JumpThreading: Remove duplicate maybe_loop_headers.

Author: dianqk

compiler/rustc_mir_transform/src/jump_threading.rs

@@ -84,7 +84,7 @@ impl<'tcx> crate::MirPass<'tcx> for JumpThreading {
             body,
             arena,
             map: Map::new(tcx, body, Some(MAX_PLACES)),
-            maybe_loop_headers: maybe_loop_headers(body),
+            maybe_loop_headers: loop_::maybe_loop_headers(body),
             opportunities: Vec::new(),
         };
 
@@ -830,29 +830,3 @@ enum Update {
     Incr,
     Decr,
 }
-
-/// Compute the set of loop headers in the given body. A loop header is usually defined as a block
-/// which dominates one of its predecessors. This definition is only correct for reducible CFGs.
-/// However, computing dominators is expensive, so we approximate according to the post-order
-/// traversal order. A loop header for us is a block which is visited after its predecessor in
-/// post-order. This is ok as we mostly need a heuristic.
-fn maybe_loop_headers(body: &Body<'_>) -> DenseBitSet<BasicBlock> {
-    let mut maybe_loop_headers = DenseBitSet::new_empty(body.basic_blocks.len());
-    let mut visited = DenseBitSet::new_empty(body.basic_blocks.len());
-    for (bb, bbdata) in traversal::postorder(body) {
-        // Post-order means we visit successors before the block for acyclic CFGs.
-        // If the successor is not visited yet, consider it a loop header.
-        for succ in bbdata.terminator().successors() {
-            if !visited.contains(succ) {
-                maybe_loop_headers.insert(succ);
-            }
-        }
-
-        // Only mark `bb` as visited after we checked the successors, in case we have a self-loop.
-        //     bb1: goto -> bb1;
-        let _new = visited.insert(bb);
-        debug_assert!(_new);
-    }
-
-    maybe_loop_headers
-}