On Fri, Dec 20, 2019 at 5:19 PM Tom Stellard <tstellar@xxxxxxxxxx> wrote: > > On 12/20/2019 02:01 PM, Neal Gompa wrote: > > On Fri, Dec 20, 2019 at 3:24 PM Tom Stellard <tstellar@xxxxxxxxxx> wrote: > >> > >> On 12/20/2019 03:33 AM, Nicolas Chauvet wrote: > >>> Le jeu. 19 déc. 2019 à 22:44, Ben Cotton <bcotton@xxxxxxxxxx> a écrit : > >>>> > >>>> https://fedoraproject.org/wiki/Changes/Use-Update-Alternatives-For-usr-bin-cc > >>>> > >>>> == Summary == > >>>> Modify the gcc package so that the /usr/bin/cc and /usr/bin/c++ > >>>> symlinks are managed by update-alternatives. > >>>> > >>>> == Owner == > >>>> * Name: [[User:tstellar| Tom Stellard]] > >>>> * Email: <tstellar@xxxxxxxxxx> > >>>> > >>>> == Detailed Description == > >>>> The gcc package currently installs symlinks to /usr/bin/cc and > >>>> /usr/bin/c++ which point to /usr/bin/gcc and /usr/bin/g++ > >>>> respectively. For this change, the gcc package will be modified so > >>>> that update-alternatives creates and manages these symlinks. > >>>> > >>>> In addition to modifying the gcc package, the clang package will be > >>>> modified so that /usr/bin/clang and /usr/bin/clang++ can be used as > >>>> alternatives for /usr/bin/cc and /usr/bin/c++. The clang alternatives > >>>> will have a lower priority than the gcc alternatives, so that by > >>>> default, gcc will provide the /usr/bin/cc and /usr/bin/c++ > >>>> implementations. > >>>> > >>>> The clang package currently has a run-time dependency on gcc, so this > >>>> ensures that gcc will always provide the default implementation, > >>>> because it's impossible to install clang without gcc. > >>>> > >>>> The only way users will be able to change the /usr/bin/cc or > >>>> /usr/bin/c++ implementations will be by explicitly using the > >>>> update-alternatives tool. > >>>> > >>>> == Benefit to Fedora == > >>>> Many build systems default to using /usr/bin/cc and /usr/bin/c++ as > >>>> the default C/C++ compilers. Being able to easily swap out these > >>>> implementation will provide a lot of flexibility within Fedora for > >>>> doing things like: > >>>> > >>>> * Setting up alternative buildroots for testing. > >>>> * Installing a gcc wrapper script to /usr/bin/cc to help migrate > >>>> packages to new compiler flags or to capture statistics about compiler > >>>> usage. > >>>> * Letting users experiment easily with alternate compilers. > >>>> * Easily switch between system gcc and a development version of gcc. > >>>> > >>>> == Scope == > >>>> * Proposal owners: The proposal owner will implement the necessary > >>>> changes in the gcc and clang packages. > >>>> > >>>> * Other developers: The gcc maintainers will be responsible for > >>>> reviewing and approving changes to the gcc package. > >>>> > >>>> * Release engineering: (a check of an impact with Release Engineering is needed) > >>>> * Policies and guidelines: No policies or guidelines will need to be > >>>> updated as a result of this change. > >>>> * Trademark approval: N/A (not needed for this Change) > >>>> > >>>> > >>>> == Upgrade/compatibility impact == > >>>> This change should not impact upgradeability. > >>>> > >>>> == How To Test == > >>>> CI tests will be added to the gcc package to ensure that /usr/bin/cc > >>>> and /usr/bin/c++ still point to /usr/bin/gcc and /usr/bin/g++ when > >>>> installed. There will also be a CI test added to the clang package to > >>>> ensure that /usr/bin/gcc and /usr/bin/g++ remain the default when > >>>> clang is installed. > >>>> > >>>> == User Experience == > >>>> This change will give users a much better way to experiment using > >>>> other compilers for their own development. They will be able to > >>>> easily switch between different compilers without having to modify > >>>> their projects build system or make non-standard changes to their > >>>> Fedora system. > >>>> > >>>> == Dependencies == > >>>> This change has no other dependencies besides the changes to the gcc > >>>> and clang packages. > >>>> > >>>> == Contingency Plan == > >>>> * Contingency mechanism: (What to do? Who will do it?) Proposal Owner > >>>> will revert changes made to gcc and clang packages and rebuild. > >>>> * Contingency deadline: If the changes are not complete by 2 weeks > >>>> before the mass rebuild, then we will consider postponing to the next > >>>> Fedora release