[LLVMdev] Vectors in structures

[Alasdair Grant]

Bob Wilson writes:
> On Sep 21, 2010, at 9:33 AM, Renato Golin wrote:
> > I was checking NEON instructions this week and the vector types seem
> > to be inside structures. If vector types are considered proper types
> > in LLVM, why pack them inside structures?
>
> Because that is what ARM has specified?  They define the vector types
> that are used with their NEON intrinsics as "containerized vectors".
> Perhaps someone on the list from ARM can explain why they did it that
> way.

"Containerized Vector" in the ARM AAPCS refers to fundamental data
types (machine types), it's the class of machine types comprising
the 64-bit and 128-bit NEON machine types.

The AAPCS defines how to pass containerized vectors and it defines
how the NEON user types map on to them.  Also it defines how to
mangle the NEON user types.  So it defines how to use NEON user
types in a public binary interface.

It also says that arm_neon.h "defines a set of internal structures
that describe the short vector types" which I guess could be read
as saying they are packed inside structures - but I don't think this
is the intention and it doesn't match implementations.
The arm_neon.h implementation in the ARM compiler defines the user
types in terms of C structs called __simd64_int8_t etc. and the
mangling originated as an artifact of this.  But the C structs aren't
wrapped vectors; they wrap double or a pair of doubles, to get the
size and alignment.  Their only purpose is to be recognized by name
by the front end and turned into a native register type.
In gcc's arm_neon.h the user types aren't structs at all, they're
defined using vector_size and the mangling is done as a special case.

So I think there's no need to wrap these types in LLVM.

Al

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