[CMake] Fix LTO option on Windows

[aganea]

Summary

Previously, the option LLVM_ENABLE_LLD was not adequatly taken into account when using LLVM_ENABLE_LTO on Windows.

The picture below shows the problem before the patch. CMAKE_LINKER says lld-link.exe, however when running configure, CMAKE_LINKER is back to link.exe (the message was printed in HandleLLVMOptions.cmake. This is probably caused by CMakeCXXCompiler.cmake which indeed overrides CMAKE_LINKER (I'm using CMake 3.12.0).

This causes LINKER_IS_LLD_LINK not being set, which in turns prevents using LTO.

[rnk]

Wow, three different ways to set the linker. I'm impressed with how many cooks there are in our kitchen.

[zturner]

Is this a bug in CMake? Maybe you should report it on their bug tracker.

[aganea]

In D51804#1227806, @zturner wrote:

Is this a bug in CMake? Maybe you should report it on their bug tracker.

I will, but I need to investigate the issue more in-depth. This happens when passing -T"LLVM-vs201X" to cmake.
I'll upgrade CMake and switch to using -Tllvm instead see if the issue is still there.

[aganea]

In D51804#1227806, @zturner wrote:

Is this a bug in CMake? Maybe you should report it on their bug tracker.

The issue have been fixed in CMake 3.12.2

[azharudd]

CMAKE_LINKER MATCHES "lld-link\.exe"

@aganea, can you please elaborate on the need for the backslash that you added?

[aganea]

In D51804#1257130, @azharudd wrote:

CMAKE_LINKER MATCHES "lld-link\.exe"

@aganea, can you please elaborate on the need for the backslash that you added?

It is not needed per se. But given than MATCHES takes a regular expression on the right side, I machinally replaced the . (maching any character) by \. (escaped dot) while looking for a bug. Is that an issue?

llvm/trunk/cmake/modules/HandleLLVMOptions.cmake

 # This CMake module is responsible for interpreting the user defined LLVM_
 # options and executing the appropriate CMake commands to realize the users'
 # selections.
 
 # This is commonly needed so make sure it's defined before we include anything
 # else.
 string(TOUPPER "${CMAKE_BUILD_TYPE}" uppercase_CMAKE_BUILD_TYPE)
 
 include(CheckCompilerVersion)
 include(HandleLLVMStdlib)
 include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)
 
-if(CMAKE_LINKER MATCHES "lld-link.exe" OR (WIN32 AND LLVM_USE_LINKER STREQUAL "lld"))
+if(CMAKE_LINKER MATCHES "lld-link\.exe" OR (WIN32 AND LLVM_USE_LINKER STREQUAL "lld") OR LLVM_ENABLE_LLD)
   set(LINKER_IS_LLD_LINK TRUE)
 else()
   set(LINKER_IS_LLD_LINK FALSE)
 endif()
 
 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)