Correct loop_headers logic.

Author: cjgillot

compiler/rustc_mir_transform/src/jump_threading.rs

@@ -743,15 +743,15 @@ enum Update {
 /// Compute the set of loop headers in the given body. We define a loop header as a block which has
 /// at least a predecessor which it dominates. This definition is only correct for reducible CFGs.
 /// But if the CFG is already irreducible, there is no point in trying much harder.
-/// is already irreducibl
+/// is already irreducible.
 fn loop_headers(body: &Body<'_>) -> BitSet<BasicBlock> {
     let mut loop_headers = BitSet::new_empty(body.basic_blocks.len());
     let dominators = body.basic_blocks.dominators();
     // Only visit reachable blocks.
     for (bb, bbdata) in traversal::preorder(body) {
         for succ in bbdata.terminator().successors() {
             if dominators.dominates(succ, bb) {
-                loop_headers.insert(bb);
+                loop_headers.insert(succ);
             }
         }
     }

tests/mir-opt/jump_threading.dfa.JumpThreading.panic-abort.diff

@@ -19,8 +19,7 @@
           StorageLive(_1);
           _1 = DFA::A;
           StorageLive(_2);
--         goto -> bb1;
-+         goto -> bb7;
+          goto -> bb1;
       }
 
       bb1: {
@@ -64,11 +63,6 @@
           _3 = const ();
           StorageDead(_7);
           goto -> bb1;
-+     }
-+ 
-+     bb7: {
-+         _4 = discriminant(_1);
-+         goto -> bb4;
       }
   }
 

tests/mir-opt/jump_threading.dfa.JumpThreading.panic-unwind.diff

@@ -19,8 +19,7 @@
           StorageLive(_1);
           _1 = DFA::A;
           StorageLive(_2);
--         goto -> bb1;
-+         goto -> bb7;
+          goto -> bb1;
       }
 
       bb1: {
@@ -64,11 +63,6 @@
           _3 = const ();
           StorageDead(_7);
           goto -> bb1;
-+     }
-+ 
-+     bb7: {
-+         _4 = discriminant(_1);
-+         goto -> bb4;
       }
   }