1 files changed, 131 insertions, 0 deletions
diff --git a/compiler/rustc_mir_transform/src/remove_noop_landing_pads.rs b/compiler/rustc_mir_transform/src/remove_noop_landing_pads.rs
new file mode 100644
index 000000000..5c441c5b1
--- /dev/null
+++ b/compiler/rustc_mir_transform/src/remove_noop_landing_pads.rs
@@ -0,0 +1,131 @@
+use crate::MirPass;
+use rustc_index::bit_set::BitSet;
+use rustc_middle::mir::patch::MirPatch;
+use rustc_middle::mir::*;
+use rustc_middle::ty::TyCtxt;
+use rustc_target::spec::PanicStrategy;
+
+/// A pass that removes noop landing pads and replaces jumps to them with
+/// `None`. This is important because otherwise LLVM generates terrible
+/// code for these.
+pub struct RemoveNoopLandingPads;
+
+impl<'tcx> MirPass<'tcx> for RemoveNoopLandingPads {
+    fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
+        sess.panic_strategy() != PanicStrategy::Abort
+    }
+
+    fn run_pass(&self, _tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        debug!("remove_noop_landing_pads({:?})", body);
+        self.remove_nop_landing_pads(body)
+    }
+}
+
+impl RemoveNoopLandingPads {
+    fn is_nop_landing_pad(
+        &self,
+        bb: BasicBlock,
+        body: &Body<'_>,
+        nop_landing_pads: &BitSet<BasicBlock>,
+    ) -> bool {
+        for stmt in &body[bb].statements {
+            match &stmt.kind {
+                StatementKind::FakeRead(..)
+                | StatementKind::StorageLive(_)
+                | StatementKind::StorageDead(_)
+                | StatementKind::AscribeUserType(..)
+                | StatementKind::Coverage(..)
+                | StatementKind::Nop => {
+                    // These are all noops in a landing pad
+                }
+
+                StatementKind::Assign(box (place, Rvalue::Use(_) | Rvalue::Discriminant(_))) => {
+                    if place.as_local().is_some() {
+                        // Writing to a local (e.g., a drop flag) does not
+                        // turn a landing pad to a non-nop
+                    } else {
+                        return false;
+                    }
+                }
+
+                StatementKind::Assign { .. }
+                | StatementKind::SetDiscriminant { .. }
+                | StatementKind::Deinit(..)
+                | StatementKind::CopyNonOverlapping(..)
+                | StatementKind::Retag { .. } => {
+                    return false;
+                }
+            }
+        }
+
+        let terminator = body[bb].terminator();
+        match terminator.kind {
+            TerminatorKind::Goto { .. }
+            | TerminatorKind::Resume
+            | TerminatorKind::SwitchInt { .. }
+            | TerminatorKind::FalseEdge { .. }
+            | TerminatorKind::FalseUnwind { .. } => {
+                terminator.successors().all(|succ| nop_landing_pads.contains(succ))
+            }
+            TerminatorKind::GeneratorDrop
+            | TerminatorKind::Yield { .. }
+            | TerminatorKind::Return
+            | TerminatorKind::Abort
+            | TerminatorKind::Unreachable
+            | TerminatorKind::Call { .. }
+            | TerminatorKind::Assert { .. }
+            | TerminatorKind::DropAndReplace { .. }
+            | TerminatorKind::Drop { .. }
+            | TerminatorKind::InlineAsm { .. } => false,
+        }
+    }
+
+    fn remove_nop_landing_pads(&self, body: &mut Body<'_>) {
+        debug!("body: {:#?}", body);
+
+        // make sure there's a resume block
+        let resume_block = {
+            let mut patch = MirPatch::new(body);
+            let resume_block = patch.resume_block();
+            patch.apply(body);
+            resume_block
+        };
+        debug!("remove_noop_landing_pads: resume block is {:?}", resume_block);
+
+        let mut jumps_folded = 0;
+        let mut landing_pads_removed = 0;
+        let mut nop_landing_pads = BitSet::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() {
+                if let Some(unwind_bb) = *unwind {
+                    if nop_landing_pads.contains(unwind_bb) {
+                        debug!("    removing noop landing pad");
+                        landing_pads_removed += 1;
+                        *unwind = None;
+                    }
+                }
+            }
+
+            for target in body[bb].terminator_mut().successors_mut() {
+                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);
+    }
+}