From: Jakub Jelinek <jakub@redhat.com>
To: Jonathan Wakely <jwakely@redhat.com>
Cc: Jason Merrill <jason@redhat.com>, Martin Jambor <mjambor@suse.cz>,
Richard Biener <rguenther@suse.de>,
Iain Buclaw <ibuclaw@gdcproject.org>,
gcc-patches@gcc.gnu.org
Subject: Re: [PATCH] c++: Add __builtin_bit_cast to implement std::bit_cast [PR93121]
Date: Thu, 27 Aug 2020 13:17:28 +0200 [thread overview]
Message-ID: <20200827111728.GK2961@tucnak> (raw)
In-Reply-To: <20200827110659.GS3400@redhat.com>
On Thu, Aug 27, 2020 at 12:06:59PM +0100, Jonathan Wakely wrote:
> On 27/08/20 12:46 +0200, Jakub Jelinek wrote:
> > On Thu, Aug 27, 2020 at 12:06:13PM +0200, Jakub Jelinek via Gcc-patches wrote:
> >
> > Oops, rewrote the testcase from __builtin_bit_cast to std::bit_cast without
> > adjusting the syntax properly.
> > Also, let's not use bitfields in there, as clang doesn't support those.
> > So, adjusted testcase below. clang++ rejects all 6 of those, but from what
> > you said, I'd expect that u and z should be well defined.
> >
> > #include <bit>
> >
> > struct S { short a; int b; };
> > struct T { int a, b; };
> >
> > constexpr int
> > foo ()
> > {
> > S a = S ();
> > S b = { 0, 0 };
> > S c = a;
> > S d;
> > S e;
> > d = a;
> > e = S ();
> > int u = std::bit_cast<T> (a).a; // Is this well defined due to value initialization of a?
>
> The standard says that padding bits in the bit_cast result are
> unspecified, so I don't think they have to be copied even if the
> source object was zero-initialized and has all-zero padding bits.
My understanding of
"Padding bits of the To object are unspecified."
is that one shouldn't treat the result of std::bit_cast as if it was e.g.
value initialization followed by member-wise assignment.
But it doesn't say anything about padding bits in the From object.
In the above testcase, T has no padding bits, only S has them.
I think the "Padding bits of the To object are unspecified." should be about:
T t = { 0, 0 };
int s = std::bit_cast<T> (std::bit_cast<S> (t)).a;
being UB, that one can't expect the padding bits in S to have a particular
value.
Jakub
next prev parent reply other threads:[~2020-08-27 11:17 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-18 18:50 Jakub Jelinek
2020-07-22 14:03 ` Paul Koning
2020-07-30 14:16 ` Jason Merrill
2020-07-30 14:57 ` Jakub Jelinek
2020-07-30 21:59 ` Jason Merrill
2020-07-31 8:19 ` Jakub Jelinek
2020-07-31 9:54 ` Jonathan Wakely
2020-07-31 10:06 ` Jakub Jelinek
2020-07-31 20:28 ` Jason Merrill
2020-08-27 10:06 ` Jakub Jelinek
2020-08-27 10:19 ` Richard Biener
2020-09-02 21:52 ` Jason Merrill
2020-08-27 10:46 ` Jakub Jelinek
2020-08-27 11:06 ` Jonathan Wakely
2020-08-27 11:17 ` Jakub Jelinek [this message]
2020-08-27 11:22 ` Jonathan Wakely
2020-08-27 10:59 ` Jonathan Wakely
2020-08-27 20:43 ` Iain Buclaw
2020-11-02 19:21 ` [PATCH] c++: v2: " Jakub Jelinek
2020-11-25 0:31 ` Jeff Law
2020-11-25 9:23 ` Jakub Jelinek
2020-11-25 10:31 ` Jonathan Wakely
2020-11-25 16:24 ` Jakub Jelinek
2020-11-25 16:28 ` Jonathan Wakely
2020-11-25 17:26 ` Jason Merrill
2020-11-25 18:50 ` Jakub Jelinek
2020-11-26 0:52 ` Jason Merrill
2020-11-26 15:09 ` [PATCH] c++: v3: " Jakub Jelinek
2020-12-03 14:24 ` Jason Merrill
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