On Thu, Oct 31, 2013 at 10:04:57AM +0100, Peter Zijlstra wrote:

On Wed, Oct 30, 2013 at 09:32:58PM -0700, Paul E. McKenney wrote:

quoted

Before C/C++11, the closest thing to such a prohibition is use of
volatile, for example, ACCESS_ONCE().  Even in C/C++11, you have to
use atomics to get anything resembing this prohibition.

If you just use normal variables, the compiler is within its rights
to transform something like the following:

	if (a)
		b = 1;
	else
		b = 42;

Into:

	b = 42;
	if (a)
		b = 1;

Many other similar transformations are permitted.  Some are used to all
vector instructions to be used -- the compiler can do a write with an
overly wide vector instruction, then clean up the clobbered variables
later, if it wishes.  Again, if the variables are not marked volatile,
or, in C/C++11, atomic.

While I've heard you tell this story before, my mind keeps boggling how
we've been able to use shared memory at all, all these years.

It seems to me stuff should have broken left, right and center if
compilers were really aggressive about this.

Sometimes having stupid compilers is a good thing.  But they really are
getting more aggressive.

							Thanx, Paul