Revision as of 00:47, 18 December 2019

Changes/LTO by default for package builds

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@redhat.com

Current status

Targeted release: Fedora 32
Last updated: 2019-12-18
Tracker bug: <will be assigned by the Wrangler>
Release notes tracker: <will be assigned by the Wrangler>

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.

This change also brings us back on-par with SuSE who enabled LTO by default for their free distribution earlier in 2019.

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.

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.

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.

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: #Releng issue number (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 ~200 package build failures. Many of those failures will be packages that need to opt-out of LTO because they use features not compatible with LTO. Other failures represent package problems that we'll work with the package owners to fix. In the event there's an issue that's unresolved, we can opt-out such packages.

Documentation

I would think we would want documentation on the opt-out method for RPM builds.

Release Notes

@@ Line 1: / Line 1: @@
 <!-- The actual name of your proposed change page should look something like: Changes/Your_Change_Proposal_Name.  This keeps all change proposals in the same namespace -->
-= LTO by default for package builds <!-- The name of your change proposal --> =
+= Changes/LTO by default for package builds <!-- The name of your change proposal --> =
 == Summary ==
 <!-- A sentence or two summarizing what this change is and what it will do. This information is used for the overall changeset summary page for each release.
 Note that motivation for the change should be in the Motivation section below, and this part should answer the question "What?" rather than "Why?". -->
-This is a proposal to enable link time optimization (LTO) of packages by default.  LTO, in general, produces smaller and faster executables.  LTO also allows for deeper compile-time analysis of code to find defects such as uninitialized variables/memory, buffer overflows, etc.
+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 ==
@@ Line 13: / Line 17: @@
 This should link to your home wiki page so we know who you are.
 -->
-* Name: law
+* Name: Jeff Law
 <!-- Include you email address that you can be reached should people want to contact you about helping with your change, status is requested, or technical issues need to be resolved. If the change proposal is owned by a SIG, please also add a primary contact person. -->
 * Email: law@redhat.com
@@ Line 40: / Line 44: @@
 <!-- Expand on the summary, if appropriate.  A couple sentences suffices to explain the goal, but the more details you can provide the better. -->
-Packages will be built with LTO by default to produce smaller and faster executables/DSOs.  The LTO bytecode itself will not be distributed as it is not stable from one GCC release to the next.
+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.
-There will be an opt-out mechanism for packages that need it.  Goal is opt-out would be a single variable set in the package's .spec file.
+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.
+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.
+This change also brings us back on-par with SuSE who enabled LTO by default for their free distribution earlier in 2019.
@@ Line 53: / Line 61: @@
 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.
-There will be packages that need to opt-out.  For example, LTO is known not to work with linker scripts that provide symbol versioning within a DSO.  More interestingly, some packages will fail to build with LTO due to improved compile time analysis for diagnostics.  Finally some packages are known to use horribly out of date configure scripts which can be compromised by LTO optimizations.  I'm happy to work with the package owners to address these issues.
+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.
+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.
+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.
-It's an isolated change in redhat-rpm-config and its post-processing scripts.  However, the change affects everything within the distribution that is compiled with GCC.  We have some prototype work already done for the redhat-rpm-config changes and the SuSE engineers have post-processing scripts to strip out the LTO bytecode at install time that we can use.
 * 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.  It would be advantageous if I could get proven packager privileges to address the simplest of these issues.
+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.
-We already know there are some issues with configure scripts generated by old versions of autoconf.  When possible those scripts would be rebuilt with modern versions of autoconf.  That may not be possible in all circumstances -- for these exceptional cases I believe the existing scripts can be fixed with a relatively simple sed script we could insert into a package's .spec file.
+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.
-I have a means by which I can continuously test Fedora builds with this feature enabled so that we can address issues proactively rather than late in the Fedora 32 release cycle.  All the work would need to land prior to the mass rebuild in Jan/Feb.
+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.
 <!-- What work do other developers have to accomplish to complete the feature in time for release?  Is it a large change affecting many parts of the distribution or is it a very isolated change? What are those changes?-->
-We know there are some problems with debugging LTO optimized code.  We will be working with the GDB team to identify these issues and fix them either in the debugger or compiler as needed.
+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: [https://pagure.io/releng/issues #Releng issue number] (a check of an impact with Release Engineering is needed) <!-- REQUIRED FOR SYSTEM WIDE CHANGES -->
@@ Line 75: / Line 96: @@
 * Policies and guidelines:
 <!-- Do the packaging guidelines or other documents need to be updated for this feature?  If so, does it need to happen before or after the implementation is done?  If a FPC ticket exists, add a link here. -->
-It would be useful to document how to opt-out of LTO in the packaging 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)
@@ Line 88: / Line 109: @@
 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 the gcc-10 effort, testing LTO at the same time is a natural extension.
+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 executables could notice differences in the debugging experience.
+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 during our testing of LTO + debugging over the next 6-8 weeks.
+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 ==
 <!-- If you cannot complete your feature by the final development freeze, what is the backup plan?  This might be as simple as "Revert the shipped configuration".  Or it might not (e.g. rebuilding a number of dependent packages).  If you feature is not completed in time we want to assure others that other parts of Fedora will not be in jeopardy.  -->
-* Contingency mechanism: (What to do?  Who will do it?) N/A (not a System Wide Change)  <!-- REQUIRED FOR SYSTEM WIDE CHANGES -->
+* Contingency mechanism: Revert the LTO flags injection  <!-- REQUIRED FOR SYSTEM WIDE CHANGES -->
 <!-- When is the last time the contingency mechanism can be put in place?  This will typically be the beta freeze. -->
-* Contingency deadline: N/A (not a System Wide Change)  <!-- REQUIRED FOR SYSTEM WIDE CHANGES -->
+* Contingency deadline: Beta freeze, but shooting for prior to mass rebuilds starting  <!-- REQUIRED FOR SYSTEM WIDE CHANGES -->
 <!-- Does finishing this feature block the release, or can we ship with the feature in incomplete state? -->
 * Blocks release? No <!-- REQUIRED FOR SYSTEM WIDE CHANGES -->
 * Blocks product? No <!-- Applicable for Changes that blocks specific product release/Fedora.next -->
-In the event a show stopping issue was found with LTO we could just revert the relevant redhat-rpm-config change which would disable the LTO flags by default.  It is more likely that some packages have bugs that are exposed by LTO that we may not be able to get the package owners to fix in time for Fedora 32.  For those packages the contingency plan would be to have those specific packages opt-out of LTO.
+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 ~200 package build failures.   Many of those failures will be packages that need to opt-out of LTO because they use features not compatible with LTO.  Other failures represent package problems that we'll work with the package owners to fix.  In the event there's an issue that's unresolved, we can opt-out such packages.
 == Documentation ==

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