This makes tracer not explode with -fno-guess-branch-probabilities. I've settled with find_best_successor/predecessor not returning anything if _any_ edge in the interesting direction doesn't have ->count () initialized (rather than ignoring such edges).
Honza - I suppose it is on purpose that functions like .to_frequency () do not ICE for uninitialized counters? It at least looks like "previous" behavior was more sane for tracer in the counts/frequencies that were exposed. Bootstrapped and tested on x86_64-unknown-linux-gnu. Honza, does this look OK to you? tracer going wild on this testcase exposes the CFG cleanup scalability issue I've posted the following RFC for: https://gcc.gnu.org/ml/gcc-patches/2018-05/msg01644.html Thanks, Richard. 2018-05-30 Richard Biener <rguent...@suse.de> PR tree-optimization/85964 * tracer.c (better_p): Drop initialized count check, we only call the function with initialized counts now. (find_best_successor): Do find a best edge if one has uninitialized count. (find_best_predecessor): Likewise. Do BB frequency check only if count is initialized. Index: gcc/tracer.c =================================================================== --- gcc/tracer.c (revision 260896) +++ gcc/tracer.c (working copy) @@ -132,8 +132,7 @@ count_insns (basic_block bb) static bool better_p (const_edge e1, const_edge e2) { - if (e1->count ().initialized_p () && e2->count ().initialized_p () - && ((e1->count () > e2->count ()) || (e1->count () < e2->count ()))) + if ((e1->count () > e2->count ()) || (e1->count () < e2->count ())) return e1->count () > e2->count (); /* This is needed to avoid changes in the decision after CFG is modified. */ @@ -152,12 +151,15 @@ find_best_successor (basic_block bb) edge_iterator ei; FOR_EACH_EDGE (e, ei, bb->succs) - if (!best || better_p (e, best)) - best = e; + { + if (!e->count ().initialized_p ()) + return NULL; + if (!best || better_p (e, best)) + best = e; + } if (!best || ignore_bb_p (best->dest)) return NULL; - if (best->probability.initialized_p () - && best->probability.to_reg_br_prob_base () <= probability_cutoff) + if (best->probability.to_reg_br_prob_base () <= probability_cutoff) return NULL; return best; } @@ -172,12 +174,17 @@ find_best_predecessor (basic_block bb) edge_iterator ei; FOR_EACH_EDGE (e, ei, bb->preds) - if (!best || better_p (e, best)) - best = e; + { + if (!e->count ().initialized_p ()) + return NULL; + if (!best || better_p (e, best)) + best = e; + } if (!best || ignore_bb_p (best->src)) return NULL; - if (EDGE_FREQUENCY (best) * REG_BR_PROB_BASE - < bb->count.to_frequency (cfun) * branch_ratio_cutoff) + if (bb->count.initialized_p () + && (best->count ().to_frequency (cfun) * REG_BR_PROB_BASE + < bb->count.to_frequency (cfun) * branch_ratio_cutoff)) return NULL; return best; }