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<p class="MsoNormal">I’m not sure, I haven’t experimented with GCC enough to know.
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<p class="MsoNormal"><b>From:</b> Finkel, Hal J. <hfinkel@anl.gov> <br>
<b>Sent:</b> Friday, May 15, 2020 8:54 AM<br>
<b>To:</b> Richard Sandiford <richard.sandiford@arm.com>; Keane, Erich via cfe-dev <cfe-dev@lists.llvm.org>; Keane, Erich <erich.keane@intel.com><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<o:p></o:p></p>
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<p class="MsoNormal"><b><span style="color:black">From:</span></b><span style="color:black"> cfe-dev <<a href="mailto:cfe-dev-bounces@lists.llvm.org">cfe-dev-bounces@lists.llvm.org</a>> on behalf of Keane, Erich via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org">cfe-dev@lists.llvm.org</a>><br>
<b>Sent:</b> Friday, May 15, 2020 9:43 AM<br>
<b>To:</b> Richard Sandiford <<a href="mailto:richard.sandiford@arm.com">richard.sandiford@arm.com</a>>; Keane, Erich via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org">cfe-dev@lists.llvm.org</a>><br>
<b>Cc:</b> Erich Focht <<a href="mailto:Erich.Focht@EMEA.NEC.COM">Erich.Focht@EMEA.NEC.COM</a>>; MARUKAWA KAZUSHI <<a href="mailto:marukawa@nec.com">marukawa@nec.com</a>>; ISHIZAKA KAZUHISA <<a href="mailto:ishizaka@nec.com">ishizaka@nec.com</a>><br>
<b>Subject:</b> Re: [cfe-dev] [RFC] Opt-in vector of bool type</span><span style="color:#666666">
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<p class="MsoNormal"><span style="color:#666666">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.<o:p></o:p></span></p>
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<p class="MsoNormal">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?<o:p></o:p></p>
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<p class="MsoNormal"> -Hal<o:p></o:p></p>
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<br>
-----Original Message-----<br>
From: Richard Sandiford <<a href="mailto:richard.sandiford@arm.com">richard.sandiford@arm.com</a>>
<br>
Sent: Friday, May 15, 2020 7:41 AM<br>
To: Keane, Erich via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org">cfe-dev@lists.llvm.org</a>><br>
Cc: Simon Moll <<a href="mailto:Simon.Moll@EMEA.NEC.COM">Simon.Moll@EMEA.NEC.COM</a>>; Craig Topper <<a href="mailto:craig.topper@gmail.com">craig.topper@gmail.com</a>>; Keane, Erich <<a href="mailto:erich.keane@intel.com">erich.keane@intel.com</a>>; MARUKAWA
KAZUSHI <<a href="mailto:marukawa@nec.com">marukawa@nec.com</a>>; ISHIZAKA KAZUHISA <<a href="mailto:ishizaka@nec.com">ishizaka@nec.com</a>>; Erich Focht <<a href="mailto:Erich.Focht@EMEA.NEC.COM">Erich.Focht@EMEA.NEC.COM</a>><br>
Subject: Re: [cfe-dev] [RFC] Opt-in vector of bool type<br>
<br>
"Keane, Erich via cfe-dev" <<a href="mailto:cfe-dev@lists.llvm.org">cfe-dev@lists.llvm.org</a>> 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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