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Re: [Qemu-devel] [PATCH for 2.5] QEMU does not care about left shifts of
From: |
Laszlo Ersek |
Subject: |
Re: [Qemu-devel] [PATCH for 2.5] QEMU does not care about left shifts of signed negative values |
Date: |
Tue, 17 Nov 2015 11:41:51 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 |
On 11/17/15 11:28, Paolo Bonzini wrote:
>
>
> On 17/11/2015 11:19, Peter Maydell wrote:
>> I think we should only take this patch if you can get a cast-iron
>> guarantee from both clang and gcc that they will never use this
>> UB to drive optimizations. As you say gcc already say this more or
>> less, but clang doesn't, and if they're warning about it that to
>> me suggests that they will feel freer to rely on the UB in future.
>
> If and when this happens we will add "-fno-strict-overflow" for clang,
> just like we are using "-fno-strict-aliasing" already.
How about adding "-fwrapv -fno-strict-overflow" right now? (Spelling out
the latter of those explicitly for pointer arithmetic.)
>> GCC is not our only supported compiler; UB is a real thing that
>> compilers in general take advantage of; we should be trying to
>> reduce our reliance on UB, not carving out extra areas where we
>> feel free to use it.
>
> We are already feeling free to use it.
>
> Paolo
>
- Re: [Qemu-devel] [PATCH for 2.5] QEMU does not care about left shifts of signed negative values, (continued)
- Re: [Qemu-devel] [PATCH for 2.5] QEMU does not care about left shifts of signed negative values, Peter Maydell, 2015/11/17
- Re: [Qemu-devel] [PATCH for 2.5] QEMU does not care about left shifts of signed negative values, Paolo Bonzini, 2015/11/17
- Re: [Qemu-devel] [PATCH for 2.5] QEMU does not care about left shifts of signed negative values, Peter Maydell, 2015/11/17
- Re: [Qemu-devel] [PATCH for 2.5] QEMU does not care about left shifts of signed negative values, Paolo Bonzini, 2015/11/17
- Re: [Qemu-devel] [PATCH for 2.5] QEMU does not care about left shifts of signed negative values, Peter Maydell, 2015/11/17
- Re: [Qemu-devel] [PATCH for 2.5] QEMU does not care about left shifts of signed negative values, Peter Maydell, 2015/11/17
- Re: [Qemu-devel] [PATCH for 2.5] QEMU does not care about left shifts of signed negative values, Paolo Bonzini, 2015/11/17
- Re: [Qemu-devel] [PATCH for 2.5] QEMU does not care about left shifts of signed negative values, Peter Maydell, 2015/11/17
- Re: [Qemu-devel] [PATCH for 2.5] QEMU does not care about left shifts of signed negative values, Paolo Bonzini, 2015/11/17
- Re: [Qemu-devel] [PATCH for 2.5] QEMU does not care about left shifts of signed negative values, Peter Maydell, 2015/11/17
- Re: [Qemu-devel] [PATCH for 2.5] QEMU does not care about left shifts of signed negative values,
Laszlo Ersek <=
- Re: [Qemu-devel] [PATCH for 2.5] QEMU does not care about left shifts of signed negative values, Paolo Bonzini, 2015/11/17
- Re: [Qemu-devel] [PATCH for 2.5] QEMU does not care about left shifts of signed negative values, Laszlo Ersek, 2015/11/17
- Re: [Qemu-devel] [PATCH for 2.5] QEMU does not care about left shifts of signed negative values, Markus Armbruster, 2015/11/17
- Re: [Qemu-devel] [PATCH for 2.5] QEMU does not care about left shifts of signed negative values, Peter Maydell, 2015/11/17
- Re: [Qemu-devel] [PATCH for 2.5] QEMU does not care about left shifts of signed negative values, Laszlo Ersek, 2015/11/17
- Re: [Qemu-devel] [PATCH for 2.5] QEMU does not care about left shifts of signed negative values, Paolo Bonzini, 2015/11/17
- Re: [Qemu-devel] [PATCH for 2.5] QEMU does not care about left shifts of signed negative values, Laszlo Ersek, 2015/11/17
Re: [Qemu-devel] [PATCH for 2.5] QEMU does not care about left shifts of signed negative values, Markus Armbruster, 2015/11/17