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Use more efficient iteration order for backward dataflow

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This applies the same basic principle as #62062 to the reverse dataflow
analysis used to compute liveness information. It is functionally
equivalent, except that post-order is used instead of reverse post-order.

Some `mir::Body`s contain basic blocks which are not reachable from the
`START_BLOCK`. We need to add them to the work queue as well to preserve
the original semantics.
This commit is contained in:
Dylan MacKenzie 2019-06-22 13:27:16 -07:00
parent a1e954e82d
commit e2479e263e
1 changed files with 18 additions and 3 deletions
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21
src/librustc_mir/util/liveness.rs
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@ -75,9 +75,24 @@ pub fn liveness_of_locals<'tcx>(
let mut bits = LiveVarSet::new_empty(num_live_vars);
// queue of things that need to be re-processed, and a set containing
// the things currently in the queue
let mut dirty_queue: WorkQueue<BasicBlock> = WorkQueue::with_all(body.basic_blocks().len());
// The dirty queue contains the set of basic blocks whose entry sets have changed since they
// were last processed. At the start of the analysis, we initialize the queue in post-order to
// make it more likely that the entry set for a given basic block will have the effects of all
// its successors in the CFG applied before it is processed.
//
// FIXME(ecstaticmorse): Reverse post-order on the reverse CFG may generate a better iteration
// order when cycles are present, but the overhead of computing the reverse CFG may outweigh
// any benefits. Benchmark this and find out.
let mut dirty_queue: WorkQueue<BasicBlock> = WorkQueue::with_none(body.basic_blocks().len());
for (bb, _) in traversal::postorder(body) {
dirty_queue.insert(bb);
}
// Add blocks which are not reachable from START_BLOCK to the work queue. These blocks will
// be processed after the ones added above.
for bb in body.basic_blocks().indices() {
dirty_queue.insert(bb);
}
let predecessors = body.predecessors();