Left shift of negative value

Andrew Haley aph@redhat.com
Tue Dec 1 17:08:00 GMT 2015


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.

Andrew.



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