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Diffstat (limited to '')
-rw-r--r-- | compiler/rustc_mir_transform/src/add_call_guards.rs | 81 |
1 files changed, 81 insertions, 0 deletions
diff --git a/compiler/rustc_mir_transform/src/add_call_guards.rs b/compiler/rustc_mir_transform/src/add_call_guards.rs new file mode 100644 index 000000000..f12c8560c --- /dev/null +++ b/compiler/rustc_mir_transform/src/add_call_guards.rs @@ -0,0 +1,81 @@ +use crate::MirPass; +use rustc_index::vec::{Idx, IndexVec}; +use rustc_middle::mir::*; +use rustc_middle::ty::TyCtxt; + +#[derive(PartialEq)] +pub enum AddCallGuards { + AllCallEdges, + CriticalCallEdges, +} +pub 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> MirPass<'tcx> for AddCallGuards { + fn run_pass(&self, _tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { + self.add_call_guards(body); + } +} + +impl AddCallGuards { + pub fn add_call_guards(&self, body: &mut Body<'_>) { + let mut pred_count: IndexVec<_, _> = + body.basic_blocks.predecessors().iter().map(|ps| ps.len()).collect(); + pred_count[START_BLOCK] += 1; + + // We need a place to store the new blocks generated + let mut new_blocks = Vec::new(); + + let cur_len = body.basic_blocks().len(); + + for block in body.basic_blocks_mut() { + match block.terminator { + Some(Terminator { + kind: TerminatorKind::Call { target: Some(ref mut destination), cleanup, .. }, + source_info, + }) if pred_count[*destination] > 1 + && (cleanup.is_some() || self == &AllCallEdges) => + { + // It's a critical edge, break it + let call_guard = BasicBlockData { + statements: vec![], + is_cleanup: block.is_cleanup, + terminator: Some(Terminator { + source_info, + kind: TerminatorKind::Goto { target: *destination }, + }), + }; + + // Get the index it will be when inserted into the MIR + let idx = cur_len + new_blocks.len(); + new_blocks.push(call_guard); + *destination = BasicBlock::new(idx); + } + _ => {} + } + } + + debug!("Broke {} N edges", new_blocks.len()); + + body.basic_blocks_mut().extend(new_blocks); + } +} |