[CMake] Link Time Code Generation (LTGC) by default, for Release configurations of Visual Studio projects?

[Cristian Adam]

Hi,

I had a quick check to see if CMake 3.10 for which compilers it has support
for Interprocedural Optimization
<https://cmake.org/cmake/help/v3.10/prop_tgt/INTERPROCEDURAL_OPTIMIZATION.html#prop_tgt:INTERPROCEDURAL_OPTIMIZATION>
(IPO, or LTCG as known on Visual C++).

Modules/Compiler had only for *GNU*, *Clang* and *PGI* the following:
set(_CMAKE_${lang}_IPO_MAY_BE_SUPPORTED_BY_COMPILER YES)

QNX with QCC didn't have it set to YES even though nowdays QNX comes with
GCC 5.4.0 on QNX 7.0, which should be just fine.

It would be nice if CMake would have support for IPO on Visual C++.

The steps would be to set the right variables into
Modules/Compiler/MSVC-CXX.cmake, change the CheckIPOSupported
<https://cmake.org/cmake/help/v3.10/module/CheckIPOSupported.html?#module:CheckIPOSupported>
module, change some flags in some places,update tests, documentation, and
so on.

Shouldn't be hard to add this feature.

Cheers,
Cristian.



On Wed, Jan 24, 2018 at 9:06 PM, Niels Dekker <
niels_dekker_address_until_2024 at xs4all.nl> wrote:

> When a Visual C++ project is created within Visual Studio (by File menu,
> New, Project...), it has Link Time Code Generation (LTGC) by default, for
> its Release configurations: Compiler option /GL and linker option /LTCG.
> However, when the Visual C++ project is generated by CMake, it does not
> have LTGC, by default. LTCG looks like a very useful feature to me. I think
> it's worth enabling the feature by default, for Release configurations.
> What do you think?
>
>
> Some details:
>
> I created a "dummy" Visual C++ project in Visual Studio 2017 (Version
> 15.5.4), by File menu, New, Project..., and I also generated one by CMake
> (version 3.10.2), using a very minimal CMakeLists.txt
>
> The Visual C++ project created by Visual Studio 2017 has the following
> compile options, by default, for Release (x64):
>
> /permissive- /GS /GL /W3 /Gy /Zc:wchar_t /Zi /Gm- /O2 /sdl
> /Fd"x64\Release\vc141.pdb" /Zc:inline /fp:precise /D "_MBCS"
> /errorReport:prompt /WX- /Zc:forScope /Gd /Oi /MD /Fa"x64\Release\" /EHsc
> /nologo /Fo"x64\Release\" /Fp"x64\Release\MyVS2015Project.pch"
> /diagnostics:classic
>
> And the following linker options:
>
> /OUT:"F:\X\Src\MyProject\MyVS2015Project\x64\Release\MyVS2015Project.exe"
> /MANIFEST /LTCG:incremental /NXCOMPAT /PDB:"F:\X\Src\MyProject\MyVS2
> 015Project\x64\Release\MyVS2015Project.pdb" /DYNAMICBASE "kernel32.lib"
> "user32.lib" "gdi32.lib" "winspool.lib" "comdlg32.lib" "advapi32.lib"
> "shell32.lib" "ole32.lib" "oleaut32.lib" "uuid.lib" "odbc32.lib"
> "odbccp32.lib" /DEBUG:FULL /MACHINE:X64 /OPT:REF
> /PGD:"F:\X\Src\MyProject\MyVS2015Project\x64\Release\MyVS2015Project.pgd"
> /MANIFESTUAC:"level='asInvoker' uiAccess='false'"
> /ManifestFile:"x64\Release\MyVS2015Project.exe.intermediate.manifest"
> /OPT:ICF /ERRORREPORT:PROMPT /NOLOGO /TLBID:1
>
> The vcxproj file created by Visual Studio has
> "<WholeProgramOptimization>true</WholeProgramOptimization>" for its
> Release configurations.
>
>
> The Visual C++ project generated by CMake (Generator: Visual Studio 15
> 2017 Win64) has the following compile options, for its Release
> configuration:
>
> /GS /TP /W3 /Zc:wchar_t /Gm- /O2 /Ob2 /Fd"MyExecutable.dir\Release\vc141.pdb"
> /Zc:inline /fp:precise /D "WIN32" /D "_WINDOWS" /D "NDEBUG" /D
> "CMAKE_INTDIR=\"Release\"" /D "_MBCS" /errorReport:prompt /WX- /Zc:forScope
> /GR /Gd /MD /Fa"Release/" /EHsc /nologo /Fo"MyExecutable.dir\Release\"
> /Fp"MyExecutable.dir\Release\MyExecutable.pch" /diagnostics:classic
>
> And the following linker options:
>
> /OUT:"F:\X\Bin\vc141-64\MyProject\Release\MyExecutable.exe" /MANIFEST
> /NXCOMPAT /PDB:"F:/X/Bin/vc141-64/MyProject/Release/MyExecutable.pdb"
> /DYNAMICBASE "kernel32.lib" "user32.lib" "gdi32.lib" "winspool.lib"
> "shell32.lib" "ole32.lib" "oleaut32.lib" "uuid.lib" "comdlg32.lib"
> "advapi32.lib" /IMPLIB:"F:/X/Bin/vc141-64/MyProject/Release/MyExecutable.lib"
> /MACHINE:X64 /INCREMENTAL:NO /PGD:"F:\X\Bin\vc141-64\MyProject\Release\MyExecutable.pgd"
> /SUBSYSTEM:CONSOLE /MANIFESTUAC:"level='asInvoker' uiAccess='false'"
> /ManifestFile:"MyExecutable.dir\Release\MyExecutable.exe.intermediate.manifest"
> /ERRORREPORT:PROMPT /NOLOGO /TLBID:1
>
> The vcxproj file generated by CMake does not have a
> "<WholeProgramOptimization>" tag.
>
>
> I hope you can consider adding LTCG to Release configurations of Visual
> C++ projects generated by CMake, by default.
>
>
> Kind regards, Niels
> --
> Niels Dekker
> Scientific programmer
> LKEB, Leiden University Medical Center
> --
>
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