rustc_mir_transform/
add_call_guards.rs

1use rustc_index::{Idx, IndexVec};
2use rustc_middle::mir::*;
3use rustc_middle::ty::TyCtxt;
4use tracing::debug;
5
6#[derive(PartialEq)]
7pub(super) enum AddCallGuards {
8    AllCallEdges,
9    CriticalCallEdges,
10}
11pub(super) use self::AddCallGuards::*;
12
13/**
14 * Breaks outgoing critical edges for call terminators in the MIR.
15 *
16 * Critical edges are edges that are neither the only edge leaving a
17 * block, nor the only edge entering one.
18 *
19 * When you want something to happen "along" an edge, you can either
20 * do at the end of the predecessor block, or at the start of the
21 * successor block. Critical edges have to be broken in order to prevent
22 * "edge actions" from affecting other edges. We need this for calls that are
23 * codegened to LLVM invoke instructions, because invoke is a block terminator
24 * in LLVM so we can't insert any code to handle the call's result into the
25 * block that performs the call.
26 *
27 * This function will break those edges by inserting new blocks along them.
28 *
29 * NOTE: Simplify CFG will happily undo most of the work this pass does.
30 *
31 */
32
33impl<'tcx> crate::MirPass<'tcx> for AddCallGuards {
34    fn run_pass(&self, _tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
35        let mut pred_count = IndexVec::from_elem(0u8, &body.basic_blocks);
36        for (_, data) in body.basic_blocks.iter_enumerated() {
37            for succ in data.terminator().successors() {
38                pred_count[succ] = pred_count[succ].saturating_add(1);
39            }
40        }
41
42        // We need a place to store the new blocks generated
43        let mut new_blocks = Vec::new();
44
45        let cur_len = body.basic_blocks.len();
46        let mut new_block = |source_info: SourceInfo, is_cleanup: bool, target: BasicBlock| {
47            let block = BasicBlockData::new(
48                Some(Terminator { source_info, kind: TerminatorKind::Goto { target } }),
49                is_cleanup,
50            );
51            let idx = cur_len + new_blocks.len();
52            new_blocks.push(block);
53            BasicBlock::new(idx)
54        };
55
56        for block in body.basic_blocks_mut() {
57            match block.terminator {
58                Some(Terminator {
59                    kind: TerminatorKind::Call { target: Some(ref mut destination), unwind, .. },
60                    source_info,
61                }) if pred_count[*destination] > 1
62                    && (generates_invoke(unwind) || self == &AllCallEdges) =>
63                {
64                    // It's a critical edge, break it
65                    *destination = new_block(source_info, block.is_cleanup, *destination);
66                }
67                Some(Terminator {
68                    kind:
69                        TerminatorKind::InlineAsm {
70                            asm_macro: InlineAsmMacro::Asm,
71                            ref mut targets,
72                            ref operands,
73                            unwind,
74                            ..
75                        },
76                    source_info,
77                }) if self == &CriticalCallEdges => {
78                    let has_outputs = operands.iter().any(|op| {
79                        matches!(op, InlineAsmOperand::InOut { .. } | InlineAsmOperand::Out { .. })
80                    });
81                    let has_labels =
82                        operands.iter().any(|op| matches!(op, InlineAsmOperand::Label { .. }));
83                    if has_outputs && (has_labels || generates_invoke(unwind)) {
84                        for target in targets.iter_mut() {
85                            if pred_count[*target] > 1 {
86                                *target = new_block(source_info, block.is_cleanup, *target);
87                            }
88                        }
89                    }
90                }
91                _ => {}
92            }
93        }
94
95        debug!("Broke {} N edges", new_blocks.len());
96
97        body.basic_blocks_mut().extend(new_blocks);
98    }
99
100    fn is_required(&self) -> bool {
101        true
102    }
103}
104
105/// Returns true if this unwind action is code generated as an invoke as opposed to a call.
106fn generates_invoke(unwind: UnwindAction) -> bool {
107    match unwind {
108        UnwindAction::Continue | UnwindAction::Unreachable => false,
109        UnwindAction::Cleanup(_) | UnwindAction::Terminate(_) => true,
110    }
111}
rustc_mir_transform/add_call_guards.rs

rustc_mir_transform/
add_call_guards.rs
