Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift

[Paolo Bonzini]

On 30/09/2015 22:24, Richard Henderson wrote:
> On 09/30/2015 11:27 PM, Paolo Bonzini wrote:
>>
>>
>> On 29/09/2015 22:34, Eduardo Habkost wrote:
>>> Fix undefined behavior detected by clang runtime check:
>>>
>>>    qemu/target-i386/cpu.c:1494:15: runtime error:
>>>      left shift of 1 by 31 places cannot be represented in type 'int'
>>>
>>> While doing that, add extra parenthesis for clarity.
>>>
>>> Reported-by: Peter Maydell <address@hidden>
>>> Signed-off-by: Eduardo Habkost <address@hidden>
>>> ---
>>>   target-i386/cpu.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/target-i386/cpu.c b/target-i386/cpu.c
>>> index 2b914b2..6af6db9 100644
>>> --- a/target-i386/cpu.c
>>> +++ b/target-i386/cpu.c
>>> @@ -1491,7 +1491,7 @@ static void
>>> report_unavailable_features(FeatureWord w, uint32_t mask)
>>>       int i;
>>>
>>>       for (i = 0; i < 32; ++i) {
>>> -        if (1 << i & mask) {
>>> +        if ((1UL << i) & mask) {
>>
>> 1U is enough.
>>
>> Paolo
>>
>> ps: Ego ceterum censeo that these warnings are useless and uglify the
>> code unnecessarily.  But it looks like I'm in a minority so the patch is
>> okay.
> 
> I totally agree.  There are no ones-compliment machines anymore, and so
> the whole point of that "undefined" in the C standard is moot.  Let's
> all accept that shifts of signed quantities do exactly what we expect.
> 
> Without looking, I don't suppose either compiler has a switch to disable
> just the shift part of ubsan?

Nope, I already asked. :)

Paolo