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mir opt: fix subtype handling

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lcnr 2023-06-09 13:13:42 +02:00
parent 46af169ec5
commit 0589cd0f0a
2 changed files with 20 additions and 0 deletions
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13
compiler/rustc_mir_transform/src/dest_prop.rs
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@ -37,6 +37,10 @@
//! if they do not consistently refer to the same place in memory. This is satisfied if they do
//! not contain any indirection through a pointer or any indexing projections.
//!
//! * `p` and `q` must have the **same type**. If we replace a local with a subtype or supertype,
//! we may end up with a differnet vtable for that local. See the `subtyping-impacts-selection`
//! tests for an example where that causes issues.
//!
//! * We need to make sure that the goal of "merging the memory" is actually structurally possible
//! in MIR. For example, even if all the other conditions are satisfied, there is no way to
//! "merge" `_5.foo` and `_6.bar`. For now, we ensure this by requiring that both `p` and `q` are
@ -134,6 +138,7 @@ use crate::MirPass;
use rustc_data_structures::fx::FxHashMap;
use rustc_index::bit_set::BitSet;
use rustc_middle::mir::visit::{MutVisitor, PlaceContext, Visitor};
use rustc_middle::mir::HasLocalDecls;
use rustc_middle::mir::{dump_mir, PassWhere};
use rustc_middle::mir::{
traversal, Body, InlineAsmOperand, Local, LocalKind, Location, Operand, Place, Rvalue,
@ -769,6 +774,14 @@ impl<'tcx> Visitor<'tcx> for FindAssignments<'_, '_, 'tcx> {
return;
}
// As described at the top of this file, we do not touch locals which have different types.
let src_ty = self.body.local_decls()[src].ty;
let dest_ty = self.body.local_decls()[dest].ty;
if src_ty != dest_ty {
trace!("skipped `{src:?} = {dest:?}` due to subtyping: {src_ty} != {dest_ty}");
return;
}
// Also, we need to make sure that MIR actually allows the `src` to be removed
if is_local_required(src, self.body) {
return;