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//! Global value numbering.
//!
//! MIR may contain repeated and/or redundant computations. The objective of this pass is to detect
//! such redundancies and re-use the already-computed result when possible.
//!
//! In a first pass, we compute a symbolic representation of values that are assigned to SSA
//! locals. This symbolic representation is defined by the `Value` enum. Each produced instance of
//! `Value` is interned as a `VnIndex`, which allows us to cheaply compute identical values.
//!
//! From those assignments, we construct a mapping `VnIndex -> Vec<(Local, Location)>` of available
//! values, the locals in which they are stored, and a the assignment location.
//!
//! In a second pass, we traverse all (non SSA) assignments `x = rvalue` and operands. For each
//! one, we compute the `VnIndex` of the rvalue. If this `VnIndex` is associated to a constant, we
//! replace the rvalue/operand by that constant. Otherwise, if there is an SSA local `y`
//! associated to this `VnIndex`, and if its definition location strictly dominates the assignment
//! to `x`, we replace the assignment by `x = y`.
//!
//! By opportunity, this pass simplifies some `Rvalue`s based on the accumulated knowledge.
//!
//! # Operational semantic
//!
//! Operationally, this pass attempts to prove bitwise equality between locals. Given this MIR:
//! ```ignore (MIR)
//! _a = some value // has VnIndex i
//! // some MIR
//! _b = some other value // also has VnIndex i
//! ```
//!
//! We consider it to be replacable by:
//! ```ignore (MIR)
//! _a = some value // has VnIndex i
//! // some MIR
//! _c = some other value // also has VnIndex i
//! assume(_a bitwise equal to _c) // follows from having the same VnIndex
//! _b = _a // follows from the `assume`
//! ```
//!
//! Which is simplifiable to:
//! ```ignore (MIR)
//! _a = some value // has VnIndex i
//! // some MIR
//! _b = _a
//! ```
//!
//! # Handling of references
//!
//! We handle references by assigning a different "provenance" index to each Ref/AddressOf rvalue.
//! This ensure that we do not spuriously merge borrows that should not be merged. Meanwhile, we
//! consider all the derefs of an immutable reference to a freeze type to give the same value:
//! ```ignore (MIR)
//! _a = *_b // _b is &Freeze
//! _c = *_b // replaced by _c = _a
//! ```
use rustc_data_structures::fx::{FxHashMap, FxIndexSet};
use rustc_data_structures::graph::dominators::Dominators;
use rustc_index::bit_set::BitSet;
use rustc_index::IndexVec;
use rustc_macros::newtype_index;
use rustc_middle::mir::visit::*;
use rustc_middle::mir::*;
use rustc_middle::ty::{self, Ty, TyCtxt};
use rustc_target::abi::{VariantIdx, FIRST_VARIANT};
use crate::ssa::SsaLocals;
use crate::MirPass;
pub struct GVN;
impl<'tcx> MirPass<'tcx> for GVN {
fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
sess.mir_opt_level() >= 4
}
#[instrument(level = "trace", skip(self, tcx, body))]
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
debug!(def_id = ?body.source.def_id());
propagate_ssa(tcx, body);
}
}
fn propagate_ssa<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
let param_env = tcx.param_env_reveal_all_normalized(body.source.def_id());
let ssa = SsaLocals::new(body);
// Clone dominators as we need them while mutating the body.
let dominators = body.basic_blocks.dominators().clone();
let mut state = VnState::new(tcx, param_env, &ssa, &dominators, &body.local_decls);
for arg in body.args_iter() {
if ssa.is_ssa(arg) {
let value = state.new_opaque().unwrap();
state.assign(arg, value);
}
}
ssa.for_each_assignment_mut(&mut body.basic_blocks, |local, rvalue, location| {
let value = state.simplify_rvalue(rvalue, location).or_else(|| state.new_opaque()).unwrap();
// FIXME(#112651) `rvalue` may have a subtype to `local`. We can only mark `local` as
// reusable if we have an exact type match.
if state.local_decls[local].ty == rvalue.ty(state.local_decls, tcx) {
state.assign(local, value);
}
});
// Stop creating opaques during replacement as it is useless.
state.next_opaque = None;
let reverse_postorder = body.basic_blocks.reverse_postorder().to_vec();
for bb in reverse_postorder {
let data = &mut body.basic_blocks.as_mut_preserves_cfg()[bb];
state.visit_basic_block_data(bb, data);
}
let any_replacement = state.any_replacement;
// For each local that is reused (`y` above), we remove its storage statements do avoid any
// difficulty. Those locals are SSA, so should be easy to optimize by LLVM without storage
// statements.
StorageRemover { tcx, reused_locals: state.reused_locals }.visit_body_preserves_cfg(body);
if any_replacement {
crate::simplify::remove_unused_definitions(body);
}
}
newtype_index! {
struct VnIndex {}
}
#[derive(Debug, PartialEq, Eq, Hash)]
enum Value<'tcx> {
// Root values.
/// Used to represent values we know nothing about.
/// The `usize` is a counter incremented by `new_opaque`.
Opaque(usize),
/// Evaluated or unevaluated constant value.
Constant(Const<'tcx>),
/// An aggregate value, either tuple/closure/struct/enum.
/// This does not contain unions, as we cannot reason with the value.
Aggregate(Ty<'tcx>, VariantIdx, Vec<VnIndex>),
/// This corresponds to a `[value; count]` expression.
Repeat(VnIndex, ty::Const<'tcx>),
/// The address of a place.
Address {
place: Place<'tcx>,
/// Give each borrow and pointer a different provenance, so we don't merge them.
provenance: usize,
},
// Extractions.
/// This is the *value* obtained by projecting another value.
