Add FMV priority syntax #404

AlfieRichardsArm · 2025-07-15T07:27:43Z

Hi all,

This is a PR to implement option 3 of #403 proposed by @labrinea .

It adds the proposed syntax, with higher priority values taking precedence.

Happy to modify wording if needed.

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AlfieRichardsArm · 2025-07-15T07:32:21Z

Lint failure is due to https://allcontributors.org/docs/en/emoji-key being dead at the moment. Not relevant to the change.

main/acle.md

labrinea · 2025-07-23T09:06:44Z

main/acle.md

+The precedence for two target version strings is determined by:
+
+1. If both target version strings specify a `priority`, with different values,
+then the version with the higher `priority` has precedence.


@jroelofs if I read your comment #371 (comment) correctly you'd also prefer higher number = higher priority?

labrinea · 2025-07-23T09:08:11Z

main/acle.md

+                            | <version string>
+<version string>           := <priority string> ';' <arch strings>
+                            | <arch strings>
+<priority string>          := 'priority=[1-31]'


@jroelofs with llvm/llvm-project#150079 we have plenty of room to go beyond 5 bits of priority. Any preference?

I personally find 6 bits the maximum sensible, but I am also happy with 5 or less.

main/acle.md

This addresses the changes requested by Chris, most importantlt the ordering of the priority string and the arch string. Otherwise its working nits.

AlfieRichardsArm · 2025-08-12T15:12:17Z

Addressed Chris' comments. Unless there's any more discussion I'd like to move towards merging this soon?

labrinea

Looks good to me.

Implements ARM-software/acle#404 This allows the user to specify "priority=[1-255];featA+featB" where priority=255 means highest priority. If the explicit priority string is omitted then the priority of "featA+featB" is implied, which is lower than priority=1. Internally this gets expanded using special FMV features P0 ... P7 which can encode up to 256-1 priority levels (excluding all zeros). Those do not have corresponding detection bit at pos FEAT_#enum so I made this field optional in FMVInfo. Also they don't affect the codegen or name mangling of versioned functions.

vhscampos · 2025-08-14T09:49:10Z

Thanks for the patch. Let's wait for further comments until end of tomorrow before we merge it.

sallyarmneale · 2025-11-03T19:45:10Z

main/acle.md

 * Added `svdot[_n_f16_mf8]_fpm` and `svdot[_n_f32_mf8]_fpm`.
 * Added Guarded Control Stack (GCS) at
  [**Beta**](#current-status-and-anticipated-changes) quality level.
+* Add Function Multi Versioning feature priority syntax.


Add Function Multi Versioning feature priority syntax.
Change to Added

Thanks Sally. I'm going to fix this.

sallyarmneale

LGTM - apart from one very minor change

Implements ARM-software/acle#404 This allows the user to specify "featA+featB;priority=[1-255]" where priority=255 means highest priority. If the explicit priority string is omitted then the priority of "featA+featB" is implied, which is lower than priority=1. Internally this gets expanded using special FMV features P0 ... P7 which can encode up to 256-1 priority levels (excluding all zeros). Those do not have corresponding detection bit at pos FEAT_#enum so I made this field optional in FMVInfo. Also they don't affect the codegen or name mangling of versioned functions.

…150267) Implements ARM-software/acle#404 This allows the user to specify "featA+featB;priority=[1-255]" where priority=255 means highest priority. If the explicit priority string is omitted then the priority of "featA+featB" is implied, which is lower than priority=1. Internally this gets expanded using special FMV features P0 ... P7 which can encode up to 256-1 priority levels (excluding all zeros). Those do not have corresponding detection bit at pos FEAT_#enum so I made this field optional in FMVInfo. Also they don't affect the codegen or name mangling of versioned functions.

Add FMV priority syntax

5875cd5

labrinea reviewed Jul 21, 2025

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labrinea reviewed Jul 23, 2025

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