From 023ca6322c74430a2605d34ab325409caf43ab44 Mon Sep 17 00:00:00 2001
From: Alexandre Ganea <alexandre.ganea@ubisoft.com>
Date: Tue, 28 May 2019 14:14:48 +0000
Subject: [PATCH] [CMake] Default options for faster executables on MSVC

Differential Revision: https://reviews.llvm.org/D55056

llvm-svn: 361826
---
 CMakeLists.txt                        |  4 ++++
 cmake/modules/ChooseMSVCCRT.cmake     | 26 +++++++++++++++++++++++++-
 cmake/modules/HandleLLVMOptions.cmake |  8 ++++++++
 3 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 895f9ab7189..5862644dff1 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -435,6 +435,10 @@ option(LLVM_ENABLE_LLD "Use lld as C and C++ linker." OFF)
 option(LLVM_ENABLE_PEDANTIC "Compile with pedantic enabled." ON)
 option(LLVM_ENABLE_WERROR "Fail and stop if a warning is triggered." OFF)
 
+if (MSVC)
+   option(LLVM_ENABLE_INCREMENTAL_LINK "Link incrementally. Enabling it might produce slower executables." OFF)
+endif()
+
 option(LLVM_ENABLE_DUMP "Enable dump functions even when assertions are disabled" OFF)
 
 if( NOT uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG" )
diff --git a/cmake/modules/ChooseMSVCCRT.cmake b/cmake/modules/ChooseMSVCCRT.cmake
index 0e6e1aa5525..50bddf83ef5 100644
--- a/cmake/modules/ChooseMSVCCRT.cmake
+++ b/cmake/modules/ChooseMSVCCRT.cmake
@@ -50,6 +50,17 @@ macro(set_flag_in_var flagsvar regex flag)
   set(${flagsvar} "${${flagsvar}}" CACHE STRING "${flagsvar_docs}" FORCE)
 endmacro(set_flag_in_var)
 
+macro(disable_MT_if_LLDB build message)
+  if (LLVM_TOOL_LLDB_BUILD)
+    if ((NOT ${build} STREQUAL "DEBUG") AND (LLVM_USE_CRT_${build} STREQUAL "MT"))
+      if (LLVM_TOOL_CLANG_BUILD OR LLVM_TOOL_LLD_BUILD)
+        set(performance " This might impact runtime performance for Clang or LLD. Preferably build them separately.")
+      endif()
+      message(WARNING "${message}.${performance}")
+      set(LLVM_USE_CRT_${build} "MD")
+    endif()
+  endif()
+endmacro(disable_MT_if_LLDB)
 
 macro(choose_msvc_crt MSVC_CRT)
   if(LLVM_USE_CRT)
@@ -66,13 +77,26 @@ variables (LLVM_USE_CRT_DEBUG, etc) instead.")
       get_current_crt(LLVM_USE_CRT_${build}
         MSVC_CRT_REGEX
         CMAKE_CXX_FLAGS_${build})
+
+      # Make /MT the default in Release builds to make them faster
+      # and avoid the DLL function thunking.
+      if ((${build} STREQUAL "MINSIZEREL") OR
+          (${build} STREQUAL "RELEASE") OR
+          (${build} STREQUAL "RELWITHDEBINFO"))
+          set(LLVM_USE_CRT_${build} "MT")
+      endif()
+
+      disable_MT_if_LLDB(${build} "Using /MD as required by LLDB")
+
       set(LLVM_USE_CRT_${build}
         "${LLVM_USE_CRT_${build}}"
         CACHE STRING "Specify VC++ CRT to use for ${build_type} configurations."
         FORCE)
       set_property(CACHE LLVM_USE_CRT_${build}
         PROPERTY STRINGS ;${${MSVC_CRT}})
-    endif(NOT LLVM_USE_CRT_${build})
+    else()
+      disable_MT_if_LLDB(${build} "Disabling /MT as required by LLDB")
+    endif()
   endforeach(build_type)
 
   foreach(build_type ${CMAKE_CONFIGURATION_TYPES} ${CMAKE_BUILD_TYPE})
diff --git a/cmake/modules/HandleLLVMOptions.cmake b/cmake/modules/HandleLLVMOptions.cmake
index cb9a01e1d39..3154c1487fa 100644
--- a/cmake/modules/HandleLLVMOptions.cmake
+++ b/cmake/modules/HandleLLVMOptions.cmake
@@ -418,6 +418,14 @@ if( MSVC )
   # "Enforce type conversion rules".
   append("/Zc:rvalueCast" CMAKE_CXX_FLAGS)
 
+  if (CMAKE_CXX_COMPILER_ID MATCHES "MSVC" AND NOT LLVM_ENABLE_INCREMENTAL_LINK)
+    foreach(CONFIG RELEASE RELWITHDEBINFO MINSIZEREL)
+      foreach(FLAG EXE MODULE SHARED STATIC)
+        string(REGEX REPLACE "[-/](INCREMENTAL:YES|INCREMENTAL:NO|INCREMENTAL)" "/INCREMENTAL:NO" CMAKE_${FLAG}_LINKER_FLAGS_${CONFIG} "${CMAKE_${FLAG}_LINKER_FLAGS_${CONFIG}}")
+      endforeach()
+    endforeach()
+  endif()
+
   if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT LLVM_ENABLE_LTO)
     # clang-cl and cl by default produce non-deterministic binaries because
     # link.exe /incremental requires a timestamp in the .obj file.  clang-cl