[PATCH] D127762: [Clang][AArch64] Add ACLE attributes for SME.

Tue Jun 21 11:52:12 PDT 2022

aaron.ballman added a comment.

In D127762#3589347 <https://reviews.llvm.org/D127762#3589347>, @sdesmalen wrote:

> Thanks @aaron.ballman for your elaborate review, that was very helpful! I'm still working through some of your suggestions (although some of them weren't entirely clear to me), but I've addressed a number of them already.
>
> In D127762#3582753 <https://reviews.llvm.org/D127762#3582753>, @aaron.ballman wrote:
>
>>> This patch adds all the language-level function attributes defined in:
>>>
>>> https://github.com/ARM-software/acle/pull/188
>>
>> This pull request has not yet been merged, so what are the chances that it gets rejected or altered?
>
> There has already been quite some discussion on the draft both online and offline, and we already started to implement it to get confidence that there's no big flaws in its design. Other than perhaps some minor changes, I expect it to land in its current form.

Good to know, thank you for the details!

>>> LLVM support for these attributes will follow later, so at the moment these attributes are non-functional.
>>
>> FWIW, I would rather we land the attributes once they're functional even if we do a bunch of the review work up front. While rare, we have run into the situation where attributes get added with intentions to make them useful later but then something comes up and we're left with half the implementation exposed to users.
>
> I think that's fair. We'll share LLVM support for these attributes shortly, but I'm happy to wait landing this work until the LLVM support has landed. It would be nice if I can get this patch in a state where you and other reviewers are happy with it, so that it only needs a quick rebase once the LLVM support is there.

SGTM!

================
Comment at: clang/include/clang/Basic/Attr.td:2322

+def ArmStreamingCompatible : DeclOrTypeAttr, TargetSpecificAttr<TargetAArch64> {
+  let Spellings = [Clang<"arm_streaming_compatible">];
----------------
sdesmalen wrote:
> aaron.ballman wrote:
> > `DeclOrTypeAttr` is only intended to be used for attributes which were historically a declaration attribute but are semantically type attributes (like calling conventions). It seems to me that each of these is purely a type attribute and we shouldn't allow them to be written in the declaration position. WDYT?
> In this case, I believe the attribute is valid on both the type and the declaration, because the SME ACLE explicitly specifies that the attributes can be applied to both declarations and types.
> 
> The [[https://github.com/ARM-software/acle/pull/188/commits/ce878cf6c43fd824ffc634526e5dfec1ddc1839e#diff-516526d4a18101dc85300bc2033d0f86dc46c505b7510a7694baabea851aedfaR8283-R8294|specification]] says:
> ```Some of the attributes described in this section apply to function types.
> Their semantics are as follows:
> 
> *   If the attribute is attached to a function declaration or definition,
>     it applies to the type of the function.
> 
> *   If the attribute is attached to a function type, it applies to that type.
> 
> *   If the attribute is attached to a pointer to a function type, it applies
>     to that function type.
> 
> *   Otherwise, the attribute is [ill-formed](#ill-formed).```
> In this case, I believe the attribute is valid on both the type and the declaration, because the SME ACLE explicitly specifies that the attributes can be applied to both declarations and types.

What are the chances that can change? Because there are problems here:

> If the attribute is attached to a function declaration or definition, it applies to the type of the function.

This is egregiously opposed to the design of `[[]]` attributes in both C and C++. We came up with `DeclOrTypeAttr` for attributes that previously existed, but that is different than introducing new attributes. It's par for the course for `__attribute__` style attributes, so I don't worry so much there.

What's the rationale for this confusing of a design? (e.g., is there some reason you basically have to do that, like historically accepting the attributes in both positions?)

================
Comment at: clang/lib/Sema/SemaType.cpp:7676
+      return false;
+    }
+
----------------
sdesmalen wrote:
> aaron.ballman wrote:
> > Unfortunately, type attributes do not yet have any of the automated checking generated by tablegen, so there are no calls to `Sema::checkCommonAttributeFeatures()`, which means you have to manually check things like mutual exclusions, not accepting arguments, etc.
> After some experimenting I found that:
> * When I add the `MutualExclusions` to Attr.td and compile `typedef __attribute__((arm_streaming, arm_streaming_compatible)) void (*ptrty1) (void);`, I still get the diagnostic for conflicting attributes 'for free', even without any code being added here to check for conflicting attributes. However, when I then apply it to a declaration (e.g. `void __attribute__((arm_streaming, arm_streaming_compatible)) foo(void);`), I get the diagnostics twice.
> * When I add some code here to mark the attribute as invalid, I get no diagnostic whatsoever unless I explicitly emit it here, rendering the use of `MutualExclusions` in Attr.td ineffective.
> 
> Based on the above observation, I've chosen to keep the code in SemaDeclAttr.cpp and not use the `MutualExclusions`, because it generates the best diagnostics (an error + a note) and only emits it once. Let me know what you think.
Yeah, I can see why we'd hit that problem -- when we go to form the function type to be able to make the declaration, we'd issue the warning once, and then after forming the declaration and trying to apply attributes to it, we'd issue the warning a second time. We do not have any other `DeclOrTypeAttr` which has mutual exclusions.

I don't want to lose the declarative nature of using `MutualExclusions` in Attr.td unless there's no other choice; that's a helpful piece of documentation to reviewers when reviewing additional attributes in the future. If you elect to keep the attribute as a declaration and type attribute, I think you'll need to try to fix this to work properly.

================
Comment at: clang/test/Sema/aarch64-sme-func-attrs.c:89
+// expected-note at +2 {{conflicting attribute is here}}
+__attribute__((arm_preserves_za, arm_new_za)) void preserves_new_za(void);
+
----------------
sdesmalen wrote:
> aaron.ballman wrote:
> > If you keep the attribute as a declaration attribute, you're also missing tests like:
> > ```
> > __attribute__((arm_new_za)) void func(void);
> > __attribute__((arm_preserves_za)) void func(void) {}
> > ```
> > 
> I'm not sure what you mean, there are tests for:
> * `__attribute__((arm_new_za)) void sme_arm_new_za(void);`
> * `__attribute__((arm_preserves_za)) void sme_arm_preserves_za(void);`
> ?
Those test applying mutually exclusive attributes to unique declarations, but I was asking for applying mutually exclusive attributes to redeclarations.

================
Comment at: clang/test/Sema/aarch64-sme-func-attrs.c:107
+typedef __attribute__((arm_new_za, arm_shared_za)) void (*fptrty9) (void);
+fptrty9 invalid_shared_za_func() { return shared_za_ptr_invalid; }
+
----------------
sdesmalen wrote:
> aaron.ballman wrote:
> > These are all being used as declaration attributes, I think you also need a test like:
> > ```
> > void (*fp)(void) [[clang::arm_new_za]] [[clang::arm_preserves_za]];
> > ```
> > 
> What exactly do you mean with 'declaration attributes' here? Are these not added to the type `fptrty9` in the test above?
> 
> Or is that the difference between having
> 
>   typedef __attribute__((arm_new_za, arm_shared_za)) void (*fptrty9) (void);
> 
> and
> 
>   typedef void (*fptrty9) (void) __attribute__((arm_new_za, arm_shared_za));
> 
> ?
> What exactly do you mean with 'declaration attributes' here? Are these not added to the type fptrty9 in the test above?

I had misread the code as if the attribute was being applied to the function declaration on the next line down. They are added to the type.

> Or is that the difference between having

No difference with the GNU spelling, sorry for the noise!

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