llvm dev - Feb 2014 - [LLVMdev] Understanding some of the recent cmake build changes

I was a bit confused with what the difference was between the old

target_link_libraries(foo bar)

and the new

target_link_libraries(foo INTERFACE|PRIVATE|PUBLIC bar)

To try to find out, I decided to look at the generated build.ninja.
The difference that shows up is far fewer order only dependencies. For
example

build lib/TableGen/CMakeFiles/LLVMTableGen.dir/Error.cpp.o:
CXX_COMPILER /home/espindola/llvm/llvm/lib/TableGen/Error.cpp ||
lib/libLLVMSupport.a

becomes just

build lib/TableGen/CMakeFiles/LLVMTableGen.dir/Error.cpp.o:
CXX_COMPILER /home/espindola/llvm/llvm/lib/TableGen/Error.cpp

The net result is that in the old system running

ninja  ./lib/TableGen/CMakeFiles/LLVMTableGen.dir/Error.cpp.o

on a tree just after running cmake would execute 91 commands to build
lib/libLLVMSupport.a and then Error.cpp.o. On the new system only one
command is executed :-)

In a more realistic situation, Error.cpp.o will be compiled in
parallel with the files needed to build lib/libLLVMSupport.a.

Cheers,
Rafael

Yep!  This is awesome.  I remember filing a CMake bug about this long ago,
and it looks like this is the answer.


On Wed, Feb 26, 2014 at 5:08 PM, Rafael Espíndola <
rafael.espindola at gmail.com> wrote:
> I was a bit confused with what the difference was between the old
>
> target_link_libraries(foo bar)
>
> and the new
>
> target_link_libraries(foo INTERFACE|PRIVATE|PUBLIC bar)
>
> To try to find out, I decided to look at the generated build.ninja.
> The difference that shows up is far fewer order only dependencies. For
> example
>
> build lib/TableGen/CMakeFiles/LLVMTableGen.dir/Error.cpp.o:
> CXX_COMPILER /home/espindola/llvm/llvm/lib/TableGen/Error.cpp ||
> lib/libLLVMSupport.a
>
> becomes just
>
> build lib/TableGen/CMakeFiles/LLVMTableGen.dir/Error.cpp.o:
> CXX_COMPILER /home/espindola/llvm/llvm/lib/TableGen/Error.cpp
>
> The net result is that in the old system running
>
> ninja  ./lib/TableGen/CMakeFiles/LLVMTableGen.dir/Error.cpp.o
>
> on a tree just after running cmake would execute 91 commands to build
> lib/libLLVMSupport.a and then Error.cpp.o. On the new system only one
> command is executed :-)
>
> In a more realistic situation, Error.cpp.o will be compiled in
> parallel with the files needed to build lib/libLLVMSupport.a.
>
> Cheers,
> Rafael
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
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Rafael, thanks to explain with clear examples.

Although target_link_libraries(INTERFACE) can cut deps among
libraries, files in add_executable() will be still serialized.
For example,

build tools/clang/utils/TableGen/CMakeFiles/clang-tblgen.dir/TableGen.cpp.o:
  CXX_COMPILER /home/chapuni/llvm-project/clang/utils/TableGen/TableGen.cpp
|| lib/libLLVMTableGen.a lib/libLLVMSupport.a

If TableGen.cpp would be made free from *.a, we should introduce
objlib stuff for add_executable().
See also my tddeps patch.


2014-02-27 10:08 GMT+09:00 Rafael Espíndola <rafael.espindola at
gmail.com>:
> I was a bit confused with what the difference was between the old
>
> target_link_libraries(foo bar)
>
> and the new
>
> target_link_libraries(foo INTERFACE|PRIVATE|PUBLIC bar)
>
> To try to find out, I decided to look at the generated build.ninja.
> The difference that shows up is far fewer order only dependencies. For
> example
>
> build lib/TableGen/CMakeFiles/LLVMTableGen.dir/Error.cpp.o:
> CXX_COMPILER /home/espindola/llvm/llvm/lib/TableGen/Error.cpp ||
> lib/libLLVMSupport.a
>
> becomes just
>
> build lib/TableGen/CMakeFiles/LLVMTableGen.dir/Error.cpp.o:
> CXX_COMPILER /home/espindola/llvm/llvm/lib/TableGen/Error.cpp
>
> The net result is that in the old system running
>
> ninja  ./lib/TableGen/CMakeFiles/LLVMTableGen.dir/Error.cpp.o
>
> on a tree just after running cmake would execute 91 commands to build
> lib/libLLVMSupport.a and then Error.cpp.o. On the new system only one
> command is executed :-)
>
> In a more realistic situation, Error.cpp.o will be compiled in
> parallel with the files needed to build lib/libLLVMSupport.a.
>
> Cheers,
> Rafael

On 27 February 2014 09:29, NAKAMURA Takumi <geek4civic at gmail.com>
wrote:
> Rafael, thanks to explain with clear examples.
>
> Although target_link_libraries(INTERFACE) can cut deps among
> libraries, files in add_executable() will be still serialized.
> For example,
>
> build
tools/clang/utils/TableGen/CMakeFiles/clang-tblgen.dir/TableGen.cpp.o:
>   CXX_COMPILER /home/chapuni/llvm-project/clang/utils/TableGen/TableGen.cpp
> || lib/libLLVMTableGen.a lib/libLLVMSupport.a
>
> If TableGen.cpp would be made free from *.a, we should introduce
> objlib stuff for add_executable().
> See also my tddeps patch.
Thanks for the example. I reported
http://llvm.org/bugs/show_bug.cgi?id=18990 to track this.

Cheers,
Rafael