Remove no-op cleanups as post-mono MIR opt

Mark-Simulacrum · Mark-Simulacrum · commit 676eed3ad948 · 2025-06-30T22:10:33.000-04:00
On cargo this cuts ~5% of the LLVM IR lines we generate (measured with
-Cno-prepopulate-passes).
diff --git a/compiler/rustc_codegen_ssa/src/mir/analyze.rs b/compiler/rustc_codegen_ssa/src/mir/analyze.rs
@@ -279,10 +279,14 @@ impl CleanupKind {
 /// MSVC requires unwinding code to be split to a tree of *funclets*, where each funclet can only
 /// branch to itself or to its parent. Luckily, the code we generates matches this pattern.
 /// Recover that structure in an analyze pass.
-pub(crate) fn cleanup_kinds(mir: &mir::Body<'_>) -> IndexVec<mir::BasicBlock, CleanupKind> {
+pub(crate) fn cleanup_kinds(
+    mir: &mir::Body<'_>,
+    live_blocks: &DenseBitSet<mir::BasicBlock>,
+) -> IndexVec<mir::BasicBlock, CleanupKind> {
     fn discover_masters<'tcx>(
         result: &mut IndexSlice<mir::BasicBlock, CleanupKind>,
         mir: &mir::Body<'tcx>,
+        live_blocks: &DenseBitSet<mir::BasicBlock>,
     ) {
         for (bb, data) in mir.basic_blocks.iter_enumerated() {
             match data.terminator().kind {
@@ -301,7 +305,9 @@ pub(crate) fn cleanup_kinds(mir: &mir::Body<'_>) -> IndexVec<mir::BasicBlock, Cl
                 | TerminatorKind::InlineAsm { unwind, .. }
                 | TerminatorKind::Assert { unwind, .. }
                 | TerminatorKind::Drop { unwind, .. } => {
-                    if let mir::UnwindAction::Cleanup(unwind) = unwind {
+                    if let mir::UnwindAction::Cleanup(unwind) = unwind
+                        && live_blocks.contains(unwind)
+                    {
                         debug!(
                             "cleanup_kinds: {:?}/{:?} registering {:?} as funclet",
                             bb, data, unwind
@@ -382,7 +388,7 @@ pub(crate) fn cleanup_kinds(mir: &mir::Body<'_>) -> IndexVec<mir::BasicBlock, Cl
 
     let mut result = IndexVec::from_elem(CleanupKind::NotCleanup, &mir.basic_blocks);
 
-    discover_masters(&mut result, mir);
+    discover_masters(&mut result, mir, &live_blocks);
     propagate(&mut result, mir);
     debug!("cleanup_kinds: result={:?}", result);
     result
diff --git a/compiler/rustc_codegen_ssa/src/mir/block.rs b/compiler/rustc_codegen_ssa/src/mir/block.rs
@@ -200,7 +200,13 @@ impl<'a, 'tcx> TerminatorCodegenHelper<'tcx> {
         }
 
         let unwind_block = match unwind {
-            mir::UnwindAction::Cleanup(cleanup) => Some(self.llbb_with_cleanup(fx, cleanup)),
+            mir::UnwindAction::Cleanup(cleanup) => {
+                if fx.live_blocks.contains(cleanup) {
+                    Some(self.llbb_with_cleanup(fx, cleanup))
+                } else {
+                    None
+                }
+            }
             mir::UnwindAction::Continue => None,
             mir::UnwindAction::Unreachable => None,
             mir::UnwindAction::Terminate(reason) => {
@@ -286,7 +292,13 @@ impl<'a, 'tcx> TerminatorCodegenHelper<'tcx> {
         mergeable_succ: bool,
     ) -> MergingSucc {
         let unwind_target = match unwind {
-            mir::UnwindAction::Cleanup(cleanup) => Some(self.llbb_with_cleanup(fx, cleanup)),
+            mir::UnwindAction::Cleanup(cleanup) => {
+                if fx.live_blocks.contains(cleanup) {
+                    Some(self.llbb_with_cleanup(fx, cleanup))
+                } else {
+                    None
+                }
+            }
             mir::UnwindAction::Terminate(reason) => Some(fx.terminate_block(reason)),
             mir::UnwindAction::Continue => None,
             mir::UnwindAction::Unreachable => None,
diff --git a/compiler/rustc_codegen_ssa/src/mir/mod.rs b/compiler/rustc_codegen_ssa/src/mir/mod.rs
@@ -97,6 +97,12 @@ pub struct FunctionCx<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> {
     /// A cold block is a block that is unlikely to be executed at runtime.
     cold_blocks: IndexVec<mir::BasicBlock, bool>,
 
