Refactor remove_noop_landing_pads in two loops.

Author: cjgillot

compiler/rustc_mir_transform/src/remove_noop_landing_pads.rs

@@ -1,7 +1,7 @@
 use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::*;
 use rustc_middle::ty::TyCtxt;
-use tracing::debug;
+use tracing::{debug, instrument};
 
 use crate::patch::MirPatch;
 
@@ -15,6 +15,7 @@ impl<'tcx> crate::MirPass<'tcx> for RemoveNoopLandingPads {
         sess.panic_strategy().unwinds()
     }
 
+    #[instrument(level = "debug", skip(self, _tcx, body))]
     fn run_pass(&self, _tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         let def_id = body.source.def_id();
         debug!(?def_id);
@@ -25,7 +26,7 @@ impl<'tcx> crate::MirPass<'tcx> for RemoveNoopLandingPads {
             .iter_enumerated()
             .any(|(_bb, block)| matches!(block.terminator().kind, TerminatorKind::UnwindResume));
         if !has_resume {
-            debug!("remove_noop_landing_pads: no resume block in MIR");
+            debug!("no resume block in MIR");
             return;
         }
 
@@ -36,42 +37,44 @@ impl<'tcx> crate::MirPass<'tcx> for RemoveNoopLandingPads {
             patch.apply(body);
             resume_block
         };
-        debug!("remove_noop_landing_pads: resume block is {:?}", resume_block);
+        debug!(?resume_block);
 
-        let mut jumps_folded = 0;
-        let mut landing_pads_removed = 0;
         let mut nop_landing_pads = DenseBitSet::new_empty(body.basic_blocks.len());
 
         // This is a post-order traversal, so that if A post-dominates B
         // then A will be visited before B.
-        let postorder: Vec<_> = traversal::postorder(body).map(|(bb, _)| bb).collect();
-        for bb in postorder {
-            debug!("  processing {:?}", bb);
-            if let Some(unwind) = body[bb].terminator_mut().unwind_mut()
+        for &bb in body.basic_blocks.reverse_postorder().iter().rev() {
+            let is_nop_landing_pad = self.is_nop_landing_pad(bb, body, &nop_landing_pads);
+            debug!("is_nop_landing_pad({bb:?}) = {is_nop_landing_pad}");
+            if is_nop_landing_pad {
+                nop_landing_pads.insert(bb);
+            }
+        }
+
+        if nop_landing_pads.is_empty() {
+            debug!("no nop landing pads in MIR");
+            return;
+        }
+
+        let basic_blocks = body.basic_blocks.as_mut();
+        for (bb, bbdata) in basic_blocks.iter_enumerated_mut() {
+            debug!("processing {:?}", bb);
+
+            if let Some(unwind) = bbdata.terminator_mut().unwind_mut()
                 && let UnwindAction::Cleanup(unwind_bb) = *unwind
                 && nop_landing_pads.contains(unwind_bb)
             {
                 debug!("    removing noop landing pad");
-                landing_pads_removed += 1;
                 *unwind = UnwindAction::Continue;
             }
 
-            body[bb].terminator_mut().successors_mut(|target| {
+            bbdata.terminator_mut().successors_mut(|target| {
                 if *target != resume_block && nop_landing_pads.contains(*target) {
                     debug!("    folding noop jump to {:?} to resume block", target);
                     *target = resume_block;
-                    jumps_folded += 1;
                 }
             });
-
-            let is_nop_landing_pad = self.is_nop_landing_pad(bb, body, &nop_landing_pads);
-            if is_nop_landing_pad {
-                nop_landing_pads.insert(bb);
-            }
-            debug!("    is_nop_landing_pad({:?}) = {}", bb, is_nop_landing_pad);
         }
-
-        debug!("removed {:?} jumps and {:?} landing pads", jumps_folded, landing_pads_removed);
     }
 
     fn is_required(&self) -> bool {