Skip caching block transfer functions for acyclic MIR

Author: ecstatic-morse

src/librustc_mir/dataflow/generic/engine.rs

@@ -44,15 +44,20 @@ where
         def_id: DefId,
         analysis: A,
     ) -> Self {
+        // If there are no back-edges in the control-flow graph, we only ever need to apply the
+        // transfer function for each block exactly once (assuming that we process blocks in RPO).
+        //
+        // In this case, there's no need to compute the block transfer functions ahead of time.
+        if !body.is_cfg_cyclic() {
+            return Self::new(tcx, body, def_id, analysis, None);
+        }
+
+        // Otherwise, compute and store the cumulative transfer function for each block.
+
         let bits_per_block = analysis.bits_per_block(body);
         let mut trans_for_block =
             IndexVec::from_elem(GenKillSet::identity(bits_per_block), body.basic_blocks());
 
-        // Compute cumulative block transfer functions.
-        //
-        // FIXME: we may want to skip this if the MIR is acyclic, since we will never access a
-        // block transfer function more than once.
-
         for (block, block_data) in body.basic_blocks().iter_enumerated() {
             let trans = &mut trans_for_block[block];
 
@@ -62,11 +67,10 @@ where
                 analysis.statement_effect(trans, statement, loc);
             }
 
-            if let Some(terminator) = &block_data.terminator {
-                let loc = Location { block, statement_index: block_data.statements.len() };
-                analysis.before_terminator_effect(trans, terminator, loc);
-                analysis.terminator_effect(trans, terminator, loc);
-            }
+            let terminator = block_data.terminator();
+            let loc = Location { block, statement_index: block_data.statements.len() };
+            analysis.before_terminator_effect(trans, terminator, loc);
+            analysis.terminator_effect(trans, terminator, loc);
         }
 
         Self::new(tcx, body, def_id, analysis, Some(trans_for_block))