<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div dir="ltr"></div><div dir="ltr">It seems simpler to map it directly to the existing IR construct (bitcast) instead of the odd IR memcpy. But 🤷‍♂️ </div><div dir="ltr"><br>On Oct 31, 2018, at 3:40 PM, Erik Pilkington <<a href="mailto:erik.pilkington@gmail.com">erik.pilkington@gmail.com</a>> wrote:<br><br></div><blockquote type="cite"><div dir="ltr">
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<div class="moz-cite-prefix">On 10/31/18 3:23 PM, Richard Smith via
cfe-dev wrote:<br>
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<div dir="ltr">On Mon, 29 Oct 2018 at 13:09, JF Bastien via
cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org" moz-do-not-send="true">cfe-dev@lists.llvm.org</a>>
wrote:<br>
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<div style="word-wrap:break-word;line-break:after-white-space">Richard
recently made __builtin_memcpy constexpr in some cases: <a href="https://reviews.llvm.org/rL338941" target="_blank" moz-do-not-send="true">https://reviews.llvm.org/rL338941</a><br>
His patch currently disallows all type-puns.
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<div>Erik is looking at implementing a bit_cast intrinsic
for clang,</div>
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<div>Is there a reason to make it a new builtin (which I think
is what you're suggesting) rather than extending the
existing __builtin_memcpy support to cover all the bit_cast
cases? (Eg, is the idea to make it exactly cover the cases
that std::bit_cast supports?) If we want both, it'd be good
to at least make the two share code.</div>
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No real reason I guess, I suppose that approach would lead to a lot
less boilerplate, so it probably makes more sense. I'll have a
chance to really dig into this in a week or two, so hopefully I'll
can put a patch up soon.<br>
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<div>at which point libc++ can use it. ADT won’t be able
to do so until we migrate to C++20 (and toolchains
support it).</div>
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<div>We also want to reach out to the RedHat maintainers
of libstdc++ to make sure we don’t adopt a weirdly
different builtin.</div>
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<div>On Oct 29, 2018, at 12:56 PM, will wray via
cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org" target="_blank" moz-do-not-send="true">cfe-dev@lists.llvm.org</a>>
wrote:</div>
<br class="m_2445159788289606080Apple-interchange-newline">
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<div>std::bit_cast was added to the C++ 20 draft
this summer:</div>
<div><a href="http://eel.is/c++draft/bit.cast" target="_blank" moz-do-not-send="true">http://eel.is/c++draft/bit.cast</a></div>
<br class="m_2445159788289606080gmail-Apple-interchange-newline">
In September JFB added a header with
implementation (in llvm namespace):<br>
llvm/include/llvm/ADT/bit.h<br>
<br>
// This file implements the C++20 <bit>
header.<br>
...<br>
// - It isn't constexpr because that requires
compiler support.<br>
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<div>What compiler support is it waiting on?
What's the status?</div>
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