[SME] Add __arm_agnostic("sme_za_state") keyword attribute #336
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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.
main/acle.md
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| @@ -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` | | |||
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Nit: would be good to keep the entries sorted by name.
main/acle.md
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| function has a “ZA-compatible interface”; see [[AAPCS64]](#AAPCS64) | ||
| for more details. | ||
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| * It is not valid for a function declaration with `__arm_agnostic("sme_za_state")` |
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__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.
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| ```"sme_za_state"``` | ||
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| * If the function is defined and PSTATE.ZA is available, the definition must |
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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
The
__arm_agnostickeyword 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_compatiblewas so that we might want use the attribute keyword for other architectural state in the future.name: Pull request
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