On 12/01/2016 09:49 AM, Martin Sebor wrote:
Right. An edge which leads to a noreturn call is predicated as
esentially "never taken". It's a good strong predictor.
Okay, thanks for the clarification. One other question though.
Why would the probability be near zero? In the absence of any
hints the expression 2 != sprintf(d, "%i", 12) should have
a very high probability of being true, near 100% in fact.
I ask because the test I referenced tries to verify the absence
of the sprintf return value optimization. Without it the
likelihood of each of the calls to abort should in the EQL kind
of test cases like the one above should be nearly 100% (but not
quite). When it's the opposite it suggests that the sprintf
optimization is providing some hint (in the form of a range of
return values) that changes the odds. I have verified that
the optimization is not performed so something else must be
setting the probability or the value isn't correct.
I think I see what's going on. It's the call to abort that GCC
uses for the probability (more precisely its attribute noreturn).
Jan would be the one to know the predictors work. It's reasonably
When I replace the abort with a function of my own that GCC knows
nothing about the probability goes up to just over 52%. Still it
seems very low given that 2 is just one of UINT_MAX values the
function can possibly return.