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Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift
From: |
Markus Armbruster |
Subject: |
Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift |
Date: |
Thu, 01 Oct 2015 22:35:08 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux) |
Peter Maydell <address@hidden> writes:
> On 1 October 2015 at 18:30, Paolo Bonzini <address@hidden> 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.
>
> I bet you QEMU breaks the 'out of bounds pointer arithmetic'
> rule all over the place. (set_prop_arraylen(), for a concrete
> example off the top of my head.)
>
> Signed integer overflow being UB is a really terrible idea which
> is one of the core cases for nailing down the UB -- everybody
> expects signed integers to behave as 2s-complement, when in
> fact what the compiler can and will do currently is just do totally
> unpredictable things...
>
>> 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.
>
> QEMU currently turns off strict-aliasing entirely, which I think
> is entirely sensible of us.
>
> A lot of the underlying intention behind the proposal (as I
> interpret it) is "consistency and predictability of behaviour
> for the programmer trumps pure performance". That sounds like
> a good idea to me.
We do not have a raging "oh my god the compiler can't sufficiently
optimize" crisis. We do have a raging "we can't get our software
sufficiently reliable" crisis.
- Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift, (continued)
- 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, 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 <=
- 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