On 01/12/15 17:10, Andrew Haley wrote: > 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. > > Yeah, sorry, I mis-parsed your original email. I'd expect that to work on practically all 2's complement machines. And I'd also expect right shifts to work correctly with GCC by doing sign-bit replication on a 2's complement machine. R.
