llvm dev - Nov 2017 - Correctly linking against libLLVM (single shared library build)

Hi,

In SUSE we have recently switched from building LLVM as multiple shared 
libraries (using BUILD_SHARED_LIBS) to building it as a single shared library 
(using LLVM_BUILD_LLVM_DYLIB).

The multiple shared libraries build was causing issues and apparently it is 
only meant for LLVM developers. Our guidelines prohibit linking against static 
libraries unless there is no other option.

After this change, some external tools failed building because they try to 
link against libraries given by the cmake function 
`llvm_map_components_to_libnames`. This function returns library names as if 
each component was in its own library. (LLVMSupport, LLVMCore, ...)
Usage of this function comes from documentation:
  https://llvm.org/docs/CMake.html#embedding-llvm-in-your-project

1) Is `llvm_map_components_to_libnames` working as expected? Shouldn't it 
return just "LLVM" if LLVM_BUILD_LLVM_DYLIB was used?

2) I saw that there is a `add_llvm_library` function that chooses between 
using `llvm_map_components_to_libnames` or just using "LLVM" directly
based on
LLVM_BUILD_LLVM_DYLIB. Is this function suitable for use by external projects? 
It doesn't seem to be documented. Is there also equivalent for executables?

Thank you,
Michal Srb

Sorry for the delayed response. I've been out of town a lot lately.

The documentation you referenced is unfortunately out of date (you might notice
it still references autoconf). I'll see if I can find time to update it, but
the guidance should be to use the `llvm_config` CMake function instead. The
proper usage of that in the example there would be to replace the call to
`llvm_map_components_to_libnames` with `llvm_config(simple-tool support core
irreader)`.

`llvm_config` should properly handle the LLVM shared library.

Thanks,
-Chris
> On Oct 23, 2017, at 7:19 AM, Michal Srb via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Hi,
> 
> In SUSE we have recently switched from building LLVM as multiple shared 
> libraries (using BUILD_SHARED_LIBS) to building it as a single shared
library
> (using LLVM_BUILD_LLVM_DYLIB).
> 
> The multiple shared libraries build was causing issues and apparently it is
> only meant for LLVM developers. Our guidelines prohibit linking against
static
> libraries unless there is no other option.
> 
> After this change, some external tools failed building because they try to 
> link against libraries given by the cmake function 
> `llvm_map_components_to_libnames`. This function returns library names as
if
> each component was in its own library. (LLVMSupport, LLVMCore, ...)
> Usage of this function comes from documentation:
>  https://llvm.org/docs/CMake.html#embedding-llvm-in-your-project
> 
> 1) Is `llvm_map_components_to_libnames` working as expected? Shouldn't
it
> return just "LLVM" if LLVM_BUILD_LLVM_DYLIB was used?
> 
> 2) I saw that there is a `add_llvm_library` function that chooses between 
> using `llvm_map_components_to_libnames` or just using "LLVM"
directly based on
> LLVM_BUILD_LLVM_DYLIB. Is this function suitable for use by external
projects?
> It doesn't seem to be documented. Is there also equivalent for
executables?
> 
> Thank you,
> Michal Srb
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

On 11/14/2017, 01:26 AM, Chris Bieneman wrote:
> Sorry for the delayed response. I've been out of town a lot lately.
> 
> The documentation you referenced is unfortunately out of date (you might
notice it still references autoconf). I'll see if I can find time to update
it, but the guidance should be to use the `llvm_config` CMake function instead.
The proper usage of that in the example there would be to replace the call to
`llvm_map_components_to_libnames` with `llvm_config(simple-tool support core
irreader)`.
> 
> `llvm_config` should properly handle the LLVM shared library.
Actually it does not work at all. It behaves exactly the same as
map_components with this CMakeLists.txt:
cmake_minimum_required(VERSION 3.4.3)
project(SimpleProject)

find_package(LLVM REQUIRED CONFIG)

message(STATUS "Found LLVM ${LLVM_PACKAGE_VERSION}")
message(STATUS "Using LLVMConfig.cmake in: ${LLVM_DIR}")

llvm_map_components_to_libnames(llvm_libs support core irreader)
message(STATUS "Components mapped to libnames: ${llvm_libs}")

add_executable(bubak a.c)

llvm_config(bubak support core irreader)
get_target_property(LL bubak LINK_LIBRARIES)
message(STATUS "Components mapped by llvm_config: ${LL}")


It results in:
-- Found LLVM 4.0.1
-- Using LLVMConfig.cmake in: /usr/lib64/cmake/llvm
-- Components mapped to libnames: LLVMSupport;LLVMCore;LLVMIRReader
-- Components mapped by llvm_config: LLVMSupport;LLVMCore;LLVMIRReader



And in turn we get linking errors:
 [ 50%] Linking C executable bubak
/usr/lib64/gcc/x86_64-suse-linux/7/../../../../x86_64-suse-linux/bin/ld:
cannot find -lLLVMSupport
/usr/lib64/gcc/x86_64-suse-linux/7/../../../../x86_64-suse-linux/bin/ld:
cannot find -lLLVMCore
/usr/lib64/gcc/x86_64-suse-linux/7/../../../../x86_64-suse-linux/bin/ld:
cannot find -lLLVMIRReader

So this does not work either :/.

thanks,
-- 
js
suse labs