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Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift |
Date: |
Fri, 2 Oct 2015 10:48:43 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 |
On 01/10/2015 20:40, Laszlo Ersek wrote:
> > Also strict aliasing improves performance noticeably at least on some
> > kind of code. The relaxation of strict aliasing that GCC does with
> > unions would be a useful addition to the C standard, though.
>
> What do you mean under "relaxation of strict aliasing that GCC does with
> unions"? I believe I know how unions affect this (although for details
> I'd obviously have to consult the standard :)), but what are the gcc
> specific parts?
I remembered wrong---it's not a relaxation of strict aliasing, it's
defining what happens when a member of a union is accessed through a
member of a different type. C89 makes that implementation-defined
(3.3.2.3) and GCC defines the behavior the sane way: "the relevant bytes
of the representation of the object are treated as an object of the type
used for the access".
C99 makes the other members "take unspecified values" (6.2.6.1). I have
always thought this to be weaker than GCC's promise, but found out that
a subsequent TR added a footnote to clarify that the desired behavior is
the sane one too. C11 also has the same footnote, and no other change
in this area.
Paolo
- [Qemu-devel] [PATCH 0/2] target-i386: Fix undefined behavior on bit shifts, Eduardo Habkost, 2015/10/08
- [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift, Eduardo Habkost, 2015/10/08
- Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift, Paolo Bonzini, 2015/10/08
- Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift, Richard Henderson, 2015/10/08
- Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift, Paolo Bonzini, 2015/10/08
- Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift, Peter Maydell, 2015/10/08
- Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift, Paolo Bonzini, 2015/10/08
- Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift, Laszlo Ersek, 2015/10/08
- Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift, Paolo Bonzini, 2015/10/08
- Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift, Laszlo Ersek, 2015/10/08
- Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift,
Paolo Bonzini <=
- Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift, Peter Maydell, 2015/10/08
- Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift, Markus Armbruster, 2015/10/08
- Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift, Laszlo Ersek, 2015/10/08
- Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift, Paolo Bonzini, 2015/10/08
- Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift, Laszlo Ersek, 2015/10/08
- Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift, Paolo Bonzini, 2015/10/08
- Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift, Kevin O'Connor, 2015/10/08
[Qemu-devel] [PATCH 2/2] target-i386: Don't left shift negative constant, Eduardo Habkost, 2015/10/08