[llvm-commits] [llvm] r107199 - /llvm/trunk/include/llvm/ADT/SmallPtrSet.h

[Jakob Stoklund Olesen]

On Jun 30, 2010, at 7:49 AM, Duncan Sands wrote:

> Hi Jakob,
> 
>>> NextPowerOfTwo was already used to ensure that SmallSizePowTwo is a power
>>> of two, no need to do it a second time (NextPowerOfTwo is idempotent).
>> 
>> No it isn't.
>> 
>> Ignoring overflow, NextPowerOfTwo(x) is always strictly greater than x.
> 
> are you sure?  C++ templates make my head hurt, but as far as I can see it
> works as follows.

Oh wait, there's a template?

I was talking about the function in MathExtras.h:

/// NextPowerOf2 - Returns the next power of two (in 64-bits)
/// that is strictly greater than A.  Returns zero on overflow.
static inline uint64_t NextPowerOf2(uint64_t A) {
  A |= (A >> 1);
  A |= (A >> 2);
  A |= (A >> 4);
  A |= (A >> 8);
  A |= (A >> 16);
  A |= (A >> 32);
  return A + 1;
}

You are right, the template is idempotent.

Clearly the meaning of the word 'next' is up for debate.

/jakob

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