Why can't gcc or clang properly @encode SIMD vector types?

Question

While doing some messing around with vector types and the ObjC runtime, I came across a very perplexing problem.

Neither clang or GCC will give the 'proper' type-encoding for any SIMD vector type, as far as I can tell.

#import <Foundation/Foundation.h>

int main() {
    typedef int int4 __attribute__((vector_size(16)));
    typedef float float4 __attribute__((vector_size(16)));

    NSLog(@"Int4: %s", @encode(int4));
    NSLog(@"Float4: %s", @encode(float4));
}

When compiled & run with either GCC or clang, I get the following output:

2014-04-09 06:21:01.102 test[1707:507] Int4:
2014-04-09 06:21:01.103 test[1707:507] Float4:

Instead of what I expect, which would be something like this:

2014-04-09 06:21:01.102 test[1707:507] Int4: ![16,16i]
2014-04-09 06:21:01.103 test[1707:507] Float4: ![16,16f]

As per the documentation here:

vectors      ‘![’ followed by the vector_size (the number of bytes composing the vector) followed by a comma, followed by the alignment (in bytes) of the vector, followed by the type of the elements followed by ‘]’

Which becomes a problem when attempting to return these types from an ObjC method, as instead of getting something logical like ![16,16i]@: for the encoding of the following method:

-(int4) foo;

I only get the string @:, which causes both NSMethodSignature and NSInvocation to essentially crap their pants, and segfault.

Is there some compiler option I can enable to restore the proper encoding of vector types? Or is the only solution here 'don't return vectors from an ObjC method'?

Note: Returning (or passing) a pointer to the vector doesn't help either, as the encoding for a pointer-to-vector is simply ^, which then greedily-matches the next argument in the list, causing my argument count to be off once again. I suppose I could cast the vector to a void *, but that really is an ugly hack at that point.

Answer 1

Looks like this is a known issue, clang has apparently been modified to show a warning in this case: https://github.com/llvm-mirror/clang/commit/024d9c65e9d3887045c82be09e4f630f19da48b4