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[SME] Add __arm_agnostic("sme_za_state") keyword attribute #336

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[SME] Add __arm_agnostic("sme_za_state") keyword attribute #336

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[SME] Add __arm_agnostic("sme_za_state") keyword attribute
sdesmalen-arm Jul 26, 2024
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sdesmalen-arm Sep 3, 2024
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42 changes: 41 additions & 1 deletion main/acle.md
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Original file line number Diff line number Diff line change
Expand Up @@ -400,6 +400,7 @@ Armv8.4-A [[ARMARMv84]](#ARMARMv84). Support is added for the Dot Product intrin
* Added a requirement for function version declaration in Function Multi Versioning.
* Fixed some rendering issues in the online Markdown documentation and fixed
a misplaced anchor.
* Added [`__arm_agnostic`](#arm_agnostic) keyword attribute

### References

Expand Down Expand Up @@ -826,6 +827,7 @@ predefine the associated macro to a nonzero value.

| **Name** | **Target** | **Predefined macro** |
| ----------------------------------------------------------- | --------------------- | --------------------------------- |
| [`__arm_agnostic`](#arm_agnostic) | function type | `__ARM_FEATURE_SME` |
| [`__arm_locally_streaming`](#arm_locally_streaming) | function declaration | `__ARM_FEATURE_LOCALLY_STREAMING` |
| [`__arm_in`](#ways-of-sharing-state) | function type | Argument-dependent |
| [`__arm_inout`](#ways-of-sharing-state) | function type | Argument-dependent |
Expand Down Expand Up @@ -4836,6 +4838,30 @@ if such a restoration is necessary. For example:
}
```

## `__arm_agnostic`

A function with the `__arm_agnostic` [keyword attribute](#keyword-attributes)
must preserve the architectural state that is specified by its arguments when
such state exists at runtime. The function is otherwise unconcerned with this
state.

The `__arm_agnostic` [keyword attribute](#keyword-attributes) applies to
**function types** and accepts the following arguments:

```"sme_za_state"```

* This attribute affects the ABI of a function, which must implement an
[agnostic-ZA interface](#agnostic-za). It is the compiler's responsibility
to ensure that the function's object code honors the ABI requirements.

* The use of `__arm_agnostic("sme_za_state")` allows writing functions that
are compatible with ZA state without having to share ZA state with the
caller, as required by `__arm_preserves`.

* It is not valid for a function declaration with
`__arm_agnostic("sme_za_state")` to [share](#shares-state) PSTATE.ZA state
with its caller.

## Mapping to the Procedure Call Standard

[[AAPCS64]](#AAPCS64) classifies functions as having one of the following
Expand All @@ -4847,6 +4873,10 @@ interfaces:

* a “shared-ZA” interface

<span id="agnostic-za"></span>

* a "agnostic-ZA" interface

If a C or C++ function F forms part of the object code's ABI, that
object code function has a shared-ZA interface if and only if at least
one of the following is true:
Expand All @@ -4855,7 +4885,17 @@ one of the following is true:

* F shares ZT0 with its caller

All other functions have a private-ZA interface.
All other functions have either a private-ZA or an agnostic-ZA interface.

If F implements an agnostic-ZA interface and PSTATE.ZA is available at runtime,
then a call to F must return with its ZA state unchanged in accordance
to the [[AAPCS64]](#AAPCS64). In practice this means that calls to F don't have
to emit code to set up a lazy-save for ZA or to preserve other state like ZT0
when such state is live at the call site.

The implementation of F must not make any assumptions on the availability of
PSTATE.ZA or any architectural state associated with it.


## Function definitions

Expand Down