Smaller and more correct generator codegen

src/librustc_mir/transform/generator.rs

@@ -991,18 +991,101 @@ fn insert_panic_block<'tcx>(
     assert_block
 }
 
+fn can_return<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>) -> bool {
+    // Returning from a function with an uninhabited return type is undefined behavior.
+    if body.return_ty().conservative_is_privately_uninhabited(tcx) {
+        return false;
+    }
+
+    // If there's a return terminator the function may return.
+    for block in body.basic_blocks() {
+        if let TerminatorKind::Return = block.terminator().kind {
+            return true;
+        }
+    }
+
+    // Otherwise the function can't return.
+    false
+}
+
+fn can_unwind<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>) -> bool {
+    // Nothing can unwind when landing pads are off.
+    if tcx.sess.no_landing_pads() {
+        return false;
+    }
+
+    // Unwinds can only start at certain terminators.
+    for block in body.basic_blocks() {
+        match block.terminator().kind {
+            // These never unwind.
+            TerminatorKind::Goto { .. }
+            | TerminatorKind::SwitchInt { .. }
+            | TerminatorKind::Abort
+            | TerminatorKind::Return
+            | TerminatorKind::Unreachable
+            | TerminatorKind::GeneratorDrop
+            | TerminatorKind::FalseEdges { .. }
+            | TerminatorKind::FalseUnwind { .. } => {}
+
+            // Resume will *continue* unwinding, but if there's no other unwinding terminator it
+            // will never be reached.
+            TerminatorKind::Resume => {}
+
+            TerminatorKind::Yield { .. } => {
+                unreachable!("`can_unwind` called before generator transform")
+            }
+
+            // These may unwind.
+            TerminatorKind::Drop { .. }
+            | TerminatorKind::DropAndReplace { .. }
+            | TerminatorKind::Call { .. }
+            | TerminatorKind::Assert { .. } => return true,
+        }
+    }
+
+    // If we didn't find an unwinding terminator, the function cannot unwind.
+    false
+}
+
 fn create_generator_resume_function<'tcx>(
     tcx: TyCtxt<'tcx>,
     transform: TransformVisitor<'tcx>,
     def_id: DefId,
     source: MirSource<'tcx>,
     body: &mut BodyAndCache<'tcx>,
+    can_return: bool,
 ) {
+    let can_unwind = can_unwind(tcx, body);
+
     // Poison the generator when it unwinds
-    for block in body.basic_blocks_mut() {
-        let source_info = block.terminator().source_info;
-        if let &TerminatorKind::Resume = &block.terminator().kind {
-            block.statements.push(transform.set_discr(VariantIdx::new(POISONED), source_info));
+    if can_unwind {
+        let poison_block = BasicBlock::new(body.basic_blocks().len());
+        let source_info = source_info(body);
+        body.basic_blocks_mut().push(BasicBlockData {
+            statements: vec![transform.set_discr(VariantIdx::new(POISONED), source_info)],
+            terminator: Some(Terminator { source_info, kind: TerminatorKind::Resume }),
+            is_cleanup: true,
+        });
+
+        for (idx, block) in body.basic_blocks_mut().iter_enumerated_mut() {
+            let source_info = block.terminator().source_info;
+
+            if let TerminatorKind::Resume = block.terminator().kind {
+                // An existing `Resume` terminator is redirected to jump to our dedicated
+                // "poisoning block" above.
+                if idx != poison_block {
+                    *block.terminator_mut() = Terminator {
+                        source_info,
+                        kind: TerminatorKind::Goto { target: poison_block },
+                    };
+                }
+            } else if !block.is_cleanup {
+                // Any terminators that *can* unwind but don't have an unwind target set are also
+                // pointed at our poisoning block (unless they're part of the cleanup path).
+                if let Some(unwind @ None) = block.terminator_mut().unwind_mut() {
+                    *unwind = Some(poison_block);
+                }
+            }
         }
     }
 
@@ -1015,8 +1098,20 @@ fn create_generator_resume_function<'tcx>(
 
     // Panic when resumed on the returned or poisoned state
     let generator_kind = body.generator_kind.unwrap();
-    cases.insert(1, (RETURNED, insert_panic_block(tcx, body, ResumedAfterReturn(generator_kind))));
-    cases.insert(2, (POISONED, insert_panic_block(tcx, body, ResumedAfterPanic(generator_kind))));
+
+    if can_unwind {
+        cases.insert(
+            1,
+            (POISONED, insert_panic_block(tcx, body, ResumedAfterPanic(generator_kind))),
+        );
+    }
+
+    if can_return {
+        cases.insert(
+            1,
+            (RETURNED, insert_panic_block(tcx, body, ResumedAfterReturn(generator_kind))),
+        );
+    }
 
     insert_switch(body, cases, &transform, TerminatorKind::Unreachable);
 
@@ -1200,6 +1295,8 @@ impl<'tcx> MirPass<'tcx> for StateTransform {
         let (remap, layout, storage_liveness) =
             compute_layout(tcx, source, &upvars, interior, movable, body);
 
+        let can_return = can_return(tcx, body);
+
         // Run the transformation which converts Places from Local to generator struct
         // accesses for locals in `remap`.
         // It also rewrites `return x` and `yield y` as writing a new generator state and returning
@@ -1243,6 +1340,6 @@ impl<'tcx> MirPass<'tcx> for StateTransform {
         body.generator_drop = Some(box drop_shim);
 
         // Create the Generator::resume function
-        create_generator_resume_function(tcx, transform, def_id, source, body);
+        create_generator_resume_function(tcx, transform, def_id, source, body, can_return);
     }
 }

src/test/mir-opt/generator-tiny.rs

@@ -0,0 +1,34 @@
+//! Tests that generators that cannot return or unwind don't have unnecessary
+//! panic branches.
+
+// compile-flags: -Zno-landing-pads
+
+#![feature(generators, generator_trait)]
+
+struct HasDrop;
+
+impl Drop for HasDrop {
+    fn drop(&mut self) {}
+}
+
+fn callee() {}
+
+fn main() {
+    let _gen = |_x: u8| {
+        let _d = HasDrop;
+        loop {
+            yield;
+            callee();
+        }
+    };
+}
+
+// END RUST SOURCE
+
+// START rustc.main-{{closure}}.generator_resume.0.mir
+// bb0: {
+//     ...
+//     switchInt(move _11) -> [0u32: bb1, 3u32: bb5, otherwise: bb6];
+// }
+// ...
+// END rustc.main-{{closure}}.generator_resume.0.mir