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? -Tom > > > > -- > 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 > _______________________________________________ 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
