> >>>>> Wilco Dijkstra wrote:
>
> > It relies on static branch probabilities, which are set completely wrong in
> > GCC,
> so it ends up optimizing the hot path in many functions for size
> rather than speed
> and visa versa.
>
> This sounds like the static branch prediction issue that we have been
> discussing with Honza. Honza discussed plans to improve this
> infrastructure during this development cycle.
In fact I just started on refactoring the profile updating code.
However I do not see how it is going to help there or what is wrong.
In the testcase:
void g(void);
int a;
int f(void)
{
g();
if (a < 0x7ffffffe) // or != 0 or < 0 or a < 0x7ffffffe
return -1;
a = 1;
return 1;
}
triggers negative return heuristics that says that with 96% probability the
function will not return negative constant (as those are usually error states).
This further combine with default compare heuristcs but it won't outvote this
one to make return -1 more likely and return 1;
With opcode, we predict that comparsions a==b are false when b is not 0. We
also predict that values are positive. This is what triggers for test a>0
What is the particular problem with this?
Honza
>
> - David
