[dragonegg PATCH] Implement __builtin_signbit

[Duncan Sands]

Hi Peter,

 >>> +      return Builder.CreateZExt(Result,
>>> +                                ConvertType(gimple_call_return_type(stmt)));
>>
>> I think the extension type should be based on the signedness of the return type,
>> rather than assuming it is an unsigned type.
>
> Result is an i1, so zero extension should cause us to return 0 or 1
> (consistent with GCC) regardless of the signedness of the return type.

it does look like GCC only cares about 0 or no-zero.  In which case I'm OK with
this.  That said, it could be more efficient.  For example, if the input type is
float, and the output i32, then you can just and with a mask that only keeps the
sign bit, reducing from bitcast+icmp+zext to bitcast+and.  What do you think of
doing a bitcast, an "and" to only keep the sign bit, then
   - if the result is wider than the float, a zext
   - if the result is the same size as the float, nothing
   - if the result is smaller than the float, right shift the bit into the top 
bit of the result

Ciao, Duncan.