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

sdesmalen-arm · 2024-07-26T16:18:50Z

The __arm_agnostic keyword attribute enables the user to specify that a function is agnostic to a specified piece of architectural state. That means that the function must preserve this state when it exists, or otherwise ignores its contents.

The reason for not naming this something like __arm_za_compatible was so that we might want use the attribute keyword for other architectural state in the future.

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The __arm_agnostic keyword attribute enables the user to specify that a function is agnostic to a specified piece of architectural state. That means that the function must preserve this state when it exists, or otherwise ignores its contents. The reason for not naming this something like '__arm_za_compatible' was so that we might want use the attribute keyword for other architectural state in the future.

sdesmalen-arm · 2024-07-26T16:25:39Z

@rsandifo-arm

rsandifo-arm

The patch should also update the “Inline assembly” section, since ZA can now be “on” on entry to an inline asm in cases that the current text doesn't cover.

rsandifo-arm · 2024-08-05T15:11:49Z

main/acle.md

@@ -832,6 +833,7 @@ predefine the associated macro to a nonzero value.
 | [`__arm_new`](#arm_new)                                     | function declaration  | Argument-dependent                |
 | [`__arm_out`](#ways-of-sharing-state)                       | function type         | Argument-dependent                |
 | [`__arm_preserves`](#ways-of-sharing-state)                 | function type         | Argument-dependent                |
+| [`__arm_agnostic`](#arm_agnostic)                           | function type         | `__ARM_FEATURE_SME`               |


Nit: would be good to keep the entries sorted by name.

rsandifo-arm · 2024-08-05T15:26:40Z

main/acle.md

+    function has a “ZA-compatible interface”; see [[AAPCS64]](#AAPCS64)
+    for more details.
+
+*   It is not valid for a function declaration with `__arm_agnostic("sme_za_state")`


__arm_new is only meaningful for definitions, not declarations. I'm not sure it should be a semantic requirement, since:

__arm_new("za") void foo(void) …stuff… { … }

isn't semantically different from:

__arm_new("za") void foo1(void) { … } void foo(void) …stuff… { foo1(); }

(It's reasonable for an implementation to say that it doesn't support __arm_new for now though. This would be the equivalent of a GCC “sorry” diagnostic, but it'd be an implementation choice rather than something that the ACLE should document.)

I think we can then say:

It is not valid … to [share](#shares-state) PSTATE.ZA state with its caller.

rsandifo-arm · 2024-08-05T15:32:53Z

main/acle.md

+
+```"sme_za_state"```
+
+*   If the function is defined and PSTATE.ZA is available, the definition must


Since the AAPCS64 patch (rightly) creates a three-way choice between private, shared & agnostic, we should update the description under “Mapping to the Procedure Call Standard” section. I think the text here should then say something like:

This attribute affects the ABI of object code functions, as described under … It is the compiler's responsibility to ensure that such object code functions honor the ABI requirements.

and then delete the following two bullet points. AFAICT, the attribute has no effect on the behaviour of the abstract machine outside of inline asms.

This implements the lowering of calls from agnostic-ZA functions to non-agnostic-ZA functions, using the ABI routines `__arm_sme_state_size`, `__arm_sme_save` and `__arm_sme_restore`. This implements the proposal described in the following PRs: * ARM-software/acle#336 * ARM-software/abi-aa#264

This adds support for parsing the attribute and codegen to map it to "aarch64_za_state_agnostic" LLVM IR attribute. This is proposed in the following PR: ARM-software/acle#336

rsandifo-arm

The previous comment about updating the inline asm documentation still stands.

rsandifo-arm · 2024-12-12T16:06:04Z

main/acle.md

@@ -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.


I think we should leave this sentence as-is and describe the agnostic-ZA case before it. I'd recomment restructuring the text as follows:

If a C or C++ function F forms part of the object code's ABI: * the object code function has a shared-ZA interface if and only if at least one of the following is true: * F shares ZA with its caller * F shares ZT0 with its caller * the object code function has an agnostic-ZA interface if and only if F's type has an `__arm_agnostic("sme_za_state")` attribute. All other functions have a private-ZA interface.

rsandifo-arm · 2024-12-12T16:06:18Z

main/acle.md

@@ -4847,6 +4873,10 @@ interfaces:

 *   a “shared-ZA” interface

+<span id="agnostic-za"></span>
+
+*   a "agnostic-ZA" interface


Suggested change

* a "agnostic-ZA" interface

* an "agnostic-ZA" interface

rsandifo-arm · 2024-12-12T16:07:27Z

main/acle.md

+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


I think we should instead add a note to the attribute documentation to say that it does not imply that SME is available.

#264) This PR adds a new "agnostic-ZA" interface which is intended to be called from any subroutine without requiring a change to PSTATE.ZA. This PR also adds new SME ABI routines to save/restore state enabled by PSTATE.ZA. The corresponding ACLE PR can be found [here](ARM-software/acle#336).

rsandifo-arm reviewed Aug 5, 2024

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vhscampos added the proposal label Sep 12, 2024

sdesmalen-arm mentioned this pull request Dec 2, 2024

[SME] Add agnostic-ZA interface and routines to save/restore SME state ARM-software/abi-aa#264

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

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

sdesmalen-arm commented Jul 26, 2024

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rsandifo-arm Dec 12, 2024


		```"sme_za_state"```

		* If the function is defined and PSTATE.ZA is available, the definition must

[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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