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coverage: Simplify the detection of reloop edges to be given expressions

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This commit is contained in:
Zalathar 2023-10-12 17:41:11 +11:00
parent 4f05e95452
commit a7ae2a6e6c
2 changed files with 56 additions and 81 deletions
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119
compiler/rustc_mir_transform/src/coverage/counters.rs
View File

@ -510,18 +510,11 @@ fn choose_preferred_expression_branch(
traversal: &TraverseCoverageGraphWithLoops,
branches: &[BcbBranch],
) -> BcbBranch {
let branch_needs_a_counter = |branch: &BcbBranch| self.branch_has_no_counter(branch);
let some_reloop_branch = self.find_some_reloop_branch(traversal, &branches);
if let Some(reloop_branch_without_counter) =
some_reloop_branch.filter(branch_needs_a_counter)
{
debug!(
"Selecting reloop_branch={:?} that still needs a counter, to get the \
`Expression`",
reloop_branch_without_counter
);
reloop_branch_without_counter
let good_reloop_branch = self.find_good_reloop_branch(traversal, &branches);
if let Some(reloop_branch) = good_reloop_branch {
assert!(self.branch_has_no_counter(&reloop_branch));
debug!("Selecting reloop branch {reloop_branch:?} to get an expression");
reloop_branch
} else {
let &branch_without_counter =
branches.iter().find(|&branch| self.branch_has_no_counter(branch)).expect(
@ -538,75 +531,52 @@ fn choose_preferred_expression_branch(
}
}
/// At most, one of the branches (or its edge, from the branching_bcb, if the branch has
/// multiple incoming edges) can have a counter computed by expression.
///
/// If at least one of the branches leads outside of a loop (`found_loop_exit` is
/// true), and at least one other branch does not exit the loop (the first of which
/// is captured in `some_reloop_branch`), it's likely any reloop branch will be
/// executed far more often than loop exit branch, making the reloop branch a better
/// candidate for an expression.
fn find_some_reloop_branch(
/// Tries to find a branch that leads back to the top of a loop, and that
/// doesn't already have a counter. Such branches are good candidates to
/// be given an expression (instead of a physical counter), because they
/// will tend to be executed more times than a loop-exit branch.
fn find_good_reloop_branch(
&self,
traversal: &TraverseCoverageGraphWithLoops,
branches: &[BcbBranch],
) -> Option<BcbBranch> {
let branch_needs_a_counter = |branch: &BcbBranch| self.branch_has_no_counter(branch);
// Consider each loop on the current traversal context stack, top-down.
for reloop_bcbs in traversal.reloop_bcbs_per_loop() {
let mut all_branches_exit_this_loop = true;
let mut some_reloop_branch: Option<BcbBranch> = None;
for context in traversal.context_stack.iter().rev() {
if let Some((backedge_from_bcbs, _)) = &context.loop_backedges {
let mut found_loop_exit = false;
for &branch in branches.iter() {
if backedge_from_bcbs.iter().any(|&backedge_from_bcb| {
self.bcb_dominates(branch.target_bcb, backedge_from_bcb)
}) {
if let Some(reloop_branch) = some_reloop_branch {
if self.branch_has_no_counter(&reloop_branch) {
// we already found a candidate reloop_branch that still
// needs a counter
continue;
}
}
// The path from branch leads back to the top of the loop. Set this
// branch as the `reloop_branch`. If this branch already has a
// counter, and we find another reloop branch that doesn't have a
// counter yet, that branch will be selected as the `reloop_branch`
// instead.
some_reloop_branch = Some(branch);
} else {
// The path from branch leads outside this loop
found_loop_exit = true;
}
if found_loop_exit
&& some_reloop_branch.filter(branch_needs_a_counter).is_some()
{
// Found both a branch that exits the loop and a branch that returns
// to the top of the loop (`reloop_branch`), and the `reloop_branch`
// doesn't already have a counter.
break;
// Try to find a branch that doesn't exit this loop and doesn't
// already have a counter.
for &branch in branches {
// A branch is a reloop branch if it dominates any BCB that has
// an edge back to the loop header. (Other branches are exits.)
let is_reloop_branch = reloop_bcbs.iter().any(|&reloop_bcb| {
self.basic_coverage_blocks.dominates(branch.target_bcb, reloop_bcb)
});
if is_reloop_branch {
all_branches_exit_this_loop = false;
if self.branch_has_no_counter(&branch) {
// We found a good branch to be given an expression.
return Some(branch);
}
// Keep looking for another reloop branch without a counter.
} else {
// This branch exits the loop.
}
if !found_loop_exit {
debug!(
"No branches exit the loop, so any branch without an existing \
counter can have the `Expression`."
);
break;
}
if some_reloop_branch.is_some() {
debug!(
"Found a branch that exits the loop and a branch the loops back to \
the top of the loop (`reloop_branch`). The `reloop_branch` will \
get the `Expression`, as long as it still needs a counter."
);
break;
}
// else all branches exited this loop context, so run the same checks with
// the outer loop(s)
}
if !all_branches_exit_this_loop {
// We found one or more reloop branches, but all of them already
// have counters. Let the caller choose one of the exit branches.
debug!("All reloop branches had counters; skip checking the other loops");
return None;
}
// All of the branches exit this loop, so keep looking for a good
// reloop branch for one of the outer loops.
}
some_reloop_branch
None
}
#[inline]
@ -652,9 +622,4 @@ fn branch_counter(&self, branch: &BcbBranch) -> Option<&BcbCounter> {
fn bcb_has_one_path_to_target(&self, bcb: BasicCoverageBlock) -> bool {
self.bcb_predecessors(bcb).len() <= 1
}
#[inline]
fn bcb_dominates(&self, dom: BasicCoverageBlock, node: BasicCoverageBlock) -> bool {
self.basic_coverage_blocks.dominates(dom, node)
}
}

