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Re: [Qemu-devel] [PATCH v2 for 2.5] QEMU does not care about left shifts
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH v2 for 2.5] QEMU does not care about left shifts of signed negative values |
Date: |
Tue, 17 Nov 2015 18:45:34 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 |
On 17/11/2015 18:39, Markus Armbruster wrote:
> The kernel switched from -fwrapv to -fno-strict-overflow in '09, because
> -fwrapv was buggy in gcc 4.1 (already old then, completely irrelevant
> now), and because it "seems to be much less disturbing to gcc too: the
> difference in the generated code by -fno-strict-overflow are smaller
> (compared to using neither flag) than when using -fwrapv", which may or
> may not be still the case with compilers that matter today.
>
> Could you briefly explain why you picked -fwrapv and not
> -fno-strict-overflow?
Because -fno-strict-overflow doesn't silence ubsan, only -fwrapv does
(it doesn't silence it for negative left shifts, but I've asked on
gcc-patches whether they'd like to have that fixed as well).
In the meanwhile I got some good news from the GCC folks:
>> I think we should remove the ", but this is subject to change" in
>> implement-c.texi (while replacing it with noting that ubsan will still
>> diagnose such cases, and they will also be diagnosed where constant
>> expressions are required).
Paolo
- [Qemu-devel] [PATCH v2 for 2.5] QEMU does not care about left shifts of signed negative values, Paolo Bonzini, 2015/11/17
- Re: [Qemu-devel] [PATCH v2 for 2.5] QEMU does not care about left shifts of signed negative values, Markus Armbruster, 2015/11/17
- Re: [Qemu-devel] [PATCH v2 for 2.5] QEMU does not care about left shifts of signed negative values,
Paolo Bonzini <=
- Re: [Qemu-devel] [PATCH v2 for 2.5] QEMU does not care about left shifts of signed negative values, Peter Maydell, 2015/11/17
- Re: [Qemu-devel] [PATCH v2 for 2.5] QEMU does not care about left shifts of signed negative values, Paolo Bonzini, 2015/11/17
- Re: [Qemu-devel] [PATCH v2 for 2.5] QEMU does not care about left shifts of signed negative values, Peter Maydell, 2015/11/17
- Re: [Qemu-devel] [PATCH v2 for 2.5] QEMU does not care about left shifts of signed negative values, Paolo Bonzini, 2015/11/17
- Re: [Qemu-devel] [PATCH v2 for 2.5] QEMU does not care about left shifts of signed negative values, Peter Maydell, 2015/11/17
- Re: [Qemu-devel] [PATCH v2 for 2.5] QEMU does not care about left shifts of signed negative values, Markus Armbruster, 2015/11/17
- Re: [Qemu-devel] [PATCH v2 for 2.5] QEMU does not care about left shifts of signed negative values, Paolo Bonzini, 2015/11/17
- Re: [Qemu-devel] [PATCH v2 for 2.5] QEMU does not care about left shifts of signed negative values, Markus Armbruster, 2015/11/17