,time_macros"
+    set(LLVM_CCACHE_PARAMS "run_second_cpp=true;hash_dir=true;sloppiness=pch_defines,time_macros"
         CACHE STRING "Parameters to pass through to ccache")
 
-    if(NOT CMAKE_SYSTEM_NAME MATCHES "Windows")
-      set(CCACHE_PROGRAM "${LLVM_CCACHE_PARAMS} ${CCACHE_PROGRAM}")
-      if (LLVM_CCACHE_MAXSIZE)
-        set(CCACHE_PROGRAM "CCACHE_MAXSIZE=${LLVM_CCACHE_MAXSIZE} ${CCACHE_PROGRAM}")
-      endif()
-      if (LLVM_CCACHE_DIR)
-        set(CCACHE_PROGRAM "CCACHE_DIR=${LLVM_CCACHE_DIR} ${CCACHE_PROGRAM}")
-      endif()
-      set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ${CCACHE_PROGRAM})
-    else()
-      if(LLVM_CCACHE_MAXSIZE OR LLVM_CCACHE_DIR OR
-         NOT LLVM_CCACHE_PARAMS MATCHES "CCACHE_CPP2=yes CCACHE_HASHDIR=yes CCACHE_SLOPPINESS=pch_defines,time_macros")
-        message(FATAL_ERROR "Ccache configuration through CMake is not supported on Windows. Please use environment variables.")
-      endif()
-      # RULE_LAUNCH_COMPILE should work with Ninja but currently has issues
-      # with cmd.exe and some MSVC tools other than cl.exe
-      set(CMAKE_C_COMPILER_LAUNCHER ${CCACHE_PROGRAM})
-      set(CMAKE_CXX_COMPILER_LAUNCHER ${CCACHE_PROGRAM})
-    endif()
+    set(launcher_params ${LLVM_CCACHE_PARAMS})
+    if (CCACHE_MAXSIZE)
+      set(launcher_params "max_size=${CCACHE_MAXSIZE};${launcher_params}")
+    endif ()
+    if (CCACHE_DIR)
+      set(launcher_params "cache_dir=${CCACHE_DIR};${launcher_params}")
+    endif ()
+    set(launcher "${CCACHE_PROGRAM};${launcher_params}")
+    set(CMAKE_C_COMPILER_LAUNCHER ${launcher})
+    set(CMAKE_CXX_COMPILER_LAUNCHER ${launcher})
   else()
     message(FATAL_ERROR "Unable to find the program ccache. Set LLVM_CCACHE_BUILD to OFF")
   endif()