Refactor AddCallGuards in two loops.

Author: cjgillot

compiler/rustc_mir_transform/src/add_call_guards.rs

@@ -1,3 +1,20 @@
+//! Breaks outgoing critical edges for call terminators in the MIR.
+//!
+//! Critical edges are edges that are neither the only edge leaving a
+//! block, nor the only edge entering one.
+//!
+//! When you want something to happen "along" an edge, you can either
+//! do at the end of the predecessor block, or at the start of the
+//! successor block. Critical edges have to be broken in order to prevent
+//! "edge actions" from affecting other edges. We need this for calls that are
+//! codegened to LLVM invoke instructions, because invoke is a block terminator
+//! in LLVM so we can't insert any code to handle the call's result into the
+//! block that performs the call.
+//!
+//! This function will break those edges by inserting new blocks along them.
+//!
+//! NOTE: Simplify CFG will happily undo most of the work this pass does.
+
 use rustc_index::{Idx, IndexVec};
 use rustc_middle::mir::*;
 use rustc_middle::ty::TyCtxt;
@@ -10,80 +27,48 @@ pub(super) enum AddCallGuards {
 }
 pub(super) use self::AddCallGuards::*;
 
-/**
- * Breaks outgoing critical edges for call terminators in the MIR.
- *
- * Critical edges are edges that are neither the only edge leaving a
- * block, nor the only edge entering one.
- *
- * When you want something to happen "along" an edge, you can either
- * do at the end of the predecessor block, or at the start of the
- * successor block. Critical edges have to be broken in order to prevent
- * "edge actions" from affecting other edges. We need this for calls that are
- * codegened to LLVM invoke instructions, because invoke is a block terminator
- * in LLVM so we can't insert any code to handle the call's result into the
- * block that performs the call.
- *
- * This function will break those edges by inserting new blocks along them.
- *
- * NOTE: Simplify CFG will happily undo most of the work this pass does.
- *
- */
-
 impl<'tcx> crate::MirPass<'tcx> for AddCallGuards {
     fn run_pass(&self, _tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         let mut pred_count = IndexVec::from_elem(0u8, &body.basic_blocks);
+        pred_count[START_BLOCK] = 1; // Add entry edge.
         for (_, data) in body.basic_blocks.iter_enumerated() {
             for succ in data.terminator().successors() {
                 pred_count[succ] = pred_count[succ].saturating_add(1);
             }
         }
 
-        // We need a place to store the new blocks generated
-        let mut new_blocks = Vec::new();
-
-        let cur_len = body.basic_blocks.len();
-        let mut new_block = |source_info: SourceInfo, is_cleanup: bool, target: BasicBlock| {
-            let block = BasicBlockData::new(
-                Some(Terminator { source_info, kind: TerminatorKind::Goto { target } }),
-                is_cleanup,
-            );
-            let idx = cur_len + new_blocks.len();
-            new_blocks.push(block);
-            BasicBlock::new(idx)
-        };
+        enum Action {
+            Call,
+            Asm { target_index: usize },
+        }
 
-        for block in body.basic_blocks_mut() {
-            match block.terminator {
-                Some(Terminator {
-                    kind: TerminatorKind::Call { target: Some(ref mut destination), unwind, .. },
-                    source_info,
-                }) if pred_count[*destination] > 1
-                    && (generates_invoke(unwind) || self == &AllCallEdges) =>
+        let mut work = Vec::with_capacity(body.basic_blocks.len());
+        for (bb, block) in body.basic_blocks.iter_enumerated() {
+            let term = block.terminator();
+            match term.kind {
+                TerminatorKind::Call { target: Some(destination), unwind, .. }
+                    if pred_count[destination] > 1
+                        && (generates_invoke(unwind) || self == &AllCallEdges) =>
                 {
                     // It's a critical edge, break it
-                    *destination = new_block(source_info, block.is_cleanup, *destination);
+                    work.push((bb, Action::Call));
                 }
-                Some(Terminator {
-                    kind:
-                        TerminatorKind::InlineAsm {
-                            asm_macro: InlineAsmMacro::Asm,
-                            ref mut targets,
-                            ref operands,
-                            unwind,
-                            ..
-                        },
-                    source_info,
-                }) if self == &CriticalCallEdges => {
+                TerminatorKind::InlineAsm {
+                    asm_macro: InlineAsmMacro::Asm,
+                    ref targets,
+                    ref operands,
+                    unwind,
+                    ..
+                } if self == &CriticalCallEdges => {
                     let has_outputs = operands.iter().any(|op| {
                         matches!(op, InlineAsmOperand::InOut { .. } | InlineAsmOperand::Out { .. })
                     });
                     let has_labels =
                         operands.iter().any(|op| matches!(op, InlineAsmOperand::Label { .. }));
                     if has_outputs && (has_labels || generates_invoke(unwind)) {
-                        for target in targets.iter_mut() {
+                        for (target_index, target) in targets.iter().enumerate() {
                             if pred_count[*target] > 1 {
-                                *target = new_block(source_info, block.is_cleanup, *target);
+                                work.push((bb, Action::Asm { target_index }));
                             }
                         }
                     }
@@ -92,9 +77,50 @@ impl<'tcx> crate::MirPass<'tcx> for AddCallGuards {
             }
         }
 
-        debug!("Broke {} N edges", new_blocks.len());
+        if work.is_empty() {
+            return;
+        }
+
+        // We need a place to store the new blocks generated
+        let mut new_blocks = Vec::new();
 
-        body.basic_blocks_mut().extend(new_blocks);
+        let cur_len = body.basic_blocks.len();
+        let mut new_block = |source_info: SourceInfo, is_cleanup: bool, target: BasicBlock| {
+            let block = BasicBlockData::new(
+                Some(Terminator { source_info, kind: TerminatorKind::Goto { target } }),
+                is_cleanup,
+            );
+            let idx = cur_len + new_blocks.len();
+            new_blocks.push(block);
+            BasicBlock::new(idx)
+        };
+
+        let basic_blocks = body.basic_blocks.as_mut();
+        for (source, action) in work {
+            let block = &mut basic_blocks[source];
+            let is_cleanup = block.is_cleanup;
+            let term = block.terminator_mut();
+            let source_info = term.source_info;
+            match action {
+                Action::Call => {
+                    let TerminatorKind::Call { target: Some(ref mut destination), .. } = term.kind
+                    else {
+                        unreachable!()
+                    };
+                    *destination = new_block(source_info, is_cleanup, *destination);
+                }
+                Action::Asm { target_index } => {
+                    let TerminatorKind::InlineAsm { ref mut targets, .. } = term.kind else {
+                        unreachable!()
+                    };
+                    targets[target_index] =
+                        new_block(source_info, is_cleanup, targets[target_index]);
+                }
+            }
+        }
+
+        debug!("Broke {} N edges", new_blocks.len());
+        basic_blocks.extend(new_blocks);
     }
 
     fn is_required(&self) -> bool {