reduce acyclic check cost

Author: lqd

compiler/rustc_borrowck/src/lib.rs

@@ -593,7 +593,20 @@ fn do_mir_borrowck<'tcx>(
     //     );
     // }
 
-    if body.basic_blocks.is_cfg_cyclic() {
+    // Loops in THIR don't always have backedges (e.g. a loop that breaks immediately), so the MIR
+    // can still be acyclic even if the THIR had loops. However, if the MIR is acyclic, it must come
+    // from loops.
+    // if body.basic_blocks.is_cfg_cyclic() {
+    //     assert_eq!(
+    //         Some(true),
+    //         body.basic_blocks.thir_had_loops,
+    //         "mismatch between THIR loop and cyclic MIR {:?}",
+    //         body.span,
+    //     );
+    // }
+
+    // if body.basic_blocks.is_cfg_cyclic() {
+    if body.basic_blocks.thir_had_loops.unwrap_or_else(|| body.basic_blocks.is_cfg_cyclic()) {
         // let (mut flow_analysis, flow_entry_states) =
         //     get_flow_results(tcx, body, &move_data, &borrow_set, &regioncx);
         // visit_results(

compiler/rustc_middle/src/mir/basic_blocks.rs

@@ -15,6 +15,7 @@ use crate::mir::{BasicBlock, BasicBlockData, START_BLOCK};
 pub struct BasicBlocks<'tcx> {
     basic_blocks: IndexVec<BasicBlock, BasicBlockData<'tcx>>,
     cache: Cache,
+    pub thir_had_loops: Option<bool>,
 }
 
 // Typically 95%+ of basic blocks have 4 or fewer predecessors.
@@ -529,7 +530,7 @@ impl<T: Idx> VecQueue<T> {
 impl<'tcx> BasicBlocks<'tcx> {
     #[inline]
     pub fn new(basic_blocks: IndexVec<BasicBlock, BasicBlockData<'tcx>>) -> Self {
-        BasicBlocks { basic_blocks, cache: Cache::default() }
+        BasicBlocks { basic_blocks, cache: Cache::default(), thir_had_loops: None }
     }
 
     /// Returns true if control-flow graph contains a cycle reachable from the `START_BLOCK`.

compiler/rustc_mir_build/src/builder/mod.rs

@@ -533,7 +533,6 @@ fn construct_fn<'tcx>(
         builder.build_drop_trees();
         return_block.unit()
     });
-
     let mut body = builder.finish();
 
     body.spread_arg = if abi == ExternAbi::RustCall {
@@ -803,6 +802,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             None,
         );
         body.coverage_info_hi = self.coverage_info.map(|b| b.into_done());
+        body.basic_blocks.thir_had_loops = Some(self.thir.exprs.iter().any(|e| {
+            matches!(e.kind, thir::ExprKind::Loop { .. } | thir::ExprKind::LoopMatch { .. })
+        }));
 
         for (index, block) in body.basic_blocks.iter().enumerate() {
             if block.terminator.is_none() {