RE: atomicity of x86 bt/bts/btr/btc?

[Jay K <jay dot krell at cornell dot edu>]

----------------------------------------
> Subject: RE: atomicity of x86 bt/bts/btr/btc?
> From: foxmuldrster
> To: jay
> CC: gcc
> Date: Tue, 19 Oct 2010 03:05:26 -0500
>
> > > They do not automatically lock the bus. They will lock the bus with the
> > > explicit LOCK prefix, and BTS is typically used for an atomic read/write
> > > operation.
>
> > Thanks Rick.
> > I'll go back to using them.
> > I'm optimizing mainly for size.
> > The comment should perhaps be amended.
> > The "since they enforce atomic operation" part seems wrong.
>
> Np. For citation, see here (page 166).
>
> http://www.intel.com/Assets/PDF/manual/253666.pdf
>
> - Rick

 
Yep that was one of my references.
 
 
It might be nice if optimizing for size would use them with code like e.g.:
 
 
void set_bit(size_t* a, size_t b)
{
  const unsigned c = sizeof(size_t) * CHAR_BIT;
  a[b / c] |= (((size_t)1) << (b % c));
}
 
void clear_bit(size_t* a, size_t b)
{
  const unsigned c = sizeof(size_t) * CHAR_BIT;
  a[b / c] &=  ~(((size_t)1) << (b % c));
}
 
int get_bit(size_t* a, size_t b)
{
  const unsigned c = sizeof(size_t) * CHAR_BIT;
  return !!(a[b / c] & (((size_t)1) << (b % c)));
}
 
 
 - Jay