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 {
48                statements: vec![],
49                is_cleanup,
50                terminator: Some(Terminator { source_info, kind: TerminatorKind::Goto { target } }),
51            };
52            let idx = cur_len + new_blocks.len();
53            new_blocks.push(block);
54            BasicBlock::new(idx)
55        };
56
57        for block in body.basic_blocks_mut() {
58            match block.terminator {
59                Some(Terminator {
60                    kind: TerminatorKind::Call { target: Some(ref mut destination), unwind, .. },
61                    source_info,
62                }) if pred_count[*destination] > 1
63                    && (generates_invoke(unwind) || self == &AllCallEdges) =>
64                {
65                    // It's a critical edge, break it
66                    *destination = new_block(source_info, block.is_cleanup, *destination);
67                }
68                Some(Terminator {
69                    kind:
70                        TerminatorKind::InlineAsm {
71                            asm_macro: InlineAsmMacro::Asm,
72                            ref mut targets,
73                            ref operands,
74                            unwind,
75                            ..
76                        },
77                    source_info,
78                }) if self == &CriticalCallEdges => {
79                    let has_outputs = operands.iter().any(|op| {
80                        matches!(op, InlineAsmOperand::InOut { .. } | InlineAsmOperand::Out { .. })
81                    });
82                    let has_labels =
83                        operands.iter().any(|op| matches!(op, InlineAsmOperand::Label { .. }));
84                    if has_outputs && (has_labels || generates_invoke(unwind)) {
85                        for target in targets.iter_mut() {
86                            if pred_count[*target] > 1 {
87                                *target = new_block(source_info, block.is_cleanup, *target);
88                            }
89                        }
90                    }
91                }
92                _ => {}
93            }
94        }
95
96        debug!("Broke {} N edges", new_blocks.len());
97
98        body.basic_blocks_mut().extend(new_blocks);
99    }
100
101    fn is_required(&self) -> bool {
102        true
103    }
104}
105
106/// Returns true if this unwind action is code generated as an invoke as opposed to a call.
107fn generates_invoke(unwind: UnwindAction) -> bool {
108    match unwind {
109        UnwindAction::Continue | UnwindAction::Unreachable => false,
110        UnwindAction::Cleanup(_) | UnwindAction::Terminate(_) => true,
111    }
112}
rustc_mir_transform/add_call_guards.rs

rustc_mir_transform/
add_call_guards.rs