and back out any changes that were made. > >>>> * Blocks release? No > >>>> * Blocks product? None > >>>> > >>>> == Documentation == > >>>> Release notes will be added for this change. > >>>> > >>>> == Release Notes == > >>>> The user /usr/bin/cc and /usr/bin/c++ symlinks are now managed by > >>>> update-alternatives. If you would like to change these symlinks to > >>>> point to another compiler, like clang, for example, you can use these > >>>> commands: > >>>> > >>>> `update-alternatives --set cc /usr/bin/clang` > >>>> > >>>> `update-alternatives --set c++ /usr/bin/clang++` > >>> > >>> Does this process even works in RPM context ? given rpm -E %{__cc} > >>> outputs gcc, I don't think /usr/bin/cc is ever used anywhere. (same > >>> for __cxx, __cpp) > >> > >> /usr/bin/cc is the default compiler for cmake projects. > >> > >>> If that's only supposed to work in a local compilation context (not > >>> with RPM), what is the benefit from using alternatives rather than > >>> export CC=clang ? > >> > >> I'm actually not sure how much better alternatives is that using only CC=clang. > >> I haven't done a full rebuild with only CC=clang and without > >> the proposed /usr/bin/cc alternative pointing to clang to see what the > >> numbers look like. > >> > >> What I have done is build all the packages that depend on gcc with /usr/bin/cc > >> pointing to clang and also CC=/usr/bin/cc (and the same for the c++ compilers). > >> With these changes at least 1281 of 4271 packages still build with gcc. > >> So the best case scenario for CC=/usr/bin/clang is that it will work for about 70% > >> of packages. > >> > >>> What about ccache ? (does it need to also be registered with alternatives) ? > >>> > >> > >> No, because if you want to use ccache with the cc compiler you run: > >> /usr/lib64/ccache/cc > >> > >> > >>> As I imagine, setting clang for a given package with such alternatives > >>> would requires to add a BR of some clang-default that will call > >>> alternatives in %post. > >>> At first sight, I would dramatically prefer to have a RPM macro that > >>> would set __cc, __cpp,__cxx and the relevant cflags ldflgas in %prep. > >>> (and eventually another macro that would set then back to default). > >>> > >> > >> This is something I have been looking at as well, but less as a solution > >> to the "mass-rebuilds with clang" problem, and more focused on getting > >> a consistent experience for packages that currently do build with clang. > >> e.g. > >> > >> %enable_cc_clang \ > >> %__cc clang \ > >> %__cxx clang++ \ > >> %__cpp clang-cpp \ > >> %global optflags %(echo %{optflags} | sed -e-e 's/-fstack-clash-protection//g') > >> > >> I think this is something packages could use in %prep, but it would be nice to > >> take this a step further and have something we could conditionally enable for all packages > >> to allow the kinds of mass rebuilds I would like to try. I just haven't been able to > >> figure out the best way to do this yet. If you have any suggestions for how to > >> make this work, I would like to hear them. > >> > > > > Since Koji 1.18, you can set macros on a Koji tag, which would > > influence the settings of the package build underneath it. If your > > mass building is done in a Koji instance, that's a way to do it. > > Alternatively, if you have an Open Build Service instance, you can set > > the macro at the OBS project level, import all the Fedora sources, and > > watch it rebuild with a different compiler automatically. > > > > This is good to know. For now I'm experimenting with mock + jenkins, but > I would like to move to koji at some point int the future. > > One question I have about a macro solution is how do I inject > export CC=%__cc into the builds? Do I need to add this > to the %prep macro? > You'd probably need to change %set_build_flags to add definitions for those. -- 真実はいつも一つ！/ Always, there's only one truth! _______________________________________________ devel mailing list -- devel@xxxxxxxxxxxxxxxxxxxxxxx To unsubscribe send an email to devel-leave@xxxxxxxxxxxxxxxxxxxxxxx Fedora Code of Conduct: https://docs.fedoraproject.org/en-US/project/code-of-conduct/ List Guidelines: https://fedoraproject.org/wiki/Mailing_list_guidelines List Archives: https://lists.fedoraproject.org/archives/list/devel@xxxxxxxxxxxxxxxxxxxxxxx
