Hi Jeff, On Thu, Dec 19, 2019 at 10:50 PM Ben Cotton <bcotton@xxxxxxxxxx> wrote: > > https://fedoraproject.org/wiki/LTOByDefault > > == Summary == > This is a proposal to enable link time optimization (LTO) of packages > built with rpmbuild by default. It's an over-simplification, but > think of LTO as deferring analysis, optimization and code generation > until creation of an executable or dynamic shared object. > > This is implemented by adding the option "-flto" the injected flags in > redhat-rpm-config. There will be a simple way for packages to opt-out > of LTO. > > == Owner == > * Name: Jeff Law > * Email: law@xxxxxxxxxx > > == Detailed Description == > Programs built with rpmbuild and which honor flags injection via > redhat-rpm-config will be built with LTO by default. A simple opt-out > mechanism will be provided for packages which use features that are > not LTO compatible. > > The LTO bytecode itself will not be distributed as it is not stable > from one GCC release to the next. This is enforced by stripping the > LTO bytecode from any installed .o/.a files. We'll use bits SuSE has > already written for redhat-rpm-config to implement this. > > Minor changes are desirable to the %configure macro in > redhat-rpm-config to fix common code idioms used by autoconf generated > scripts which are compromised by the additional optimization enabled > by LTO. Minor updates to various packages will be needed to opt-out > of LTO or fix bugs exposed by LTO. > > == Benefit to Fedora == > > The primary benefits of building with LTO enabled are smaller, faster > executables/DSOs. A secondary benefit is LTO allows deeper analysis > of package source code at compile time which can improve various GCC > diagnostics and thus improve our ability to catch bugs at compile time > such as uninitialized objects, buffer overflows, unterminated strings, > restrict violations, etc. It would be very nice to get more specific analysis data. Like running some benchmarks of big applications, size comparisons (of binaries and libraries) and compile time. > This change also brings us back on-par with SuSE who enabled LTO by > default for their free distribution earlier in 2019. You probably should have said openSUSE rather here. > > == Scope == > * Proposal owners: > The primary change is to redhat-rpm-config to add LTO to the default > compile/link flags as well as a conditional which allows easy opt-out > on a package by package basis. Additionally the post-build scripts > need to strip the LTO bytecodes from any installed .o/.a files. What does that mean? Which post-build scripts are needed and where that needs to be done? How does it affect build time? > Additionally, we know there are many packages with configure scripts > that are compromised by LTO. I have tweaks to the %configure macro in > redhat-rpm-config which fixes the vast majority of these problems with > a few simple sed scripts on the generated output. Like the basic > support for injecting the LTO flags, this will require coordination > with the redhat-rpm-config maintainers. Packages which call configure > directly and have compromised tests will need a one line change to > their .spec files to fix their configure scripts. "simple sed scripts" are probably not so simple :) > Some packages will need to opt-out of using LTO at this time. The > most common case are packages that use symbol versioning or toplevel > ASM statements. While there is a new mechanism to make LTO work with > symbol versioning, I don't think any packages have been updated to use > that mechanism. This will require a one line change to 50-75 packages > (my script to find these is still running). Hmm, I think we have bunch of packages (more than 75 of them) which use symbol versioning. Would be useful to give some links in the change proposal to "how to update to use that mechanism". > Finally, some packages will fail to build with LTO due to deeper > analysis for compile-time diagnostics catching programming mistakes > that have gone unnoticed until now. I'll obviously be working with > package maintainers on all of these issues. > > Note that even though the changes are fairly well localized in > redhat-rpm-config and a small number of packages, the real scope of > this change is much larger since it affects all packages in the > distribution that are compiled with GCC and which honor the flags > injection by redhat-rpm-config. > > > * Other developers: > As I mentioned, I'm happy to contact package owners that need to > modify their packages and suggest how their package needs to be fixed. > As a multi-decade GCC developer, I'm particularly well suited to > describe LTO, its limitations and how LTO impacts the diagnostics from > GCC to any package owner that needs additional information. > > I'm also capable and available to address any GCC issues that we may > arise as a result of this change. I don't expect much of the latter > as SuSE has already enabled this feature for their distribution and > thus weeded out most of the issues. > > The highest level of coordination will