18
compiler/rustc_mir_transform/src/coverage/graph.rs
View File

@ -386,17 +386,17 @@ fn bcb_filtered_successors<'a, 'tcx>(
#[derive(Debug)]
pub(super) struct TraversalContext {
/// From one or more backedges returning to a loop header.
pub loop_backedges: Option<(Vec<BasicCoverageBlock>, BasicCoverageBlock)>,
loop_backedges: Option<(Vec<BasicCoverageBlock>, BasicCoverageBlock)>,
/// worklist, to be traversed, of CoverageGraph in the loop with the given loop
/// backedges, such that the loop is the inner inner-most loop containing these
/// CoverageGraph
pub worklist: Vec<BasicCoverageBlock>,
worklist: Vec<BasicCoverageBlock>,
}
pub(super) struct TraverseCoverageGraphWithLoops {
pub backedges: IndexVec<BasicCoverageBlock, Vec<BasicCoverageBlock>>,
pub context_stack: Vec<TraversalContext>,
backedges: IndexVec<BasicCoverageBlock, Vec<BasicCoverageBlock>>,
context_stack: Vec<TraversalContext>,
visited: BitSet<BasicCoverageBlock>,
}
@ -414,6 +414,16 @@ pub fn new(basic_coverage_blocks: &CoverageGraph) -> Self {
Self { backedges, context_stack, visited }
}
/// For each loop on the loop context stack (top-down), yields a list of BCBs
/// within that loop that have an outgoing edge back to the loop header.
pub(super) fn reloop_bcbs_per_loop(&self) -> impl Iterator<Item = &[BasicCoverageBlock]> {
self.context_stack
.iter()
.rev()
.filter_map(|context| context.loop_backedges.as_ref())
.map(|(from_bcbs, _to_bcb)| from_bcbs.as_slice())
}
pub fn next(&mut self, basic_coverage_blocks: &CoverageGraph) -> Option<BasicCoverageBlock> {
debug!(
"TraverseCoverageGraphWithLoops::next - context_stack: {:?}",