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Move ZST check inside UsedLocals

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Simon Vandel Sillesen 2021-01-03 14:51:24 +01:00 committed by Erik Desjardins
parent 4e901be57d
commit bb36e3c7e7
2 changed files with 54 additions and 50 deletions
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95
compiler/rustc_mir/src/transform/simplify.rs
View File

@ -31,10 +31,10 @@ use crate::transform::MirPass;
use rustc_index::vec::{Idx, IndexVec};
use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor};
use rustc_middle::mir::*;
use rustc_middle::ty::ParamEnv;
use rustc_middle::ty::TyCtxt;
use smallvec::SmallVec;
use std::borrow::Cow;
use std::convert::TryInto;
use std::{borrow::Cow, convert::TryInto};
pub struct SimplifyCfg {
label: String,
@ -326,17 +326,18 @@ impl<'tcx> MirPass<'tcx> for SimplifyLocals {
pub fn simplify_locals<'tcx>(body: &mut Body<'tcx>, tcx: TyCtxt<'tcx>) {
// First, we're going to get a count of *actual* uses for every `Local`.
let mut used_locals = UsedLocals::new(body);
let mut used_locals = UsedLocals::new(body, tcx);
// Next, we're going to remove any `Local` with zero actual uses. When we remove those
// `Locals`, we're also going to subtract any uses of other `Locals` from the `used_locals`
// 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, tcx);
remove_unused_definitions(&mut used_locals, body);
// 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);
let arg_count = body.arg_count.try_into().unwrap();
let map = make_local_map(&mut body.local_decls, &used_locals, arg_count);
// Only bother running the `LocalUpdater` if we actually found locals to remove.
if map.iter().any(Option::is_none) {
@ -349,54 +350,61 @@ pub fn simplify_locals<'tcx>(body: &mut Body<'tcx>, tcx: TyCtxt<'tcx>) {
}
/// Construct the mapping while swapping out unused stuff out from the `vec`.
fn make_local_map<V>(
fn make_local_map<'tcx, V>(
local_decls: &mut IndexVec<Local, V>,
used_locals: &UsedLocals,
used_locals: &UsedLocals<'tcx>,
arg_count: u32,
) -> IndexVec<Local, Option<Local>> {
let mut map: IndexVec<Local, Option<Local>> = IndexVec::from_elem(None, &*local_decls);
let mut map: IndexVec<Local, Option<Local>> = IndexVec::from_elem(None, local_decls);
let mut used = Local::new(0);
for alive_index in local_decls.indices() {
// `is_used` treats the `RETURN_PLACE` and arguments as used.
if !used_locals.is_used(alive_index) {
continue;
}
// When creating the local map treat the `RETURN_PLACE` and arguments as used.
if alive_index.as_u32() <= arg_count || used_locals.is_used(alive_index) {
map[alive_index] = Some(used);
if alive_index != used {
local_decls.swap(alive_index, used);
}
used.increment_by(1);
}
}
local_decls.truncate(used.index());
map
}
/// Keeps track of used & unused locals.
struct UsedLocals {
struct UsedLocals<'tcx> {
increment: bool,
arg_count: u32,
use_count: IndexVec<Local, u32>,
is_static: bool,
local_decls: IndexVec<Local, LocalDecl<'tcx>>,
param_env: ParamEnv<'tcx>,
tcx: TyCtxt<'tcx>,
}
impl UsedLocals {
impl UsedLocals<'tcx> {
/// Determines which locals are used & unused in the given body.
fn new(body: &Body<'_>) -> Self {
fn new(body: &Body<'tcx>, tcx: TyCtxt<'tcx>) -> Self {
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 this = Self {
increment: true,
arg_count: body.arg_count.try_into().unwrap(),
use_count: IndexVec::from_elem(0, &body.local_decls),
is_static,
local_decls,
param_env,
tcx,
};
this.visit_body(body);
this
}
/// Checks if local is used.
///
/// Return place and arguments are always considered used.
fn is_used(&self, local: Local) -> bool {
trace!("is_used({:?}): use_count: {:?}", local, self.use_count[local]);
local.as_u32() <= self.arg_count || self.use_count[local] != 0
self.use_count[local] != 0
}
/// Updates the use counts to reflect the removal of given statement.
@ -424,7 +432,7 @@ impl UsedLocals {
}
}
impl Visitor<'_> for UsedLocals {
impl Visitor<'tcx> for UsedLocals<'tcx> {
fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
match statement.kind {
StatementKind::LlvmInlineAsm(..)
@ -451,7 +459,23 @@ impl Visitor<'_> for UsedLocals {
}
}
fn visit_local(&mut self, local: &Local, _ctx: PlaceContext, _location: Location) {
fn visit_local(&mut self, local: &Local, ctx: PlaceContext, _location: Location) {
debug!("local: {:?} is_static: {:?}, ctx: {:?}", local, self.is_static, ctx);
// Do not count a local as used in `_local = <rhs>` if RHS is a ZST.
let store = matches!(ctx, PlaceContext::MutatingUse(MutatingUseContext::Store));
// Do not count _0 as a used in `return;` if it is a ZST.
let return_place = *local == RETURN_PLACE
&& matches!(ctx, PlaceContext::NonMutatingUse(visit::NonMutatingUseContext::Move));
if !self.is_static && (store || return_place) {
let ty = self.local_decls[*local].ty;
let param_env_and = self.param_env.and(ty);
if let Ok(layout) = self.tcx.layout_of(param_env_and) {
debug!("layout.is_zst: {:?}", layout.is_zst());
if layout.is_zst() {
return;
}
}
}
if self.increment {
self.use_count[*local] += 1;
} else {
@ -463,21 +487,14 @@ 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,
used_locals: &'a mut UsedLocals<'tcx>,
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;
@ -489,21 +506,7 @@ fn remove_unused_definitions<'a, 'tcx>(
StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
used_locals.is_used(*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::Assign(box (place, _)) => used_locals.is_used(place.local),
StatementKind::SetDiscriminant { ref place, .. } => {
used_locals.is_used(place.local)

1
src/test/mir-opt/inline/issue_76997_inline_scopes_parenting.main.Inline.after.mir
View File

@ -28,6 +28,7 @@ fn main() -> () {
StorageLive(_5); // scope 1 at $DIR/issue-76997-inline-scopes-parenting.rs:6:5: 6:10
_5 = move (_3.0: ()); // scope 1 at $DIR/issue-76997-inline-scopes-parenting.rs:6:5: 6:10
StorageLive(_6); // scope 2 at $DIR/issue-76997-inline-scopes-parenting.rs:6:5: 6:10
_6 = const (); // scope 2 at $DIR/issue-76997-inline-scopes-parenting.rs:6:5: 6:10
StorageDead(_6); // scope 2 at $DIR/issue-76997-inline-scopes-parenting.rs:6:5: 6:10
StorageDead(_5); // scope 1 at $DIR/issue-76997-inline-scopes-parenting.rs:6:5: 6:10
StorageDead(_4); // scope 1 at $DIR/issue-76997-inline-scopes-parenting.rs:6:9: 6:10