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Preprocess dominator tree to answer queries in O(1)

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Tomasz Miąsko 2023-01-21 00:00:00 +00:00 • committed by Camille GILLOT
parent 6c64870fa6
commit aa1267f630
7 changed files with 137 additions and 55 deletions
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46
compiler/rustc_mir_transform/src/coverage/graph.rs
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@ -2,13 +2,14 @@ use super::Error;
use itertools::Itertools;
use rustc_data_structures::fx::FxHashMap;
use rustc_data_structures::graph::dominators::{self, Dominators};
use rustc_data_structures::graph::dominators::{self, DominatorTree, Dominators};
use rustc_data_structures::graph::{self, GraphSuccessors, WithNumNodes, WithStartNode};
use rustc_index::bit_set::BitSet;
use rustc_index::{IndexSlice, IndexVec};
use rustc_middle::mir::coverage::*;
use rustc_middle::mir::{self, BasicBlock, BasicBlockData, Terminator, TerminatorKind};
use std::cmp::Ordering;
use std::ops::{Index, IndexMut};
const ID_SEPARATOR: &str = ",";
@ -24,6 +25,7 @@ pub(super) struct CoverageGraph {
bb_to_bcb: IndexVec<BasicBlock, Option<BasicCoverageBlock>>,
pub successors: IndexVec<BasicCoverageBlock, Vec<BasicCoverageBlock>>,
pub predecessors: IndexVec<BasicCoverageBlock, Vec<BasicCoverageBlock>>,
dominator_tree: Option<DominatorTree<BasicCoverageBlock>>,
dominators: Option<Dominators<BasicCoverageBlock>>,
}
@ -66,9 +68,17 @@ impl CoverageGraph {
}
}
let mut basic_coverage_blocks =
Self { bcbs, bb_to_bcb, successors, predecessors, dominators: None };
let dominators = dominators::dominators(&basic_coverage_blocks);
let mut basic_coverage_blocks = Self {
bcbs,
bb_to_bcb,
successors,
predecessors,
dominator_tree: None,
dominators: None,
};
let dominator_tree = dominators::dominator_tree(&basic_coverage_blocks);
let dominators = dominators::dominators(&dominator_tree);
basic_coverage_blocks.dominator_tree = Some(dominator_tree);
basic_coverage_blocks.dominators = Some(dominators);
basic_coverage_blocks
}
@ -212,8 +222,12 @@ impl CoverageGraph {
}
#[inline(always)]
pub fn dominators(&self) -> &Dominators<BasicCoverageBlock> {
self.dominators.as_ref().unwrap()
pub fn rank_partial_cmp(
&self,
a: BasicCoverageBlock,
b: BasicCoverageBlock,
) -> Option<Ordering> {
self.dominator_tree.as_ref().unwrap().rank_partial_cmp(a, b)
}
}
@ -650,26 +664,6 @@ pub(super) fn find_loop_backedges(
let mut backedges = IndexVec::from_elem_n(Vec::<BasicCoverageBlock>::new(), num_bcbs);
// Identify loops by their backedges.
//
// The computational complexity is bounded by: n(s) x d where `n` is the number of
// `BasicCoverageBlock` nodes (the simplified/reduced representation of the CFG derived from the
// MIR); `s` is the average number of successors per node (which is most likely less than 2, and
// independent of the size of the function, so it can be treated as a constant);
// and `d` is the average number of dominators per node.
//
// The average number of dominators depends on the size and complexity of the function, and
// nodes near the start of the function's control flow graph typically have less dominators
// than nodes near the end of the CFG. Without doing a detailed mathematical analysis, I
// think the resulting complexity has the characteristics of O(n log n).
//
// The overall complexity appears to be comparable to many other MIR transform algorithms, and I
// don't expect that this function is creating a performance hot spot, but if this becomes an
// issue, there may be ways to optimize the `dominates` algorithm (as indicated by an
// existing `FIXME` comment in that code), or possibly ways to optimize it's usage here, perhaps
// by keeping track of results for visited `BasicCoverageBlock`s if they can be used to short
// circuit downstream `dominates` checks.
//
// For now, that kind of optimization seems unnecessarily complicated.
for (bcb, _) in basic_coverage_blocks.iter_enumerated() {
for &successor in &basic_coverage_blocks.successors[bcb] {
if basic_coverage_blocks.dominates(successor, bcb) {