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Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift
From: |
Laszlo Ersek |
Subject: |
Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift |
Date: |
Thu, 1 Oct 2015 20:40:04 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 |
On 10/01/15 19:30, Paolo Bonzini wrote:
>
>
> On 01/10/2015 19:07, Laszlo Ersek wrote:
>>> In addition, C89 didn't say at all what the result was for signed data
>>> types, so technically we could compile QEMU with -std=gnu89 (the default
>>> until GCC5) and call it a day.
>>>
>>> Really the C standard should make this implementation-defined.
>>
>> Obligatory link: http://blog.regehr.org/archives/1180
>
> Many ideas in there are good (e.g. mem*() being defined for invalid
> argument and zero lengths, and of course item 7 which is the issue at
> hand). In many cases it's also good to change undefined behavior to
> unspecified values, however I think that goes too far.
>
> For example I'm okay with signed integer overflow being undefined
> behavior, and I also disagree with "It is permissible to compute
> out-of-bounds pointer values including performing pointer arithmetic on
> the null pointer". Using uintptr_t is just fine.
>
> 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?
Thanks!
Laszlo
- [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 <=
- 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, 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