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Auto merge of #106908 - cjgillot:copyprop-ssa, r=oli-obk

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Implement simple CopyPropagation based on SSA analysis

This PR extracts the "copy propagation" logic from https://github.com/rust-lang/rust/pull/106285.

MIR may produce chains of assignment between locals, like `_x = move? _y`.
This PR attempts to remove such chains by unifying locals.

The current implementation is a bit overzealous in turning moves into copies, and in removing storage statements.
This commit is contained in:
bors 2023-01-29 13:01:06 +00:00
commit 2a4b00beaa
68 changed files with 1948 additions and 278 deletions
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5
compiler/rustc_mir_transform/src/dest_prop.rs
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@ -328,7 +328,8 @@ impl<'a, 'tcx> MutVisitor<'tcx> for Merger<'a, 'tcx> {
match &statement.kind {
StatementKind::Assign(box (dest, rvalue)) => {
match rvalue {
Rvalue::Use(Operand::Copy(place) | Operand::Move(place)) => {
Rvalue::CopyForDeref(place)
| Rvalue::Use(Operand::Copy(place) | Operand::Move(place)) => {
// These might've been turned into self-assignments by the replacement
// (this includes the original statement we wanted to eliminate).
if dest == place {
@ -755,7 +756,7 @@ impl<'tcx> Visitor<'tcx> for FindAssignments<'_, '_, 'tcx> {
fn visit_statement(&mut self, statement: &Statement<'tcx>, _: Location) {
if let StatementKind::Assign(box (
lhs,
Rvalue::Use(Operand::Copy(rhs) | Operand::Move(rhs)),
Rvalue::CopyForDeref(rhs) | Rvalue::Use(Operand::Copy(rhs) | Operand::Move(rhs)),
)) = &statement.kind
{
let Some((src, dest)) = places_to_candidate_pair(*lhs, *rhs, self.body) else {