From 220d923e65f47737f4f6718705d44c1bb3153d3d Mon Sep 17 00:00:00 2001
From: "alexey.lysiuk" <alexey.lysiuk@gmail.com>
Date: Thu, 19 Apr 2018 10:16:04 +0300
Subject: [PATCH] Do not write g_pch.cpp if it's already up-to-date

This eliminates rebuilding of main executable by MSVC on every project generation by CMake
---
 cmake/precompiled_headers.cmake | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/cmake/precompiled_headers.cmake b/cmake/precompiled_headers.cmake
index 0130c1533..ed1f62521 100644
--- a/cmake/precompiled_headers.cmake
+++ b/cmake/precompiled_headers.cmake
@@ -14,8 +14,19 @@ function(enable_precompiled_headers PRECOMPILED_HEADER SOURCE_VARIABLE_NAME)
     get_filename_component(pch_basename ${PRECOMPILED_HEADER} NAME_WE)
     set(pch_abs ${CMAKE_CURRENT_SOURCE_DIR}/${PRECOMPILED_HEADER})
     set(pch_unity ${CMAKE_CURRENT_BINARY_DIR}/${pch_basename}.cpp)
-    FILE(WRITE ${pch_unity} "// Precompiled header unity generated by CMake\n")
-    FILE(APPEND ${pch_unity} "#include <${pch_abs}>\n")
+    set(pch_content "// Precompiled header unity generated by CMake\n#include <${pch_abs}>\n")
+    # Read .cpp if exists
+    if(EXISTS ${pch_unity})
+        file(READ ${pch_unity} pch_content_prev)
+    endif()
+    # Compare existing .cpp content with the actual one
+    if (pch_content_prev AND pch_content STREQUAL pch_content_prev)
+        unset(pch_content)
+    endif()
+    # Write .cpp if it's out-of-date
+    if (pch_content)
+        FILE(WRITE ${pch_unity} "${pch_content}")
+    endif()
     set_source_files_properties(${pch_unity}  PROPERTIES COMPILE_FLAGS "/Yc\"${pch_abs}\"")
     
     # Update properties of source files to use the precompiled header.