be with the redhat-rpm-config > maintainers. > > I will also be coordinating with the GDB team to address debugging > issues related to LTO. The most important issue is to ensure that we > can pass the GDB testsuite with and without the -flto option being > enabled. Failure to meet this goal would be considered a blocking > issue for LTO enablement. > > I'm also already in contact with SuSE and Debian/Ununtu engineers to > discuss issues with gcc-10 with and without LTO. > > We know there are some problems with debugging LTO code. I will be > working with the GDB team to identify these issues and fix them either > in the debugger or compiler as needed. > > I have prototype code for the required redhat-rpm-config changes and > I'll coordinate with the redhat-rpm-config maintainer to get them into > the desired final form. > > I also know every package that fails with LTO enabled. I'm still > categorizing those failures. Many will ultimately need to use the > opt-out mechanism because they use features that are not compatible > with LTO. I expect to have all this ready to go the first work week > of the new year. I will coordinate with package owners to either add > the opt-out markers or fix issues in the package as needed. > > > * Release engineering: (a check of an impact with Release Engineering is needed) > Aside from the redhat-rpm-config changes, I do not expect any work > from releng to be necessary. However, they need to be aware of the > change and who to contact in case of issues. > > * Policies and guidelines: It would be useful to document how to > opt-out of LTO in the packaging guidelines. > > * Trademark approval: N/A (not needed for this Change) > > > == Upgrade/compatibility impact == > Should not affect compatibility. Stripping of the LTO bytecode is > critical to ensure there are not long term compatibility issues. > > > == How To Test == > In the short term, I'm happy to expose a repository with a gcc-10 > snapshot and updated redhat-rpm-config. Developers could then use > that repo to pick up gcc-10 and LTO optimizations for testing > purposes. I'm already doing this internally for x86_64 and exposing > it to the world would be trivial. > > Given such a repository, another developer would merely use that repo > when building their package. No special hardware is needed. The most > useful testing is first to identify FTBFS issues and get them > proactively fixed. I'm happy to own that since I'm already doing that > for baseline gcc-10 issues as well as gcc-10 + LTO issues. > > Doing the same testing on other architectures would definitely be > useful. I'd be particularly concerned about large packages on the > 32bit architectures. I wouldn't be surprised if we find some packages > need to opt-out of LTO because they run out of memory at link/compile > time. I'm already in contact with some Debian maintainers who want > to do testing around this issue as they're investigating a similar > change for Debian. > > I'm already building all of Fedora with the weekly gcc-10 snapshots > (including LTO builds starting the week of 12/15). This is primarily > to proactive find/address issues with the gcc-10 transition, but > verification of LTO state pretty much piggy backs for free on the > gcc-10 work. > > == User Experience == > In theory, the only noticeable difference to users would be smaller, > faster binaries and DSOs. However, a developer that uses rpmbuild to > build their own code may see their package fail to build if it's got > errors or uses certain features that do not work with LTO. > > Users who try to debug Fedora shipped executables could notice > differences in the debugging experience. > > == Dependencies == > None expected beyond addressing FTBFS issues and coordination between > GCC and GDB teams on any debugging issues we find over the next few > weeks. > > == Contingency Plan == > * Contingency mechanism: Revert the LTO flags injection > * Contingency deadline: Beta freeze, but shooting for prior to mass > rebuilds starting > * Blocks release? No > * Blocks product? No > > Most critically, if we don't address the GDB testsuite issue noted > above, our fallback position would be to simply disable the LTO > injection globally and re-evaluate for Fedora 33, similarly if we were > to find some show-stopping LTO issue. > > Otherwise the plan is to analyze the remaining 100-125 package build > failures. These are likely a mixture of configure issues that can't > be trivially fixed via %configure, new diagnostics exposed by the > deeper analysis from LTO, and other small issues. > > == Documentation == > I would think we would want documentation on the opt-out method for RPM builds. > > -- > Ben Cotton > He / Him / His > Fedora Program Manager > Red Hat > TZ=America/Indiana/Indianapolis > _______________________________________________ > devel-announce mailing list -- devel-announce@xxxxxxxxxxxxxxxxxxxxxxx > To unsubscribe send an email to devel-announce-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-announce@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
