Left shift of negative value
Andrew Haley
aph@redhat.com
Tue Dec 1 17:10:00 GMT 2015
On 12/01/2015 05:08 PM, Andrew Haley wrote:
> On 12/01/2015 05:01 PM, Richard Earnshaw wrote:
>> On 01/12/15 16:07, Andrew Haley wrote:
>>> Do we support left shift of a negative value? Technically it's
>>> undefined and ubsan warns about it. However, a ton of software would
>>> surely break if we didn't do the obvious right thing. I can't find
>>> anything in the GCC manual.
>>
>> I don't believe we do, or can as of today. Firstly, there are two
>> possible 'obvious' behaviours: 1) The value is treated as signed and a
>> negative shift turns into a positive shift in the other direction; 2)
>> The value is treated as unsigned and the value is treated by a massive
>> shift in the same direction that exceeds the normally supported range of
>> the type. Which one is the default will probably depend on your hardware.
>>
>> Consider the case where the value is in a variable. To support negative
>> shifts as reversed shifts on machines where they don't automatically
>> convert negative values to reversed shifts you'd have to do an explicit
>> range check before every shift using that value. Something I know we
>> don't do today since I've never seen such checks come out on ARM, nor
>> are there any hooks in the compiler to deal with it.
>
> No, a shift *of* a negative value, not a shift *by* a negative value!
> i.e. an arithmetic right shift.
That explanation was even more confusing. Sorry.
I mean something like -12345 << 4
Andrew.
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