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

[Johnny Chen]

Did you mean:

You can shift left an N bit number between 0 and (32-N) bits safely without overflow.

in the case of SignExtend32<N>(x)?
(N must be > 1 and < 32)

On Apr 4, 2010, at 9:09 PM, Chris Lattner wrote:

> 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