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Auto merge of #83177 - erikdesjardins:zstassign, r=oli-obk

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Remove assignments to ZST places instead of marking ZST return place as unused

partially reverts #83118

requested by `@tmiasko` in https://github.com/rust-lang/rust/pull/83118#issuecomment-799692574

r? `@oli-obk`
This commit is contained in:
bors 2021-03-23 12:23:27 +00:00
commit 79e5814f45
86 changed files with 180 additions and 178 deletions
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2
compiler/rustc_mir/src/transform/mod.rs
View file

@ -44,6 +44,7 @@ pub mod promote_consts;
pub mod remove_noop_landing_pads;
pub mod remove_storage_markers;
pub mod remove_unneeded_drops;
pub mod remove_zsts;
pub mod required_consts;
pub mod rustc_peek;
pub mod simplify;
@ -494,6 +495,7 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
// The main optimizations that we do on MIR.
let optimizations: &[&dyn MirPass<'tcx>] = &[
&remove_storage_markers::RemoveStorageMarkers,
&remove_zsts::RemoveZsts,
&const_goto::ConstGoto,
&remove_unneeded_drops::RemoveUnneededDrops,
&match_branches::MatchBranchSimplification,

57
compiler/rustc_mir/src/transform/remove_zsts.rs Normal file
View file

@ -0,0 +1,57 @@
//! Removes assignments to ZST places.
use crate::transform::MirPass;
use rustc_middle::mir::{Body, StatementKind};
use rustc_middle::ty::{self, Ty, TyCtxt};
pub struct RemoveZsts;
impl<'tcx> MirPass<'tcx> for RemoveZsts {
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
if tcx.sess.mir_opt_level() < 3 {
return;
}
let param_env = tcx.param_env(body.source.def_id());
let (basic_blocks, local_decls) = body.basic_blocks_and_local_decls_mut();
for block in basic_blocks.iter_mut() {
for statement in block.statements.iter_mut() {
match statement.kind {
StatementKind::Assign(box (place, _)) => {
let place_ty = place.ty(local_decls, tcx).ty;
if !maybe_zst(place_ty) {
continue;
}
let layout = match tcx.layout_of(param_env.and(place_ty)) {
Ok(layout) => layout,
Err(_) => continue,
};
if !layout.is_zst() {
continue;
}
if tcx.consider_optimizing(|| {
format!(
"RemoveZsts - Place: {:?} SourceInfo: {:?}",
place, statement.source_info
)
}) {
statement.make_nop();
}
}
_ => {}
}
}
}
}
}
/// A cheap, approximate check to avoid unnecessary `layout_of` calls.
fn maybe_zst(ty: Ty<'_>) -> bool {
match ty.kind() {
// maybe ZST (could be more precise)
ty::Adt(..) | ty::Array(..) | ty::Closure(..) | ty::Tuple(..) | ty::Opaque(..) => true,
// definitely ZST
ty::FnDef(..) | ty::Never => true,
// unreachable or can't be ZST
_ => false,
}
}

79
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, convert::TryInto};
use std::borrow::Cow;
use std::convert::TryInto;
pub struct SimplifyCfg {
label: String,
@ -326,7 +326,7 @@ 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, tcx);
let mut used_locals = UsedLocals::new(body);
// 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`
@ -336,8 +336,7 @@ pub fn simplify_locals<'tcx>(body: &mut Body<'tcx>, tcx: TyCtxt<'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 arg_count = body.arg_count.try_into().unwrap();
let map = make_local_map(&mut body.local_decls, &used_locals, arg_count);
let map = make_local_map(&mut body.local_decls, &used_locals);
// Only bother running the `LocalUpdater` if we actually found locals to remove.
if map.iter().any(Option::is_none) {
@ -350,61 +349,54 @@ 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<'tcx, V>(
fn make_local_map<V>(
local_decls: &mut IndexVec<Local, V>,
used_locals: &UsedLocals<'tcx>,
arg_count: u32,
used_locals: &UsedLocals,
) -> 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() {
// 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);
// `is_used` treats the `RETURN_PLACE` and arguments as used.
if !used_locals.is_used(alive_index) {
continue;
}
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<'tcx> {
struct UsedLocals {
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<'tcx> {
impl UsedLocals {
/// Determines which locals are used & unused in the given body.
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();
fn new(body: &Body<'_>) -> Self {
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]);
self.use_count[local] != 0
local.as_u32() <= self.arg_count || self.use_count[local] != 0
}
/// Updates the use counts to reflect the removal of given statement.
@ -434,7 +426,7 @@ impl UsedLocals<'tcx> {
}
}
impl Visitor<'tcx> for UsedLocals<'tcx> {
impl Visitor<'_> for UsedLocals {
fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
match statement.kind {
StatementKind::LlvmInlineAsm(..)
@ -461,21 +453,7 @@ impl Visitor<'tcx> for UsedLocals<'tcx> {
}
}
fn visit_local(&mut self, local: &Local, ctx: PlaceContext, _location: Location) {
debug!("local: {:?} is_static: {:?}, ctx: {:?}", local, self.is_static, ctx);
// 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 && 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;
}
}
}
fn visit_local(&mut self, local: &Local, _ctx: PlaceContext, _location: Location) {
if self.increment {
self.use_count[*local] += 1;
} else {
@ -486,10 +464,7 @@ impl Visitor<'tcx> for UsedLocals<'tcx> {
}
/// Removes unused definitions. Updates the used locals to reflect the changes made.
fn remove_unused_definitions<'a, 'tcx>(
used_locals: &'a mut UsedLocals<'tcx>,
body: &mut Body<'tcx>,
) {
fn remove_unused_definitions<'a, 'tcx>(used_locals: &'a mut UsedLocals, body: &mut Body<'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