[llvm] [cmake] Pass arguments to ccache on Windows (PR #218791)

[via llvm-commits]

https://github.com/R-Goc created https://github.com/llvm/llvm-project/pull/218791

Since ccache 4.8 it is possible to pass options as arguments. As such now PCH files can be enabled along with ccache on windows when using clang as the compiler. The original behavior was left on other systems to preserve compatibility with older ccache versions.

>From a15de4fd71084ae299face444d9dac264b930398 Mon Sep 17 00:00:00 2001
From: R-Goc <ryszardgoc at gmail.com>
Date: Tue, 25 Aug 2026 23:46:36 +0200
Subject: [PATCH] [cmake] Pass arguments to ccache on Windows

Since ccache 4.8 it is possible to pass options as arguments. As such
now PCH files can be enabled along with ccache on windows when using
clang as the compiler. The original behavior was left on other systems
to preserve compatibility with older ccache versions.
---
 llvm/CMakeLists.txt | 58 ++++++++++++++++++++-------------------------
 1 file changed, 26 insertions(+), 32 deletions(-)

diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
index 2dbc122bb79f3..20f111187ca95 100644
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -335,48 +335,42 @@ if(LLVM_CCACHE_BUILD)
     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_HOST_WIN32)
-      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 ${CCACHE_PROGRAM})
-
-      if(NOT CMAKE_CXX_COMPILER_ID MATCHES "Clang")
-        # ccache with PCH can lead to false-positives when only a macro
-        # definition changes with non-Clang compilers, because macro definitions
-        # are not compared in preprocessed mode.
-        # See: https://github.com/ccache/ccache/issues/1668
-        if(NOT DEFINED CMAKE_DISABLE_PRECOMPILE_HEADERS)
-          message(NOTICE "Using ccache with precompiled headers with non-Clang "
+    if (LLVM_CCACHE_MAXSIZE)
+      set(LLVM_CCACHE_PARAMS "CCACHE_MAXSIZE=${LLVM_CCACHE_MAXSIZE} ${LLVM_CCACHE_PARAMS}")
+    endif()
+    if (LLVM_CCACHE_DIR)
+      set(LLVM_CCACHE_PARAMS "CCACHE_DIR=${LLVM_CCACHE_DIR} ${LLVM_CCACHE_PARAMS}")
+    endif()
+
+    if(NOT CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+      # ccache with PCH can lead to false-positives when only a macro
+      # definition changes with non-Clang compilers, because macro definitions
+      # are not compared in preprocessed mode.
+      # See: https://github.com/ccache/ccache/issues/1668
+      if(NOT DEFINED CMAKE_DISABLE_PRECOMPILE_HEADERS)
+        message(NOTICE "Using ccache with precompiled headers with non-Clang "
             "compilers is not supported. CMAKE_DISABLE_PRECOMPILE_HEADERS will be set to ON. "
             "Pass -DCMAKE_DISABLE_PRECOMPILE_HEADERS=OFF to override this.")
-          set(CMAKE_DISABLE_PRECOMPILE_HEADERS "ON")
-        elseif(NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
-          message(WARNING "Using ccache with precompiled headers with non-Clang "
+        set(CMAKE_DISABLE_PRECOMPILE_HEADERS "ON")
+      elseif(NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
+        message(WARNING "Using ccache with precompiled headers with non-Clang "
             "compilers is not supported.")
-        endif()
       endif()
+    endif()
+
+    if (NOT CMAKE_HOST_WIN32)
+      # On platforms other than Windows pass the arguments as an env var to support older ccache versions
+      set(CCACHE_PROGRAM "${LLVM_CCACHE_PARAMS} ${CCACHE_PROGRAM}")
     else()
-      # Until a way to reliably configure ccache on Windows is found,
-      # disable precompiled headers for Windows + ccache builds
-      if(NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
-        message(WARNING "Using ccache with precompiled headers on Windows is currently not supported.
-          CMAKE_DISABLE_PRECOMPILE_HEADERS will be set to ON.")
-        set(CMAKE_DISABLE_PRECOMPILE_HEADERS "ON")
-      endif()
-      if(LLVM_CCACHE_MAXSIZE OR LLVM_CCACHE_DIR OR
-         NOT LLVM_CCACHE_PARAMS MATCHES "CCACHE_CPP2=yes CCACHE_HASHDIR=yes CCACHE_SLOPPINESS=pch_defines,time_macros")
-        message(FATAL_ERROR "Ccache configuration through CMake is not supported on Windows. Please use environment variables.")
-      endif()
+      # Since ccache 4.8 it is possible to pass options as arguments
+      set(CCACHE_PROGRAM "${CCACHE_PROGRAM} ${LLVM_CCACHE_PARAMS}")
       # 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_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ${CCACHE_PROGRAM})
   else()
     message(FATAL_ERROR "Unable to find the program ccache. Set LLVM_CCACHE_BUILD to OFF")
   endif()