Bug 2224925

Summary:	Build Clang with Profile-Guided Optimization (PGO)
Product:	Red Hat Enterprise Linux 9	Reporter:	Alexander Zaitsev <zamazan4ik>
Component:	clang	Assignee:	Tom Stellard <tstellar>
Status:	CLOSED MIGRATED	QA Contact:	qe-baseos-tools-bugs
Severity:	unspecified	Docs Contact:
Priority:	unspecified
Version:	CentOS Stream	CC:	bstinson, jchecahi, jwboyer, mprchlik, sipoyare
Target Milestone:	rc	Keywords:	FutureFeature, MigratedToJIRA, Triaged
Target Release:	---	Flags:	pm-rhel: mirror+
Hardware:	Unspecified
OS:	Unspecified
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Last Closed:	2023-09-22 17:07:38 UTC	Type:	Story
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oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
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Description Alexander Zaitsev 2023-07-24 00:18:31 UTC

Description of problem:

Clang supports (https://llvm.org/docs/HowToBuildWithPGO.html) bootstrapping with Profile-Guided Optimization (PGO). It increases Clang performance a lot (+20% according to the official docs and my own tests). The same thing is already enabled for GCC in Fedora (profiledbootstrap option). I think the same thing could be done for Clang since in the current build script I don't see that Clang is built with PGO.

Actual result:
Clang package is not PGO-optimized.

Expected results:
Provided Clang binaries with Fedora are built with PGO.

Additional option:
As an additional option, you could try to build binary with LLVM BOLT (https://github.com/facebookincubator/BOLT/blob/main/bolt/docs/OptimizingClang.md) but I guess it's another question and it's much more complicated topic.

More about PGO-related optimizations for other software you can find here: https://github.com/zamazan4ik/awesome-pgo

More about PGO optimizations for other LLVM-based projects (you can be interested in them too) you can find here: https://github.com/llvm/llvm-project/issues/63486

Also, I created a similar request in other Linux distributions:
- Fedora: https://bugzilla.redhat.com/show_bug.cgi?id=2156679
- Rocky Linux: https://bugs.rockylinux.org/view.php?id=1618
- AlmaLinux: https://bugs.almalinux.org/view.php?id=350

Comment 1 Alexander Zaitsev 2023-09-01 02:09:29 UTC

By the way, there is an already supported by Clang extra optimization step after PGO - optimizing Clang with LLVM BOLT.

Here you can read the results of optimizing Clang with PGO + LLVM BOLT compared to PGO:
* Slides: https://llvm.org/devmtg/2022-11/slides/Lightning15-OptimizingClangWithBOLTUsingCMake.pdf
* Results on building Clang: https://github.com/ptr1337/llvm-bolt-scripts/blob/master/results.md
* Linaro results: https://android-review.linaro.org/plugins/gitiles/toolchain/llvm_android/+/f36c64eeddf531b7b1a144c40f61d6c9a78eee7a
* on AMD 7950X3D: https://github.com/llvm/llvm-project/issues/65010#issuecomment-1701255347

Related discussion in the upstream: https://github.com/llvm/llvm-project/issues/65010

Comment 2 RHEL Program Management 2023-09-22 17:05:41 UTC

Issue migration from Bugzilla to Jira is in process at this time. This will be the last message in Jira copied from the Bugzilla bug.

Comment 3 RHEL Program Management 2023-09-22 17:07:38 UTC

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