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Check which places are dead

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Fixes #78192
This commit is contained in:
Simon Vandel Sillesen 2020-10-22 22:22:17 +02:00
parent a9cd294cf2
commit 57d01a9aee
6 changed files with 55 additions and 9 deletions
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20
compiler/rustc_mir/src/transform/instcombine.rs
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@ -119,11 +119,6 @@ impl OptimizationFinder<'b, 'tcx> {
}
fn find_deref_of_address(&mut self, rvalue: &Rvalue<'tcx>, location: Location) -> Option<()> {
// FIXME(#78192): This optimization can result in unsoundness.
if !self.tcx.sess.opts.debugging_opts.unsound_mir_opts {
return None;
}
// Look for the sequence
//
// _2 = &_1;
@ -137,6 +132,8 @@ impl OptimizationFinder<'b, 'tcx> {
_ => None,
}?;
let mut dead_locals_seen = vec![];
let stmt_index = location.statement_index;
// Look behind for statement that assigns the local from a address of operator.
// 6 is chosen as a heuristic determined by seeing the number of times
@ -160,6 +157,11 @@ impl OptimizationFinder<'b, 'tcx> {
BorrowKind::Shared,
place_taken_address_of,
) => {
// Make sure that the place has not been marked dead
if dead_locals_seen.contains(&place_taken_address_of.local) {
return None;
}
self.optimizations
.unneeded_deref
.insert(location, *place_taken_address_of);
@ -178,13 +180,19 @@ impl OptimizationFinder<'b, 'tcx> {
// Inline asm can do anything, so bail out of the optimization.
rustc_middle::mir::StatementKind::LlvmInlineAsm(_) => return None,
// Remember `StorageDead`s, as the local being marked dead could be the
// place RHS we are looking for, in which case we need to abort to avoid UB
// using an uninitialized place
rustc_middle::mir::StatementKind::StorageDead(dead) => {
dead_locals_seen.push(*dead)
}
// Check that `local_being_deref` is not being used in a mutating way which can cause misoptimization.
rustc_middle::mir::StatementKind::Assign(box (_, _))
| rustc_middle::mir::StatementKind::Coverage(_)
| rustc_middle::mir::StatementKind::Nop
| rustc_middle::mir::StatementKind::FakeRead(_, _)
| rustc_middle::mir::StatementKind::StorageLive(_)
| rustc_middle::mir::StatementKind::StorageDead(_)
| rustc_middle::mir::StatementKind::Retag(_, _)
| rustc_middle::mir::StatementKind::AscribeUserType(_, _)
| rustc_middle::mir::StatementKind::SetDiscriminant { .. } => {