[PATCH] D30240: enable building with LTO on Windows using clang-cl and lld

[Bob Haarman via Phabricator via llvm-commits]

inglorion created this revision.
Herald added subscribers: mehdi_amini, mgorny.

With clang-cl gaining support for link-time optimization, we can now enable builds using LTO when using clang-cl and lld on Windows. To do this, we must not pass the -flto flag to the linker; lld-link does not understand it, but will perform LTO automatically when it encounters bitcode files. We also don't pass /Brepro when using LTO - the compiler doesn't generate object files for LTO, so passing the flag would only result in a warning about it being unused.


https://reviews.llvm.org/D30240

Files:
  cmake/modules/HandleLLVMOptions.cmake


Index: cmake/modules/HandleLLVMOptions.cmake
===================================================================
--- cmake/modules/HandleLLVMOptions.cmake
+++ cmake/modules/HandleLLVMOptions.cmake
@@ -407,11 +407,13 @@
   # "Enforce type conversion rules".
   append("/Zc:rvalueCast" CMAKE_CXX_FLAGS)
 
-  if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+  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
     # has the flag /Brepro to force deterministic binaries. We want to pass that
-    # whenever you're building with clang unless you're passing /incremental.
+    # whenever you're building with clang unless you're passing /incremental
+    # or using LTO (/Brepro with LTO would result in a warning about the flag
+    # being unused, because we're not generating object files).
     # This checks CMAKE_CXX_COMPILER_ID in addition to check_cxx_compiler_flag()
     # because cl.exe does not emit an error on flags it doesn't understand,
     # letting check_cxx_compiler_flag() claim it understands all flags.
@@ -700,19 +702,25 @@
 set(LLVM_ENABLE_LTO OFF CACHE STRING "Build LLVM with LTO. May be specified as Thin or Full to use a particular kind of LTO")
 string(TOUPPER "${LLVM_ENABLE_LTO}" uppercase_LLVM_ENABLE_LTO)
 if(uppercase_LLVM_ENABLE_LTO STREQUAL "THIN")
-  append("-flto=thin" CMAKE_CXX_FLAGS CMAKE_C_FLAGS
-                      CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
+  append("-flto=thin" CMAKE_CXX_FLAGS CMAKE_C_FLAGS)
+  if(NOT CMAKE_LINKER MATCHES "link.exe$")
+    append("-flto=thin" CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
+  endif()
   # On darwin, enable the lto cache. This improves initial build time a little
   # since we re-link a lot of the same objects, and significantly improves
   # incremental build time.
   append_if(APPLE "-Wl,-cache_path_lto,${PROJECT_BINARY_DIR}/lto.cache"
             CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
 elseif(uppercase_LLVM_ENABLE_LTO STREQUAL "FULL")
-  append("-flto=full" CMAKE_CXX_FLAGS CMAKE_C_FLAGS
-                 CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
+  append("-flto=full" CMAKE_CXX_FLAGS CMAKE_C_FLAGS)
+  if(NOT CMAKE_LINKER MATCHES "link.exe$")
+    append("-flto=full" CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
+  endif()
 elseif(LLVM_ENABLE_LTO)
-  append("-flto" CMAKE_CXX_FLAGS CMAKE_C_FLAGS
-                 CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
+  append("-flto" CMAKE_CXX_FLAGS CMAKE_C_FLAGS)
+  if(NOT CMAKE_LINKER MATCHES "link.exe$")
+    append("-flto" CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
+  endif()
 endif()
 
 # This option makes utils/extract_symbols.py be used to determine the list of


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