+    /// A set of blocks which are live.
+    ///
+    /// This is constrained by the reachable block set, currently specifically targeting replacing
+    /// UnwindAction::Cleanup(target) with UnwindAction::Continue if target is dead.
+    live_blocks: DenseBitSet<mir::BasicBlock>,
+
     /// The location where each MIR arg/var/tmp/ret is stored. This is
     /// usually an `PlaceRef` representing an alloca, but not always:
     /// sometimes we can skip the alloca and just store the value
@@ -176,8 +182,7 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
 
     let mut mir = tcx.instance_mir(instance.def);
 
-    let fn_abi = cx.fn_abi_of_instance(instance, ty::List::empty());
-    debug!("fn_abi: {:?}", fn_abi);
+    let live_blocks = find_noop_cleanup::<Bx>(cx, &mir, instance);
 
     if tcx.features().ergonomic_clones() {
         let monomorphized_mir = instance.instantiate_mir_and_normalize_erasing_regions(
@@ -188,19 +193,24 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         mir = tcx.arena.alloc(optimize_use_clone::<Bx>(cx, monomorphized_mir));
     }
 
+    let fn_abi = cx.fn_abi_of_instance(instance, ty::List::empty());
+    debug!("fn_abi: {:?}", fn_abi);
+
     let debug_context = cx.create_function_debug_context(instance, fn_abi, llfn, &mir);
 
     let start_llbb = Bx::append_block(cx, llfn, "start");
     let mut start_bx = Bx::build(cx, start_llbb);
 
-    if mir.basic_blocks.iter().any(|bb| {
-        bb.is_cleanup || matches!(bb.terminator().unwind(), Some(mir::UnwindAction::Terminate(_)))
+    if mir::traversal::mono_reachable(&mir, tcx, instance).any(|(bb, block)| {
+        live_blocks.contains(bb)
+            && (block.is_cleanup
+                || matches!(block.terminator().unwind(), Some(mir::UnwindAction::Terminate(_))))
     }) {
         start_bx.set_personality_fn(cx.eh_personality());
     }
 
-    let cleanup_kinds =
-        base::wants_new_eh_instructions(tcx.sess).then(|| analyze::cleanup_kinds(&mir));
+    let cleanup_kinds = base::wants_new_eh_instructions(tcx.sess)
+        .then(|| analyze::cleanup_kinds(&mir, &live_blocks));
 
     let cached_llbbs: IndexVec<mir::BasicBlock, CachedLlbb<Bx::BasicBlock>> =
         mir.basic_blocks
@@ -228,6 +238,7 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         debug_context,
         per_local_var_debug_info: None,
         caller_location: None,
+        live_blocks,
     };
 
     // It may seem like we should iterate over `required_consts` to ensure they all successfully
@@ -239,7 +250,8 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         fx.compute_per_local_var_debug_info(&mut start_bx).unzip();
     fx.per_local_var_debug_info = per_local_var_debug_info;
 
-    let traversal_order = traversal::mono_reachable_reverse_postorder(mir, tcx, instance);
+    let mut traversal_order = traversal::mono_reachable_reverse_postorder(mir, tcx, instance);
+    traversal_order.retain(|bb| fx.live_blocks.contains(*bb));
     let memory_locals = analyze::non_ssa_locals(&fx, &traversal_order);
 
     // Allocate variable and temp allocas
@@ -371,6 +383,135 @@ fn optimize_use_clone<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     mir
 }
 
