Re: [Qemu-devel] [PATCH 3/3] disas/arm: avoid clang shifting negative signed warning

[Markus Armbruster]

Steven Noonan <address@hidden> writes:

> On Tue, Nov 10, 2015 at 10:52 AM, Markus Armbruster <address@hidden> wrote:
>> Peter Maydell <address@hidden> writes:
>>
>>> On 10 November 2015 at 17:33, Paolo Bonzini <address@hidden> wrote:
>>>>
>>>>
>>>> On 10/11/2015 16:57, Stefan Hajnoczi wrote:
>>>>> clang 3.7.0 on x86_64 warns about the following:
>>>>>
>>>>>   disas/arm.c:1782:17: warning: shifting a negative signed value is 
>>>>> undefined [-Wshift-negative-value]
>>>>>     imm |= (-1 << 7);
>>>>>             ~~ ^
>>>>>
>>>>> Note that this patch preserves the tab indent in this source file
>>>>> because the surrounding code still uses tabs.
>>>>>
>>>>> Signed-off-by: Stefan Hajnoczi <address@hidden>
>>>>
>>>> I would like to know a case where (except with ubsan) clang actually
>>>> uses the optimization.
>>>>
>>>> If not, this is just error message theatre (which is not news for clang)
>>>> and shouldn't have been part of -Wall.
>>>
>>> It could be they're attempting to warn us now about the possibility
>>> that in a future version of clang they will start using this UB
>>> to optimize with.
>>>
>>> http://stackoverflow.com/questions/22883790/left-shift-of-negative-values
>>> reports that Intel's ICC will use this in dead-code-elimination
>>> optimization. One day clang might do that too.
>>
>> Nice example of a compiler being gratuitously nasty.
>>
>
> I don't read this warning as "clang will do crazy things with your
> code eventually". Clang has always been very verbose when it comes to
> undefined behavior, and I don't think that's really a bad thing to do.
[...]

Misunderstanding?

Clang's warning is at worst annoying, but nowhere near nasty.

ICC concluding that code executing a left shift of a negative value must
be unreachable is gratuitously nasty.