[llvm-commits] CVS: llvm/include/llvm/ADT/APInt.h

[Reid Spencer]

On Sat, 2007-03-24 at 16:31 -0700, Chris Lattner wrote:
> >    /// @param loBit the index of the lowest bit set.
> >    /// @returns An APInt value with the requested bits set.
> >    /// @brief Get a value with a block of bits set.
> > -  static APInt getBitsSet(uint32_t numBits, uint32_t hiBit,  
> > uint32_t loBit = 0);
> > +  static APInt getBitsSet(uint32_t numBits, uint32_t hiBit,  
> > uint32_t loBit = 0){
> > +    assert(hiBit < numBits && "hiBit out of range");
> > +    assert(loBit < numBits && "loBit out of range");
> > +    if (hiBit < loBit)
> 
> Hrm?  Why would you allow hiBit < loBit?  This seems like something  
> that should be asserted against.

Read the definition of the function in the documentation. It allows you
to construct things like:
0xFF0000FF with getBitsSet(32, 8, 24);

> 
> > +      return getLowBitsSet(numBits, hiBit+1) |
> > +             getHighBitsSet(numBits, numBits-loBit+1);
> > +    else if (loBit == 0)
> > +      return getLowBitsSet(numBits, hiBit+1);
> 
> Do you need this special case for correctness?

No, I was trying to saves a rather expensive shl for a common case.
However, I decided that before I wrote getLowBitsSet (which is the
equivalent). I'll remove that case and the default parameter.

> 
> -Chris
> 
> > +    return getLowBitsSet(numBits, hiBit-loBit+1).shl(loBit);
> > +  }
> >
> >    /// Constructs an APInt value that has the top hiBitsSet bits set.
> >    /// @param numBits the bitwidth of the result
> >
> >
> >
> > _______________________________________________
> > llvm-commits mailing list
> > llvm-commits at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
> 
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