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

[Igor Munkin <imun@tarantool.org>]

Olya,

Thanks for the fixes! There are still a couple nits below.

On 14.04.20, Olga Arkhangelskaia wrote:
> Hi once again!
> 
> I have fixed your comments and pushed once again. See below.
> 
> 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.
> >
> 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
> ---
>   cmake/luajit.cmake | 23 +++++++++++++++++++++++
>   1 file changed, 23 insertions(+)
> 
> 
> >>           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 ${CMAKE_CXX_COMPILER_VERSION} VERSION_LESS 3.4)
> > So, if clang toolchain is used but version is less than 3.4, then gcc
> > toolchain settings are used. It's either totally wrong or definitely
> > need to be clarified with a corresponding comment.
> >
> > Also, AFAICS LTO support for toolchain is checked via CheckIPOSupported
> > CMake module. So additional check here looks to be excess or also need
> > to be clarified.
> >
> >> +	    if (${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?
> >
> >> +	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 ${luajit_cflags} -flto -fuse-linker-plugin -fno-fat-lto-objects)
> >> +	   endif()
> >> +	   set (CMAKE_AR gcc-ar)
> > Could you please clarify why ar is set explicitly here?
> >
> >> +        endif()
> >> +    endif()
> +    # Add LTO option to luajit
> +    if (CMAKE_INTERPROCEDURAL_OPTIMIZATION)
> +           message("Setting LTO flags for building luajit")
> +       # Clang opt to support LTO
> +       if (CMAKE_COMPILER_IS_CLANG)
> +           if (${CMAKE_CXX_COMPILER_VERSION} VERSION_LESS 3.9)
> +               set(luajit_cflags ${luajit_cflags} -flto=full)
> + else()
> +               set(luajit_cflags ${luajit_cflags} -flto=thin)
> + endif()

Moved our discussion below:

================================================================================

> > I see no difference in the branches above.
> 
> I have a mistake here (Thin LTO) - will wix it
> 
> If version is above than 3.9 we can use -flto=thin

What is the difference between "full" and "thin" values? Please drop a
few words regarding it.

================================================================================

> +          # Depending of compiler we need to set appropriate 
> llvm/gcc-ar lib
> +          set (CMAKE_AR llvm-ar)

Moved our discussion below:

================================================================================

> > 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>

It would be nice to leave a comment regarding it.

================================================================================

> +       # GNU opts to support lto

Minor: Please move this comment under <else()> statement as the
corresponding clang-related comment.

> + else()
> +          #Due to some problems (bugs, slow work, etc) we support LTO 
> only for 5.0+
> +          #The same is for binutils prior 2.27

Typo: Please separate # from the comment with a space.

> +          if (NOT ${CMAKE_CXX_COMPILER_VERSION} VERSION_LESS 5.0
> +              AND NOT ${linker_version} VERSION_LESS 2.27)

Are there any versions supporting LTO/IPO less than gcc-5.0 and ld-2.27?
If yes, please mention why they don't fit our purposes. If not, I see no
rationale for this condition.

> +              set(luajit_cflags ${luajit_cflags} -flto 
> -fuse-linker-plugin -fno-fat-lto-objects)
> + endif()
> +          set (CMAKE_AR gcc-ar)

Minor: there are some <set> commands with a whitespace prior to
parentheses and some without. Please adjust those you introduced within
your changes to a single style.

> + 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.
> >
> >>       # 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)
> >>

-- 
Best regards,
IM