[PATCH] D80425: Fix LLVM/Clang builds with mingw toolchain

[Martin Storsjö via Phabricator via cfe-commits]

mstorsjo added inline comments.


================
Comment at: llvm/cmake/modules/HandleLLVMOptions.cmake:967
            CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
-  elseif(LINKER_IS_LLD_LINK)
+  elseif(LINKER_IS_LLD_LINK AND NOT MINGW)
     append("/lldltocache:${PROJECT_BINARY_DIR}/lto.cache"
----------------
mati865 wrote:
> mstorsjo wrote:
> > mati865 wrote:
> > > thieta wrote:
> > > > mstorsjo wrote:
> > > > > Do you happen to know why `LINKER_IS_LLD_LINK` gets set in this case? `ld.lld` (the ELF linker interface, which then the MinGW driver remaps onto the COFF backend with the `lld-link` interface) certainly doesn't take `lld-link` style options. I believe (without diving further into it) that we shouldn't be setting this flag in this combination, but with the option implemented, we should fit it into the case further above, `elseif((UNIX OR MINGW) AND LLVM_USE_LINKER STREQUAL "lld")`
> > > > Yeah I bet that variable is set because I pass `LLVM_USE_LINKER=lld` but I haven't digged to deeply. I can rework the if statement here when we have the lld option in there.
> > > > Yeah I bet that variable is set because I pass LLVM_USE_LINKER=lld but I haven't digged to deeply. 
> > > 
> > > It does use `lld-link` when you use `LLVM_USE_LINKER=lld`.
> > > 
> > > The problem lies in this line:
> > > ```
> > > append("/lldltocache:${PROJECT_BINARY_DIR}/lto.cache"
> > > ```
> > > For MinGW that should read:
> > > ```
> > > append("-Wl,/lldltocache:${PROJECT_BINARY_DIR}/lto.cache"
> > > ```
> > > That's because you are passing this flag to GCC/Clang.
> > > For MinGW that should read:
> > > 
> > >     append("-Wl,/lldltocache:${PROJECT_BINARY_DIR}/lto.cache"
> > 
> > We're adding this option properly in the mingw frontend, so we shouldn't do the hacky way of passing lld-link options via the mingw frontend by passing them as `/option`.
> > 
> > And the reason `LINKER_IS_LLD_LINK` is set seems to be this:
> > 
> > ```
> > if(CMAKE_LINKER MATCHES "lld-link" OR (WIN32 AND LLVM_USE_LINKER STREQUAL "lld") OR LLVM_ENABLE_LLD)
> > ```
> > 
> > Perhaps that should be changed into
> > 
> > ```
> > if(CMAKE_LINKER MATCHES "lld-link" OR (WIN32 AND LLVM_USE_LINKER STREQUAL "lld" AND NOT MINGW) OR LLVM_ENABLE_LLD)
> > ```
> > 
> > We're adding this option properly in the mingw frontend, so we shouldn't do the hacky way of passing lld-link options via the mingw frontend by passing them as /option.
> 
> I was about to add that and one more LTO related option but thought "what if LLVM should link with lld-link on MinGW?" and eventually forgot about it.
> 
> 
> 
> > And the reason LINKER_IS_LLD_LINK is set seems to be this:
> > ```
> > if(CMAKE_LINKER MATCHES "lld-link" OR (WIN32 AND LLVM_USE_LINKER STREQUAL "lld") OR LLVM_ENABLE_LLD)
> > ```
> > Perhaps that should be changed into
> > ```
> > if(CMAKE_LINKER MATCHES "lld-link" OR (WIN32 AND LLVM_USE_LINKER STREQUAL "lld" AND NOT MINGW) OR LLVM_ENABLE_LLD)
> > ```
> 
> It won't work if somebody uses `LLVM_ENABLE_LLD=ON` instead of `LLVM_USE_LINKER=lld` which is supposed to be equivalent.
> That raises question how `LLVM_ENABLE_LLD=ON` does even work on UNIX platforms.
> 
> IMO this would be better:
> ```
> if(CMAKE_LINKER MATCHES "lld-link" OR MSVC AND (LLVM_USE_LINKER STREQUAL "lld" OR LLVM_ENABLE_LLD))
> ```
> I was about to add that and one more LTO related option but thought "what if LLVM should link with lld-link on MinGW?" and eventually forgot about it.

I don't think that really is a supposedly supported setup - lld-link wouldn't find any libs to link against etc, as the compiler driver is supposed to pass those as `-L` options in mingw/unix style tools.

> It won't work if somebody uses LLVM_ENABLE_LLD=ON instead of LLVM_USE_LINKER=lld which is supposed to be equivalent.
> That raises question how LLVM_ENABLE_LLD=ON does even work on UNIX platforms.

Either it doesn't work and nobody actually use it that way, or the effects of `LINKER_IS_LLD_LINK` are mostly harmless.

> IMO this would be better:
> 
> `if(CMAKE_LINKER MATCHES "lld-link" OR MSVC AND (LLVM_USE_LINKER STREQUAL "lld" OR LLVM_ENABLE_LLD))`

Yeah, that looks sensible.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D80425/new/

https://reviews.llvm.org/D80425