On 12/20/2019 03:20 PM, Neal Gompa wrote: > 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. > Will that take affect for all builds? I only see it being used by the %configure 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
