[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
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 13:18:36 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 |
On 11/17/15 12:59, Markus Armbruster wrote:
> Laszlo Ersek <address@hidden> writes:
>
>> 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.)
>
> One of them, not both.
>
> Quote gcc manual:
>
> Using -fwrapv means that integer signed overflow is fully defined:
> it wraps. When -fwrapv is used, there is no difference between
> -fstrict-overflow and -fno-strict-overflow for integers. With
> -fwrapv certain types of overflow are permitted. For example, if
> the compiler gets an overflow when doing arithmetic on constants,
> the overflowed value can still be used with -fwrapv, but not
> otherwise.
>
> https://gcc.gnu.org/onlinedocs/gcc-5.2.0/gcc/Optimize-Options.html#index-fstrict-overflow-1050
>
> For what it's worth, the kernel uses -fno-strict-overflow
> -fno-strict-aliasing. It doesn't use -fwrapv. If optimization is good
> enough for the kernel, it's probably good enough for us. I recommend to
> follow the kernel's lead here.
>
> Relevant kernel commits:
>
> commit a137802ee839ace40079bebde24cfb416f73208a
> Author: Linus Torvalds <address@hidden>
> Date: Sun Jul 12 11:25:04 2009 -0700
>
> Don't use '-fwrapv' compiler option: it's buggy in gcc-4.1.x
OMG.
I guess "whatever works" then. :/
Laszlo
>
> This causes kernel images that don't run init to completion with certain
> broken gcc versions.
>
> This fixes kernel bugzilla entry:
> http://bugzilla.kernel.org/show_bug.cgi?id=13012
>
> I suspect the gcc problem is this:
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28230
>
> Fix the problem by using the -fno-strict-overflow flag instead, which
> not only does not exist in the known-to-be-broken versions of gcc (it
> was introduced later than fwrapv), but 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.
>
> Reported-by: Barry K. Nathan <address@hidden>
> Pushed-by: Frans Pop <address@hidden>
> Cc: Andrew Morton <address@hidden>
> Cc: address@hidden
> Signed-off-by: Linus Torvalds <address@hidden>
>
> commit 68df3755e383e6fecf2354a67b08f92f18536594
> Author: Linus Torvalds <address@hidden>
> Date: Thu Mar 19 11:10:17 2009 -0700
>
> Add '-fwrapv' to gcc CFLAGS
>
> This makes sure that gcc doesn't try to optimize away wrapping
> arithmetic, which the kernel occasionally uses for overflow testing, ie
> things like
>
> if (ptr + offset < ptr)
>
> which technically is undefined for non-unsigned types. See
>
> http://bugzilla.kernel.org/show_bug.cgi?id=12597
>
> for details.
>
> Not all versions of gcc support it, so we need to make it conditional
> (it looks like it was introduced in gcc-3.4).
>
> Reminded-by: Alan Cox <address@hidden>
> Signed-off-by: Linus Torvalds <address@hidden>
>
> I don't think we care for gcc 4.1.x anymore, but the kernels long use of
> -fno-strict-overflow has provided substantial testing, which -fwrapv may
> not have.
>
> [...]
>
- 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, 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
- 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 <=
Re: [Qemu-devel] [PATCH for 2.5] QEMU does not care about left shifts of signed negative values, Markus Armbruster, 2015/11/17