------- Comment #13 from amylaar at gcc dot gnu dot org 2007-10-22 23:17
-------
(In reply to comment #0)
> See http://openmp.org/pipermail/omp/2007/000840.html
> and the rest of the lengthy threads:
> Memory consistency contradiction between 2.5 specification and GCC
> OpenMP spec 2.5 seems to have incorrect flush example on page 12
> Two simpler examples (Re: OpenMP spec 2.5 seems to have incorrect flush
> example
> on page 12)
>
> Some GCC optimizations are harmful for threaded code, e.g. loop invariant
> motion
> of global variables:
>
> int var;
> void
> foo (int x)
> {
> int i;
> for (i = 0; i < 100; i++)
> {
> if (i > x)
> var = i;
> }
> }
>
> When some other thread modifies var at the same time while foo (200) is
> executed,
> the compiler inserted a race which doesn't really exist in the original
> program,
> as it will do reg = var; ... var = reg; even when var was never modified.
>
This is not even a particular good choice of transformation for performance
reasons. We incur memory read latency when there is no need to.
Better is:
auto int dummy, *addr;
addr = &dummy;
if (i > x)
addr = &var;
*addr = i;
Or for the entire function:
void
foo (int x)
{
int dummy;
int *addr = &dummy;
if (99 > x)
addr = &var;
*addr = 99;
}
--
amylaar at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |missed-optimization
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31862