+//
+/// Detect cases where monomorphized MIR has a cleanup block (or series of blocks) that never does
+/// anything, just resumes unwinding.
+///
+/// This usually results from pre-mono MIR having a no-op drop(...) for a specific type.
+///
+/// Returns a set with all basic blocks that should be treated as dead code (i.e., not codegen'd and
+/// any Cleanup branch to them should instead UnwindAction::Continue). This doesn't mutate the MIR
+/// because that can be quite expensive.
+fn find_noop_cleanup<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
+    cx: &'a Bx::CodegenCx,
+    mir: &Body<'tcx>,
+    instance: Instance<'tcx>,
+) -> DenseBitSet<mir::BasicBlock> {
+    let tcx = cx.tcx();
+
+    // First we construct a bitset containing basic blocks that do something (`any_action`). These
+    // are live code that can't be skipped at codegen time.
+    let mut any_action = DenseBitSet::new_empty(mir.basic_blocks.len());
+    for (bb, block) in mir.basic_blocks.iter_enumerated() {
+        if !block.is_cleanup {
+            // We don't care about non-cleanup blocks.
+            any_action.insert(bb);
+            continue;
+        }
+
+        let mut has_actions = false;
+        for stmt in &block.statements {
+            match stmt.kind {
+                mir::StatementKind::SetDiscriminant { .. }
+                | mir::StatementKind::Deinit(..)
+                | mir::StatementKind::StorageLive(..)
+                | mir::StatementKind::StorageDead(..)
+                | mir::StatementKind::Retag(..)
+                | mir::StatementKind::Coverage(..)
+                | mir::StatementKind::Intrinsic(..)
+                | mir::StatementKind::Assign(..) => {
+                    has_actions = true;
+                    break;
+                }
+                mir::StatementKind::FakeRead(..)
+                | mir::StatementKind::PlaceMention(..)
+                | mir::StatementKind::AscribeUserType(..)
+                | mir::StatementKind::ConstEvalCounter
+                | mir::StatementKind::Nop
+                | mir::StatementKind::BackwardIncompatibleDropHint { .. } => {}
+            }
+        }
+        match block.terminator().kind {
+            mir::TerminatorKind::Goto { .. }
+            | mir::TerminatorKind::SwitchInt { .. }
+            | mir::TerminatorKind::UnwindResume
+            | mir::TerminatorKind::Unreachable => {}
+
+            mir::TerminatorKind::Call { .. }
+            | mir::TerminatorKind::Assert { .. }
+            | mir::TerminatorKind::Yield { .. }
+            | mir::TerminatorKind::InlineAsm { .. }
+            | mir::TerminatorKind::CoroutineDrop
+            | mir::TerminatorKind::TailCall { .. }
+            | mir::TerminatorKind::UnwindTerminate(..)
+            | mir::TerminatorKind::Return => has_actions = true,
+
+            mir::TerminatorKind::Drop { place, .. } => {
+                let ty = place.ty(mir, tcx).ty;
+                debug!("monomorphize: instance={:?}", instance);
+                let ty = instance.instantiate_mir_and_normalize_erasing_regions(
+                    tcx,
+                    cx.typing_env(),
+                    ty::EarlyBinder::bind(ty),
+                );
+                let drop_fn = Instance::resolve_drop_in_place(tcx, ty);
+                if let ty::InstanceKind::DropGlue(_, None) = drop_fn.def {
+                    // no need to drop anything
+                } else {
+                    has_actions = true;
+                }
+            }
+
+            mir::TerminatorKind::FalseEdge { .. } | mir::TerminatorKind::FalseUnwind { .. } => {
+                bug!("not present in optimized mir")
+            }
+        }
+
+        if has_actions {
+            any_action.insert(bb);
+        }
+    }
+
+    let mut visited = DenseBitSet::new_empty(mir.basic_blocks.len());
+    let mut stack = vec![mir::START_BLOCK];
+    while let Some(next) = stack.pop() {
+        // Mark blocks live as we go.
+        if !visited.insert(next) {
+            continue;
+        }
+
+        // To find successors, we consider whether to skip the target of the
+        // UnwindAction::Cleanup(...) edge. If it hits blocks which do something (are in
+        // any_action), then the edge must be kept and those blocks visited. Otherwise the
+        // blocks can be considered dead.
+        if let Some(mir::UnwindAction::Cleanup(cleanup_target)) =
+            mir.basic_blocks[next].terminator().unwind()
+        {
+            let found_action = mir::traversal::Postorder::new(
+                &mir.basic_blocks,
+                *cleanup_target,
+                Some((tcx, instance)),
+            )
+            .any(|bb| any_action.contains(bb));
+            if !found_action {
+                // Do not traverse into the cleanup target if no action is found.
+                stack.extend(
+                    mir.basic_blocks[next]
+                        .mono_successors(tcx, instance)
+                        .filter(|v| v != cleanup_target),
+                );
+
+                // Avoid hitting the extend below.
+                continue;
+            }
+        }
+
+        stack.extend(mir.basic_blocks[next].mono_successors(tcx, instance));
+    }
+
+    visited
+}
+
 /// Produces, for each argument, a `Value` pointing at the
 /// argument's value. As arguments are places, these are always
 /// indirect.
diff --git a/tests/codegen/mem-replace-big-type.rs b/tests/codegen/mem-replace-big-type.rs
@@ -25,7 +25,7 @@ pub fn replace_big(dst: &mut Big, src: Big) -> Big {
 // CHECK-NOT: call void @llvm.memcpy
 
 // For a large type, we expect exactly three `memcpy`s
-// CHECK-LABEL: define internal void @{{.+}}mem{{.+}}replace{{.+}}(ptr
+// CHECK-LABEL: define void @{{.+}}mem{{.+}}replace{{.+}}(ptr
 // CHECK-SAME: sret([56 x i8]){{.+}}[[RESULT:%.+]], ptr{{.+}}%dest, ptr{{.+}}%src)
 // CHECK-NOT: call void @llvm.memcpy
 // CHECK: call void @llvm.memcpy.{{.+}}(ptr align 8 [[RESULT]], ptr align 8 %dest, i{{.*}} 56, i1 false)
