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Extend SimplifyLocals to remove ZST writes

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Simon Vandel Sillesen 2020-12-29 23:32:31 +01:00 • committed by Erik Desjardins
parent e7e1dc158c
commit 4e901be57d
34 changed files with 40 additions and 40 deletions
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30
compiler/rustc_mir/src/transform/simplify.rs
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@ -333,7 +333,7 @@ pub fn simplify_locals<'tcx>(body: &mut Body<'tcx>, tcx: TyCtxt<'tcx>) {
// count. For example, if we removed `_2 = discriminant(_1)`, then we'll subtract one from
// `use_counts[_1]`. That in turn might make `_1` unused, so we loop until we hit a
// fixedpoint where there are no more unused locals.
remove_unused_definitions(&mut used_locals, body);
remove_unused_definitions(&mut used_locals, body, tcx);
// Finally, we'll actually do the work of shrinking `body.local_decls` and remapping the `Local`s.
let map = make_local_map(&mut body.local_decls, &used_locals);
@ -462,12 +462,22 @@ impl Visitor<'_> for UsedLocals {
}
/// Removes unused definitions. Updates the used locals to reflect the changes made.
fn remove_unused_definitions<'a, 'tcx>(used_locals: &'a mut UsedLocals, body: &mut Body<'tcx>) {
fn remove_unused_definitions<'a, 'tcx>(
used_locals: &'a mut UsedLocals,
body: &mut Body<'tcx>,
tcx: TyCtxt<'tcx>,
) {
// The use counts are updated as we remove the statements. A local might become unused
// during the retain operation, leading to a temporary inconsistency (storage statements or
// definitions referencing the local might remain). For correctness it is crucial that this
// computation reaches a fixed point.
let def_id = body.source.def_id();
let is_static = tcx.is_static(def_id);
let param_env = tcx.param_env(def_id);
let local_decls = body.local_decls.clone();
let mut modified = true;
while modified {
modified = false;
@ -479,7 +489,21 @@ fn remove_unused_definitions<'a, 'tcx>(used_locals: &'a mut UsedLocals, body: &m
StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
used_locals.is_used(*local)
}
StatementKind::Assign(box (place, _)) => used_locals.is_used(place.local),
StatementKind::Assign(box (place, _)) => {
let used = used_locals.is_used(place.local);
let mut is_zst = false;
// ZST locals can be removed
if used && !is_static {
let ty = local_decls[place.local].ty;
let param_env_and = param_env.and(ty);
if let Ok(layout) = tcx.layout_of(param_env_and) {
is_zst = layout.is_zst();
}
}
used && !is_zst
}
StatementKind::SetDiscriminant { ref place, .. } => {
used_locals.is_used(place.local)