[cfe-dev] [RFC] Opt-in vector of bool type

[Keane, Erich via cfe-dev]

Right, I mentioned that:
> Note we don't allow taking a non-const ref or address of a vector element, but  GCC does, though presumably that is something we should fix.



-----Original Message-----
From: Richard Sandiford <richard.sandiford at arm.com> 
Sent: Friday, May 15, 2020 7:41 AM
To: Keane, Erich via cfe-dev <cfe-dev at lists.llvm.org>
Cc: Simon Moll <Simon.Moll at EMEA.NEC.COM>; Craig Topper <craig.topper at gmail.com>; Keane, Erich <erich.keane at intel.com>; MARUKAWA KAZUSHI <marukawa at nec.com>; ISHIZAKA KAZUHISA <ishizaka at nec.com>; Erich Focht <Erich.Focht at EMEA.NEC.COM>
Subject: Re: [cfe-dev] [RFC] Opt-in vector of bool type

"Keane, Erich via cfe-dev" <cfe-dev at lists.llvm.org> writes:
>> The way i see it you are open to supporting this feature in Clang but there are LLVM bugs for <N x i1> types, which we may hit more often as a result, and then there is this unrelated Clang lvalue bug for attribute((vector_size)).
>
> I don't take this as a proper summary of my position. I was warning you about the issues in LLVM, however the biggest issue is the fact that a vector of i1s isn't individually addressable.  Unless you have a way to produce an address for each individual element (which we don't, and is why std::vector<bool> uses a proxy return type), I don't think this fits in the type system.

FWIW, I think taking the address of a vector_size element is already an error in clang.  E.g.

-----------------------------------------------------------------
typedef unsigned int vec __attribute__((vector_size(16))); vec v; unsigned int *foo(void) { return &v[1]; }
-----------------------------------------------------------------

gives:

-----------------------------------------------------------------
foo.c:3:34: error: address of vector element requested unsigned int *foo(void) { return &v[1]; }
                                 ^~~~~
1 error generated.
-----------------------------------------------------------------

GCC does accept this, but like others have said, I think the natural thing would be to make it an error for bools even if it isn't for other types.  The set of operations supported by a vector type is inevitably going to be influenced by the element type.

Definitely agree that the semantics need to be spelled out though. :-)

Thanks,
Richard