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8009120: Fuzz instruction scheduling in HotSpot compilers

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Reviewed-by: kvn, vlivanov
This commit is contained in:
Aleksey Shipilev 2013-03-05 04:24:50 -08:00
parent 999837eb30
commit 4b638cf955
5 changed files with 61 additions and 7 deletions
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35
hotspot/src/share/vm/opto/compile.cpp
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@ -3669,3 +3669,38 @@ void Compile::add_expensive_node(Node * n) {
n->set_req(0, NULL);
}
}
// Auxiliary method to support randomized stressing/fuzzing.
//
// This method can be called the arbitrary number of times, with current count
// as the argument. The logic allows selecting a single candidate from the
// running list of candidates as follows:
// int count = 0;
// Cand* selected = null;
// while(cand = cand->next()) {
// if (randomized_select(++count)) {
// selected = cand;
// }
// }
//
// Including count equalizes the chances any candidate is "selected".
// This is useful when we don't have the complete list of candidates to choose
// from uniformly. In this case, we need to adjust the randomicity of the
// selection, or else we will end up biasing the selection towards the latter
// candidates.
//
// Quick back-envelope calculation shows that for the list of n candidates
// the equal probability for the candidate to persist as "best" can be
// achieved by replacing it with "next" k-th candidate with the probability
// of 1/k. It can be easily shown that by the end of the run, the
// probability for any candidate is converged to 1/n, thus giving the
// uniform distribution among all the candidates.
//
// We don't care about the domain size as long as (RANDOMIZED_DOMAIN / count) is large.
#define RANDOMIZED_DOMAIN_POW 29
#define RANDOMIZED_DOMAIN (1 << RANDOMIZED_DOMAIN_POW)
#define RANDOMIZED_DOMAIN_MASK ((1 << (RANDOMIZED_DOMAIN_POW + 1)) - 1)
bool Compile::randomized_select(int count) {
assert(count > 0, "only positive");
return (os::random() & RANDOMIZED_DOMAIN_MASK) < (RANDOMIZED_DOMAIN / count);
}