> 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.
Lauro
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