[PATCH] Add cmake flags to disable LLD and LLDB from build.

[Bob Campbell]

> On Jan 12, 2015, at 11:49 AM, Rick Foos <rfoos at codeaurora.org> wrote:
> 
> Hi Bob,
> I was adding these following the WITH_POLLY model and hoping the external projects macros would take care of everything.

Looking around a bit WITH_POLLY, I think it is more involved that what LLD or LLDB needs,
(but I don't claim to be an expert). WITH_POLLY effects more than adding the external project, it also modifies
places where libraries are included. I don't claim to fully understand the polly build.

I am thinking that for lld, lldb, and compiler-rt nothing needed
to be guarded beyond the call to add_llvm_external_project().

Since add_llvm_external_project() already has a guarding method we should
just using it instead of creating yet another one.

That said it would be nice to make it easier for people to find these settings,
I know that it was only when I started research how to make the same changes you
are making that I discovered I did not actually need to do anything.

> Are you saying to replace LLVM_BUILD_EXTERNAL_COMPILER_RT with WITH_COMPILER_RT?

I find the existence of the symbols LLVM_BUILD_EXTERNAL_COMPILER_RT and LLVM_EXTERNAL_COMPILER_RT_BUILD
which have inverted logic a bit confusing:

1 - setting LLVM_BUILD_EXTERNAL_COMPILER_RT to ON disables building compiler-rt as part of llvm
    (I think this is indicating that compiler-rt will be built independently)
2 - setting LLVM_EXTERNAL_COMPILER_RT_BUILD to OFF
    (this is the test in add_llvm_external_project() for LLVM_EXTERNAL_${nameUPPER}_BUILD

But, I would not change the existing symbols as I don't know who is using them or how.

That said the symbol "WITH_LLDB" is much clearer to me than "LLVM_EXTERNAL_LLDB_BUILD", one could
add WITH_LLDB to add_llvm_external_project() something like

    option(LLVM_EXTERNAL_${nameUPPER}_BUILD
           "Whether to build ${name} as part of LLVM" ON)
    option(WITH_${nameUPPER}_BUILD
           "Whether to build ${name} as part of LLVM" ON)
    if (LLVM_EXTERNAL_${nameUPPER}_BUILD AND WITH_${nameUPPER})
      add_subdirectory(${LLVM_EXTERNAL_${nameUPPER}_SOURCE_DIR} ${add_llvm_external_dir})
    endif()

I would not do it, and wonder if there is a way to better document the existing feature.


> projects/CMakeLists.txt
> 
>  if(${LLVM_BUILD_RUNTIME})
>    if(WITH_COMPILER_RT)
>      add_llvm_external_project(compiler-rt)
>    else(WITH_COMPILER_RT)
>      list(APPEND LLVM_IMPLICIT_PROJECT_IGNORE "${LLVM_MAIN_SRC_DIR}/projects/compiler-rt")
>    endif(WITH_COMPILER_RT)
>  endif()

Reading this bit of code had me wondering about what LLVM_IMPLICIT_PROJECT_IGNORE does,
and it turns out that in my opinion LLVM_BUILD_EXTERNAL_COMPILER_RT=ON does not work
since it does not add the compiler-rt to the LLVM_IMPLICIT_PROJECT_IGNORE.

This is because add_llvm_implicit_external_projects() will find the CMakeLists.txt and
add it if the directory is not in LLVM_IMPLICIT_PROJECT_IGNORE.

On the other hand LLVM_BUILD_COMPILER_RT_EXTERNAL=OFF does work because add_llvm_external_project()
always adds compiler-rt to the LLVM_IMPLICIT_PROJECT_IGNORE list.

> Or I should leave compier-rt alone?

I believe that for CMake this change is not needed for compiler-rt, lld or lldb, but I am not an expert.

I have spent about 3 hours tonight looking at this, and I think I understand it, but I have not extensively
tested every combination.

I have built with ninja and XCode on Mac OS setting LLVM_EXTERNAL_LLD_BUILD, LLVM_EXTERNAL_LLDB_BUILD and LLVM_EXTERNAL_COMPILER_RT_BUILD
to OFF. It seemes to me that these symbols were working.

> 
> Rick
> compiler-rt is a little different in autoconf than in cmake. Have to think a little more to handle it.
> 
> projects/Makefile
> 
>  ;; Don't build compiler-rt, it isn't designed to be built directly.
>  DIRS := $(filter-out compiler-rt,$(DIRS))
> 
> 
> REPOSITORY
>  rL LLVM
> 
> http://reviews.llvm.org/D6909
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