Re: [Tarantool-patches] [PATCH] cmake: add LTO support for building luajit

[Olga Arkhangelskaia <arkholga@tarantool.org>]

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Hello Igor,

thanks for the review!

14.04.2020 12:18, Igor Munkin пишет:
> Olya,
>
> Thanks for the patch! I left several comments below, please consider
> them.
>
> On 12.03.20, Olga Arkhangelskaia wrote:
>> Tarantool has LTO support, however while building luajit this opt. was
>> omitted. Patch adds necessary flag to turn it on.
>>
>> Minimum compiler/linker versions: clang 3.4, gcc 5.0+, ld 2.27+.
>>
>> Closes #3743
>> ---
>> Branch: OKriw/gh-3743-LuaJIT-does-not-use-LTO-with-DENABLE_LTO=ON-full-ci
>>   cmake/lto.cmake    |  3 ++-
>>   cmake/luajit.cmake | 26 ++++++++++++++++++++++++++
>>   2 files changed, 28 insertions(+), 1 deletion(-)
>>
>> diff --git a/cmake/lto.cmake b/cmake/lto.cmake
>> index 95ade75f4..9f29f3324 100644
>> --- a/cmake/lto.cmake
>> +++ b/cmake/lto.cmake
>> @@ -78,7 +78,8 @@ if (NOT TARGET_OS_DARWIN)
>>           if (linker_version VERSION_LESS "2.31")
>>               message(FATAL_ERROR "ld.bfd >= 2.31 is needed for LTO")
>>           endif()
>> -    elseif(matched_gold)
>> +
>> +	elseif(matched_gold)
> Typo: there is a mess with whitespace above.
>
> However this chunk provides no changes except whitespace and newline.
> Please drop it.
Will fix1
>
>>           set(linker_version ${CMAKE_MATCH_1})
>>           message(STATUS "Found ld.gold version: ${linker_version}")
>>   
>> diff --git a/cmake/luajit.cmake b/cmake/luajit.cmake
>> index 072db8269..da1b4926d 100644
>> --- a/cmake/luajit.cmake
>> +++ b/cmake/luajit.cmake
>> @@ -225,6 +225,32 @@ macro(luajit_build)
>>           set(luajit_xcflags ${luajit_xcflags} -D${def})
>>       endforeach()
>>   
>> +    # Add LTO option to luajit
>> +    if (CMAKE_INTERPROCEDURAL_OPTIMIZATION)
>> +	    message("Setting LTO flags for building luajit")
>> +	# Clang opt to support LTO
>> +	if (CMAKE_COMPILER_IS_CLANG AND
>> +	    NOT ${CMAKE_CXX_COMPILER_VERSION} VERSION_LESS 3.4)
> So, if clang toolchain is used but version is less than 3.4, then gcc
> toolchain settings are used. It's either totally wrong or definitely
> need to be clarified with a corresponding comment.
>
> Also, AFAICS LTO support for toolchain is checked via CheckIPOSupported
> CMake module. So additional check here looks to be excess or also need
> to be clarified.

I have a mistake here (Thin LTO) - will wix it

If version is above than 3.9 we can use -flto=thin

>
>> +	    if (${CMAKE_CXX_COMPILER_VERSION} VERSION_LESS 3.9)
>> +		set(luajit_cflags ${luajit_cflags} -flto=full)
>> +	    else()
>> +	        set(luajit_cflags ${luajit_cflags} -flto=full)
>> +	    endif()
> I see no difference in the branches above.
>
>> +	if (NOT TARGET_OS_DARWIN)
>> +	   # Depending of compiler we need to set appropriate llvm/gcc-ar lib
>> +	   set (CMAKE_AR llvm-ar)
> What ar is used on systems different from Darwin based ones when clang
> toolchain is used?


gcc-ar/ llvm-ar  is wrapper on ar that simply passes —plugin option. We 
need it for LTO.

( See https://bugzilla.redhat.com/show_bug.cgi?id=1678826

https://lists.llvm.org/pipermail/llvm-dev/2018-March/122018.html) 
<https://lists.llvm.org/pipermail/llvm-dev/2018-March/122018.html>


>
>> +	endif()
>> +	# GNU opts to support lto
>> +	else()
>> +	   #Due to some problems (bugs, slow work, etc) we support LTO only for 5.0+
>> +	   #The same is for binutils prior 2.27
>> +	   if (NOT ${CMAKE_CXX_COMPILER_VERSION} VERSION_LESS 5.0
>> +	       AND NOT ${linker_version} VERSION_LESS 2.27)
>> +	       set(luajit_cflags ${luajit_cflags} -flto -fuse-linker-plugin -fno-fat-lto-objects)
>> +	   endif()
>> +	   set (CMAKE_AR gcc-ar)
> Could you please clarify why ar is set explicitly here?

>
>> +        endif()
>> +    endif()
> Typo: there is a mess with indent above. Unfortunately I failed to find
> our CMake style guide, so please adjust your changes considering the
> code nearby.
Will fix
>
>>       # Pass the same toolchain that is used for building of
>>       # tarantool itself, because tools from different toolchains
>>       # can be incompatible. A compiler and a linker are already set
>> -- 
>> 2.20.1 (Apple Git-117)
>>

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