[llvm-commits] proposed patch: shift when shift amount >= bit width

[Chris Lattner]

On Jun 13, 2007, at 12:06 PM, Lauro Ramos Venancio wrote:

>> I don't think this is appropriate Lauro.  The program needs to be
>> fixed, it's not just a codegen or optimizer issue, everything in the
>> compiler assumes that these shifts are undefined.  Programs cannot
>> reliably expect a compiler to do the right thing here, and your patch
>> isn't a fix.
>
> Nowadays LLVM doesn't assume, in all places, that these shifts are  
> undefined.
> See the following code (APInt.cpp):
>
> // If all the bits were shifted out, the result is 0. This avoids  
> issues
>    // with shifting by the size of the integer type, which produces  
> undefined
>    // results. We define these "undefined results" to always be 0.
>   if (shiftAmt == BitWidth)
>      return APInt(BitWidth, 0);
>
> and this code:
>
> // If all the bits were shifted out, the result is, technically,  
> undefined.
>    // We return -1 if it was negative, 0 otherwise. We check this  
> early to avoid
>    // issues in the algorithm below.
>   if (shiftAmt == BitWidth) {
>      if (isNegative())
>        return APInt(BitWidth, -1ULL);
>      else
>        return APInt(BitWidth, 0);
>    }
>
> I think to implement the gcc behavior would make easier the migration
> from gcc to llvm.

APInt can't return undef, so it has to pick a deterministic value.   
LLVM values are a different story.

-Chris