Current approach doesn't work well in cases when multiple paths are predicted to be "cold". By "cold" paths I mean those containing "unreachable" instruction, call marked with 'cold' attribute and 'unwind' handler of 'invoke' instruction. The issue is that heuristics are applied one by one until the first match and essentially ignores relative hotness/coldness
of other paths.
New approach unifies processing of "cold" paths by assigning predefined absolute weight to each block estimated to be "cold". Then we propagate these weights up/down IR similarly to existing approach. And finally set up edge probabilities based on estimated block weights.
One important difference is how we propagate weight up. Existing approach propagates the same weight to all blocks that are post-dominated by a block with some "known" weight. This is useless at least because it always gives 50\50 distribution which is assumed by default anyway. Worse, it causes the algorithm to skip further heuristics and can miss setting more accurate probability. New algorithm propagates the weight up only to the blocks that dominates and post-dominated by a block with some "known" weight. In other words, those blocks that are either always executed or not executed together.
In addition new approach processes loops in an uniform way as well. Essentially loop exit edges are estimated as "cold" paths relative to back edges and should be considered uniformly with other coldness/hotness markers.
Reviewed By: yrouban
Differential Revision: https://reviews.llvm.org/D79485
When instructions are cloned from block BB to PredBB in the method
DuplicateCondBranchOnPHIIntoPred() number of successors of PredBB
changes from 1 to number of successors of BB. So we have to copy
branch probabilities from BB to PredBB.
Reviewed By: Kazu Hirata
Differential Revision: https://reviews.llvm.org/D90841
This patch teaches the jump threading pass to set edge probabilities
whenever the pass creates new basic blocks.
Without this patch, the compiler sometimes produces non-deterministic
results. The non-determinism comes from the jump threading pass using
stale edge probabilities in BranchProbabilityInfo. Specifically, when
the jump threading pass creates a new basic block, we don't initialize
its outgoing edge probability.
Edge probabilities are maintained in:
DenseMap<Edge, BranchProbability> Probs;
in class BranchProbabilityInfo, where Edge is an ordered pair of
BasicBlock * and a successor index declared as:
using Edge = std::pair<const BasicBlock *, unsigned>;
Probs maps edges to their corresponding probabilities.
Now, we rarely remove entries from this map, so if we happen to
allocate a new basic block at the same address as a previously deleted
basic block with an edge probability assigned, the newly created basic
block appears to have an edge probability, albeit a stale one.
This patch fixes the problem by explicitly setting edge probabilities
whenever the jump threading pass creates new basic blocks.
Differential Revision: https://reviews.llvm.org/D90106
Evgeniy Brevnov