rustc_mir_transform/
simplify_branches.rs

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1use rustc_middle::mir::*;
2use rustc_middle::ty::TyCtxt;
3use tracing::trace;
4
5pub(super) enum SimplifyConstCondition {
6    AfterConstProp,
7    Final,
8}
9
10/// A pass that replaces a branch with a goto when its condition is known.
11impl<'tcx> crate::MirPass<'tcx> for SimplifyConstCondition {
12    fn name(&self) -> &'static str {
13        match self {
14            SimplifyConstCondition::AfterConstProp => "SimplifyConstCondition-after-const-prop",
15            SimplifyConstCondition::Final => "SimplifyConstCondition-final",
16        }
17    }
18
19    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
20        trace!("Running SimplifyConstCondition on {:?}", body.source);
21        let typing_env = body.typing_env(tcx);
22        'blocks: for block in body.basic_blocks_mut() {
23            for stmt in block.statements.iter_mut() {
24                // Simplify `assume` of a known value: either a NOP or unreachable.
25                if let StatementKind::Intrinsic(box ref intrinsic) = stmt.kind
26                    && let NonDivergingIntrinsic::Assume(discr) = intrinsic
27                    && let Operand::Constant(c) = discr
28                    && let Some(constant) = c.const_.try_eval_bool(tcx, typing_env)
29                {
30                    if constant {
31                        stmt.make_nop();
32                    } else {
33                        block.statements.clear();
34                        block.terminator_mut().kind = TerminatorKind::Unreachable;
35                        continue 'blocks;
36                    }
37                }
38            }
39
40            let terminator = block.terminator_mut();
41            terminator.kind = match terminator.kind {
42                TerminatorKind::SwitchInt {
43                    discr: Operand::Constant(ref c), ref targets, ..
44                } => {
45                    let constant = c.const_.try_eval_bits(tcx, typing_env);
46                    if let Some(constant) = constant {
47                        let target = targets.target_for_value(constant);
48                        TerminatorKind::Goto { target }
49                    } else {
50                        continue;
51                    }
52                }
53                TerminatorKind::Assert {
54                    target, cond: Operand::Constant(ref c), expected, ..
55                } => match c.const_.try_eval_bool(tcx, typing_env) {
56                    Some(v) if v == expected => TerminatorKind::Goto { target },
57                    _ => continue,
58                },
59                _ => continue,
60            };
61        }
62    }
63
64    fn is_required(&self) -> bool {
65        false
66    }
67}
rustc_mir_transform/simplify_branches.rs

rustc_mir_transform/
simplify_branches.rs
