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

[Kajetan Puchalski via llvm-commits]

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

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

>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] [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