From daedb463ee2388697089973affce48cd29ea4bb4 Mon Sep 17 00:00:00 2001
From: Alexandre Ganea <alexandre.ganea@ubisoft.com>
Date: Tue, 18 Dec 2018 18:17:00 +0000
Subject: [PATCH] [CMake] Default options for faster executables on MSVC

- Disable incremental linking by default. /INCREMENTAL adds extra thunks in the EXE, which makes execution slower.
- Set /MT (static CRT lib) by default instead of CMake's default /MD (dll CRT lib). The previous default /MD makes all DLL functions to be thunked, thus making execution slower (memcmp, memset, etc.)
- Adds LLVM_ENABLE_INCREMENTAL_LINK which is set to OFF by default.

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


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@349517 91177308-0d34-0410-b5e6-96231b3b80d8
---
 CMakeLists.txt                        | 4 ++++
 cmake/modules/ChooseMSVCCRT.cmake     | 9 +++++++++
 cmake/modules/HandleLLVMOptions.cmake | 8 ++++++++
 3 files changed, 21 insertions(+)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index e2e35dcebdb..15f95d2e580 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -370,6 +370,10 @@ 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..cdd7deec4c8 100644
--- a/cmake/modules/ChooseMSVCCRT.cmake
+++ b/cmake/modules/ChooseMSVCCRT.cmake
@@ -66,6 +66,15 @@ 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()
+
       set(LLVM_USE_CRT_${build}
         "${LLVM_USE_CRT_${build}}"
         CACHE STRING "Specify VC++ CRT to use for ${build_type} configurations."
diff --git a/cmake/modules/HandleLLVMOptions.cmake b/cmake/modules/HandleLLVMOptions.cmake
index 49db3088bbc..e8b0e27e000 100644
--- a/cmake/modules/HandleLLVMOptions.cmake
+++ b/cmake/modules/HandleLLVMOptions.cmake
@@ -381,6 +381,14 @@ 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
-- 
2.50.1