[llvm-commits] [llvm] r100233 - in /llvm/trunk: ./ include/llvm/Support/ lib/Target/ARM/ lib/Target/ARM/Disassembler/ test/MC/Disassembler/ utils/TableGen/

[Chris Lattner]

On Apr 4, 2010, at 9:03 PM, Jakob Stoklund Olesen wrote:
>>>> Shift left + shift right is more easily recognized by the code generator as a sext.
>>> 
>>> Sure, but if we're not building with -fwrapv, it overflows the int and
>>> causes undefined behaviour.
>> 
>> Huh?  In what case?
> 
> If you left-shift a positive number until it goes negative, that's an
> overflow. At least that's how I understand it.

You can shift left an N bit number between 0 and N-1 bits safely without overflow.

-Chris