[llvm-commits] CVS: llvm/include/llvm/DerivedTypes.h

[Chris Lattner]

>> +  /// getSignBit - Return a uint64_t with just the most  
>> significant bit set (the
>> +  /// sign bit, if the value is treated as a signed number).
>> +  uint64_t getSignBit() const {
>> +    return 1ULL << (getBitWidth()-1);
>> +  }
>> +
>
> I'm trying to get rid of such functions and you're adding more of
> them :)

The thought occurred to me :).  However, this is used in  
InstCombiner::SimplifyDemandedBits, which will require significant  
work to APInt'ify.  Using a function like this (instead of localized  
bit-twiddling) will at least help distill the intention out of the code.

> Please note that in many places (like the place you intend to use
> getSignBit), it may be necessary to use APInt::getSignedMinValue() for
> this value to enable it to work with > 64 bits. The exception to  
> this is
> GEP indexing which are known to be 32/64 bit indices. If your use is
> similar, I suppose this is fine. If not, please note that when
> InstructionCombining is done (Sheng's working on it), I intend to pass
> through LLVM and get rid of calls to both of these functions, if
> appropriate.

I strongly encourage Sheng to do instcombine one piece at a time: say  
visitAND, then visitOR (etc) and submit them as separate patches.

Blasting me with one huge patch is not going to earn brownie points,  
and if/when it breaks something, it won't be easy to track down what  
part of the huge change did it.

-Chris