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.
