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

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman added a comment.

In D127762#4102436 <https://reviews.llvm.org/D127762#4102436>, @erichkeane wrote:

> Finally, and this is something @aaron.ballman probably wants to answer:  Is this sufficiently important that we're willing to take the additional overhead of 8 bits for each function type (plus an extra 8 bits for its prototype?).  Memory wise, this seems a little costly.

It'd be nice to have a rough idea of what the overhead costs are. I think we can steal a few bits back to reduce memory overhead if necessary.



================
Comment at: clang/include/clang/AST/Type.h:4123
     bool HasTrailingReturn : 1;
+    unsigned AArch64SMEAttributes : 8;
     Qualifiers TypeQuals;
----------------
We could steal two bits back here and make this a 6-bit bit-field (we'd probably want to adjust `SME_AttributeMask` accordingly as well).


================
Comment at: clang/include/clang/Basic/AttrDocs.td:6322
+
+Only when Clang defines __ARM_FEATURE_SME, the support for this feature is
+considered to be stable and complete.
----------------
aaron.ballman wrote:
> Double-checking: that macro is missing the trailing double underscore, is that expected?
Still wondering on this.


================
Comment at: clang/lib/Sema/SemaDecl.cpp:3746-3753
+  // It is not allowed to redeclare an SME function with different SME
+  // attributes.
+  if (IsInvalidSMECallConversion(Old->getType(), New->getType())) {
+    Diag(New->getLocation(), diag::err_sme_attr_mismatch)
+        << New->getType() << Old->getType();
+    Diag(OldLocation, diag::note_previous_declaration);
+    return true;
----------------
sdesmalen wrote:
> aaron.ballman wrote:
> > How should these work with template (partial) specializations? Should it be invalid to write this on the primary template? Should it be invalid if the specialization doesn't match the primary template?
> Can you share an example of what you're thinking of?
> 
> Do you mean something like:
> 
>   template<typename T> void (*a)(T, T);
>   template<> __attribute__((arm_streaming)) void (*a<int>)(int, int);
> 
> where calling `a<short>(0, 0)` and `a<int>(0, 0)` have different streaming-mode behaviour? If so, then that is the intent. The SME attributes behave similar to a calling convention attributes, with the difference that more than one attribute can be applied.
Yup, that's the scenario I was wondering about -- thanks for the answer (and I see test coverage for the situation as well).


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https://reviews.llvm.org/D127762