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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" color="#000000" style="font-size:11pt"><b>From:</b> cfe-dev <cfe-dev-bounces@lists.llvm.org> on behalf of Keane, Erich via cfe-dev <cfe-dev@lists.llvm.org><br>
<b>Sent:</b> Friday, May 15, 2020 9:43 AM<br>
<b>To:</b> Richard Sandiford <richard.sandiford@arm.com>; Keane, Erich via cfe-dev <cfe-dev@lists.llvm.org><br>
<b>Cc:</b> Erich Focht <Erich.Focht@EMEA.NEC.COM>; MARUKAWA KAZUSHI <marukawa@nec.com>; ISHIZAKA KAZUHISA <ishizaka@nec.com><br>
<b>Subject:</b> Re: [cfe-dev] [RFC] Opt-in vector of bool type</font>
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<div class="PlainText">Right, I mentioned that:<br>
> 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.<br>
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<div class="PlainText">I think that this is another thing that falls into the category of: only if we can define the semantics in a way that makes sense. For example, are the addresses of two adjacent vector elements adjacent? Can you do pointer arithmetic
to get from one to the other?</div>
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<div class="PlainText"> -Hal</div>
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-----Original Message-----<br>
From: Richard Sandiford <richard.sandiford@arm.com> <br>
Sent: Friday, May 15, 2020 7:41 AM<br>
To: Keane, Erich via cfe-dev <cfe-dev@lists.llvm.org><br>
Cc: Simon Moll <Simon.Moll@EMEA.NEC.COM>; Craig Topper <craig.topper@gmail.com>; Keane, Erich <erich.keane@intel.com>; MARUKAWA KAZUSHI <marukawa@nec.com>; ISHIZAKA KAZUHISA <ishizaka@nec.com>; Erich Focht <Erich.Focht@EMEA.NEC.COM><br>
Subject: Re: [cfe-dev] [RFC] Opt-in vector of bool type<br>
<br>
"Keane, Erich via cfe-dev" <cfe-dev@lists.llvm.org> writes:<br>
>> 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)).<br>
><br>
> 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.<br>
<br>
FWIW, I think taking the address of a vector_size element is already an error in clang. E.g.<br>
<br>
-----------------------------------------------------------------<br>
typedef unsigned int vec __attribute__((vector_size(16))); vec v; unsigned int *foo(void) { return &v[1]; }<br>
-----------------------------------------------------------------<br>
<br>
gives:<br>
<br>
-----------------------------------------------------------------<br>
foo.c:3:34: error: address of vector element requested unsigned int *foo(void) { return &v[1]; }<br>
^~~~~<br>
1 error generated.<br>
-----------------------------------------------------------------<br>
<br>
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.<br>
<br>
Definitely agree that the semantics need to be spelled out though. :-)<br>
<br>
Thanks,<br>
Richard<br>
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