About

These are tasks carried forward from 2H2023 and some additional work identified in 2024

The extension-dce work that was started in 2H2024. This is a new optimization pass which is designed to track liveness of the result of sign/zero extension instructions. If the bits set by a sign/zero extension are never read and never escape (implicitly read), then the extension is useless and can potentially be removed. This is expected to improve x264's scalar implementation by roughly 1%. It is also expected to be a very small improvement to the coremark benchmark. Other codes show very small improvements. This code was originally targeted at gcc-14 (Spring 2024), but it was decided to wait rather than push integration of the code well after feature freeze.

Stakeholders/Partners

RISE:

Ventana: 1 FTE Jivan Hakobyan and Jeff Law

Rivos: Vineet (part time)

External:

The original idea for and implementation of the ext-dce pass was from Joern Rennecke at EMBECOSM.

Dependencies

Status

Development	COMPLETE
Development Timeline	1H2024
Upstreaming	COMPLETED
Upstream Version	gcc-15 Spring 2025
Contacts	Jeff Law (Ventana)
Dependencies	None

Updates

08 Jul 2024

New ext-dce pass has been integrated.

29 Jun 2024

Breaking out additional items as 2H2024 work

05 Apr 2024

Note additional case we should add to ext-dce.
Note Jivan has taken care of extensions following fp→int conversions. It will be submitted once gcc-15 is open for development.

14 Mar 2024

Note the two additional identified opportunities to remove redundant sign extensions. Change development state to in-progress.
Added yet another idea for ABI defined extension elimination in REE.

29 Jan 2024

Carried over unfinished items from 2H2023 plan
Jivan's patch for catching corner cases of promoted objects integrated into tester

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