[PATCH] D55056: [CMake] Default options for faster executables on MSVC

[Alexandre Ganea via Phabricator via llvm-commits]

aganea created this revision.
aganea added reviewers: beanz, rnk.
Herald added subscribers: llvm-commits, mgorny.

This patch:

- Disables incremental linking by default. /INCREMENTAL <https://docs.microsoft.com/en-us/cpp/build/reference/incremental-link-incrementally?view=vs-2017> adds extra thunks in the EXE, which makes execution slower.
- Sets /MT (static CRT lib) by default instead of CMake's default /MD (dll CRT lib). The previous default /MD <https://docs.microsoft.com/en-us/cpp/build/reference/md-mt-ld-use-run-time-library?view=vs-2017> 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.

DLL thunks:
F7623159: dll_thunk.png <https://reviews.llvm.org/F7623159>

/INCREMENTAL thunks:
F7623162: incremental_thunk.png <https://reviews.llvm.org/F7623162>

I thought it'd be better if Clang / LLVM / LLD would be fast out-of-the-box, instead of people having to dig for "good" settings.

Below are timings for linking a large DLL with LLD. The type merging pass is dominant, and is affected by those two options, because of the tight merge loop. I currently have several optimisations in that loop that tend to increase the gap.

Without this patch (default cmake settings):

  Type Merging:           23174 ms ( 56.9%)

With this patch (new default cmake settings):

  Type Merging:           22338 ms ( 52.2%)

This is a MSVC-only change.


Repository:
  rL LLVM

https://reviews.llvm.org/D55056

Files:
  llvm/trunk/CMakeLists.txt
  llvm/trunk/cmake/modules/ChooseMSVCCRT.cmake
  llvm/trunk/cmake/modules/HandleLLVMOptions.cmake


Index: llvm/trunk/cmake/modules/HandleLLVMOptions.cmake
===================================================================
--- llvm/trunk/cmake/modules/HandleLLVMOptions.cmake
+++ llvm/trunk/cmake/modules/HandleLLVMOptions.cmake
@@ -381,6 +381,14 @@
   # "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_${CONFIG}}")
+	  endforeach()
+	endforeach()
+  endif()
+
   if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT LLVM_ENABLE_LTO)
     # clang-cl and cl by default produce non-deterministic binaries because
     # link.exe /incremental requires a timestamp in the .obj file.  clang-cl
Index: llvm/trunk/cmake/modules/ChooseMSVCCRT.cmake
===================================================================
--- llvm/trunk/cmake/modules/ChooseMSVCCRT.cmake
+++ llvm/trunk/cmake/modules/ChooseMSVCCRT.cmake
@@ -66,6 +66,15 @@
       get_current_crt(LLVM_USE_CRT_${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."
Index: llvm/trunk/CMakeLists.txt
===================================================================
--- llvm/trunk/CMakeLists.txt
+++ llvm/trunk/CMakeLists.txt
@@ -365,6 +365,10 @@
 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" )


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