[PATCH] D91262: [AArch64][SVE] Allow C-style casts between fixed-size and scalable vectors

[Cullen Rhodes via Phabricator via cfe-commits]

c-rhodes added a comment.

@joechrisellis thanks for the patch Joe, I've added a few comments. I also noticed this only covers C++, do we want to test C as well?



================
Comment at: clang/lib/Sema/SemaCast.cpp:2222-2227
+    // Allow bitcasting if either the source or destination is a scalable
+    // vector.
+    if (SrcType->isSizelessBuiltinType() || DestType->isSizelessBuiltinType()) {
+      Kind = CK_BitCast;
+      return TC_Success;
+    }
----------------
This is a bit ambiguous, it'll allow bitcasting between things like a fixed predicate and any sizeless type which I don't think makes sense. I think it'll also allow bitcasting between any scalable and GNU vectors regardless of vector length, e.g.:

```
typedef int32_t int32x4_t __attribute__((vector_size(16)));

void foo() {
svint32_t s32;
int32x4_t g32;
g32 = (int32x4_t)s32;
s32 = (svint32_t)g32;
}
```

the above should only work when `-msve-vector-bits=128` but will currently be allowed for any N.


================
Comment at: clang/test/Sema/aarch64-sve-explicit-casts-fixed-size.cpp:1
+// RUN: %clang_cc1 -triple aarch64-none-linux-gnu -target-feature +sve -msve-vector-bits=512 -flax-vector-conversions=none -fallow-half-arguments-and-returns -ffreestanding -fsyntax-only -verify %s
+
----------------
C++ sema tests should be under `clang/test/SemaCXX/`


================
Comment at: clang/test/Sema/aarch64-sve-explicit-casts.cpp:1-36
+// RUN: %clang_cc1 -triple aarch64-none-linux-gnu -target-feature +sve -msve-vector-bits=512 -flax-vector-conversions=none -fallow-half-arguments-and-returns -ffreestanding -fsyntax-only -verify %s
+
+#include <arm_sve.h>
+
+// SVE types cannot be C-style casted to one another.
+// "To avoid any ambiguity [between the two operations], the ACLE does not allow C-style casting from one
+// vector type to another." ~ACLE Section 3.4 (Vector Types)
----------------
`clang/test/SemaCXX/sizeless-1.cpp` already contains a test checking C-style casts between distinct sizeless types aren't supported, I'm not sure if adding this is necessary.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D91262/new/

https://reviews.llvm.org/D91262