APX vs "length changing prefix"
Project / Subsystem
binutils / binutils
Date
2026-09-07
Proposer
"Jiang, Haochen" <haochen.jiang@intel.com>
Source type
public_inbox
Consensus
Proposed
Sentiment
—/10
Technical tradeoffs
- • Potential performance penalties for using operand size prefixes with EVEX encoding.
- • Code size may increase if prefixes are used to achieve smaller immediates.
- • Requires careful consideration by developers optimizing for APX.
All attributes
- project
- binutils
- subsystem
- binutils
- patch_id
- —
- discussion_id
- SJ5PPF77D28E3C25DC59AE37A1283402ABEECB22@SJ5PPF77D28E3C2.namprd11.prod.outlook.com
- source_type
- public_inbox
- title
- APX vs "length changing prefix"
- headline
- APX instruction encoding and performance penalty
- tldr
- Intel clarifies that APX EVEX-encoded instructions with operand size prefixes may incur performance penalties.
- proposer
- "Jiang, Haochen" <haochen.jiang@intel.com>
- consensus
- Proposed
- outcome
- proposed
- sentiment_score
- —
- technical_tradeoffs
-
- • Potential performance penalties for using operand size prefixes with EVEX encoding.
- • Code size may increase if prefixes are used to achieve smaller immediates.
- • Requires careful consideration by developers optimizing for APX.
- series_id
- —
- series_role
- standalone
- series_parts
- []
- tags
-
- • APX
- • x86
- • performance
- • instruction encoding
- bugzilla_url
- —
- date
- 2026-09-07T00:00:00.000Z
APX vs "length changing prefix"
This discussion clarifies a performance aspect of Intel’s APX (Advanced Performance Extensions) instruction set. A reviewer inquired whether EVEX-encoded instructions, similar to legacy encodings, would suffer performance penalties if operand size prefixes change the immediate size. The response confirms that overriding the default operand size, which changes the overall instruction length, can indeed lead to a penalty.