Re: Fix libgomp semaphores

[Alan Modra <amodra at gmail dot com>]

On Mon, Nov 28, 2011 at 05:23:37PM +0100, Jakub Jelinek wrote:
> On Mon, Nov 28, 2011 at 09:23:43AM +1030, Alan Modra wrote:
> > +  int count = *sem;
> > +
> > +  while ((count & 0x7fffffff) != 0)
> > +    {
> > +      int oldval = count;
> > +      __atomic_compare_exchange_4 (sem, &oldval, count - 1,
> > +				   false, MEMMODEL_ACQUIRE, MEMMODEL_RELAXED);
> > +      if (__builtin_expect (oldval == count, 1))
> > +	return;
> 
> Why aren't you instead testing the return value of
> __atomic_compare_exchange_4 (here and in other cases)?

If you use the return value on powerpc, you find that requires two
load immediate instructions (loading zero and one), and a compare
against zero.  That makes three fewer instructions as written, because
the oldval == count comparison has already been done inside the atomic
sequence.  I'd expect fewer on most other architectures unless they
happen to have a compare and exchange instruction that sets condition
bits (ie. Intel).  Even on Intel the way I've written the code
shouldn't take more instructions with a properly written cmpxchg rtl
description.  Does it?

Hmm, I suppose you could argue that powerpc and others ought to not
generate those three extra instructions when using the return value.
I'll see about fixing powerpc.

-- 
Alan Modra
Australia Development Lab, IBM