Projection(VnIndex, ProjectionElem<VnIndex, Ty<'tcx>>),
/// Discriminant of the given value.
Discriminant(VnIndex),
/// Length of an array or slice.
Len(VnIndex),
// Operations.
NullaryOp(NullOp<'tcx>, Ty<'tcx>),
UnaryOp(UnOp, VnIndex),
BinaryOp(BinOp, VnIndex, VnIndex),
CheckedBinaryOp(BinOp, VnIndex, VnIndex),
Cast {
kind: CastKind,
value: VnIndex,
from: Ty<'tcx>,
to: Ty<'tcx>,
},
}
struct VnState<'body, 'tcx> {
tcx: TyCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
local_decls: &'body LocalDecls<'tcx>,
/// Value stored in each local.
locals: IndexVec<Local, Option<VnIndex>>,
/// First local to be assigned that value.
rev_locals: FxHashMap<VnIndex, Vec<Local>>,
values: FxIndexSet<Value<'tcx>>,
/// Counter to generate different values.
/// This is an option to stop creating opaques during replacement.
next_opaque: Option<usize>,
ssa: &'body SsaLocals,
dominators: &'body Dominators<BasicBlock>,
reused_locals: BitSet<Local>,
any_replacement: bool,
}
impl<'body, 'tcx> VnState<'body, 'tcx> {
fn new(
tcx: TyCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
ssa: &'body SsaLocals,
dominators: &'body Dominators<BasicBlock>,
local_decls: &'body LocalDecls<'tcx>,
) -> Self {
VnState {
tcx,
param_env,
local_decls,
locals: IndexVec::from_elem(None, local_decls),
rev_locals: FxHashMap::default(),
values: FxIndexSet::default(),
next_opaque: Some(0),
ssa,
dominators,
reused_locals: BitSet::new_empty(local_decls.len()),
any_replacement: false,
}
}
#[instrument(level = "trace", skip(self), ret)]
fn insert(&mut self, value: Value<'tcx>) -> VnIndex {
let (index, _) = self.values.insert_full(value);
VnIndex::from_usize(index)
}
/// Create a new `Value` for which we have no information at all, except that it is distinct
/// from all the others.
#[instrument(level = "trace", skip(self), ret)]
fn new_opaque(&mut self) -> Option<VnIndex> {
let next_opaque = self.next_opaque.as_mut()?;
let value = Value::Opaque(*next_opaque);
*next_opaque += 1;
Some(self.insert(value))
}
/// Create a new `Value::Address` distinct from all the others.
#[instrument(level = "trace", skip(self), ret)]
fn new_pointer(&mut self, place: Place<'tcx>) -> Option<VnIndex> {
let next_opaque = self.next_opaque.as_mut()?;
let value = Value::Address { place, provenance: *next_opaque };
*next_opaque += 1;
Some(self.insert(value))
}
fn get(&self, index: VnIndex) -> &Value<'tcx> {
self.values.get_index(index.as_usize()).unwrap()
}
/// Record that `local` is assigned `value`. `local` must be SSA.
#[instrument(level = "trace", skip(self))]
fn assign(&mut self, local: Local, value: VnIndex) {
self.locals[local] = Some(value);
// Only register the value if its type is `Sized`, as we will emit copies of it.
let is_sized = !self.tcx.features().unsized_locals
|| self.local_decls[local].ty.is_sized(self.tcx, self.param_env);
if is_sized {
self.rev_locals.entry(value).or_default().push(local);
}
}
/// Represent the *value* which would be read from `place`, and point `place` to a preexisting
/// place with the same value (if that already exists).
#[instrument(level = "trace", skip(self), ret)]
fn simplify_place_value(
&mut self,
place: &mut Place<'tcx>,
location: Location,
) -> Option<VnIndex> {
// Invariant: `place` and `place_ref` point to the same value, even if they point to
// different memory locations.
let mut place_ref = place.as_ref();
// Invariant: `value` holds the value up-to the `index`th projection excluded.
let mut value = self.locals[place.local]?;
for (index, proj) in place.projection.iter().enumerate() {
if let Some(local) = self.try_as_local(value, location) {
// Both `local` and `Place { local: place.local, projection: projection[..index] }`
// hold the same value. Therefore, following place holds the value in the original
// `place`.
place_ref = PlaceRef { local, projection: &place.projection[index..] };
}
let proj = match proj {
ProjectionElem::Deref => {
let ty = Place::ty_from(
place.local,
&place.projection[..index],
self.local_decls,
self.tcx,
)
.ty;
if let Some(Mutability::Not) = ty.ref_mutability()
&& let Some(pointee_ty) = ty.builtin_deref(true)
&& pointee_ty.ty.is_freeze(self.tcx, self.param_env)
{
// An immutable borrow `_x` always points to the same value for the
// lifetime of the borrow, so we can merge all instances of `*_x`.
ProjectionElem::Deref
} else {
return None;
}
}
ProjectionElem::Field(f, ty) => ProjectionElem::Field(f, ty),
ProjectionElem::Index(idx) => {
let idx = self.locals[idx]?;
ProjectionElem::Index(idx)
}
ProjectionElem::ConstantIndex { offset, min_length, from_end } => {
ProjectionElem::ConstantIndex { offset, min_length, from_end }
}
ProjectionElem::Subslice { from, to, from_end } => {
ProjectionElem::Subslice { from, to, from_end }
}
ProjectionElem::Downcast(name, index) => ProjectionElem::Downcast(name, index),
ProjectionElem::OpaqueCast(ty) => ProjectionElem::OpaqueCast(ty),
ProjectionElem::Subtype(ty) => ProjectionElem::Subtype(ty),
};
value = self.insert(Value::Projection(value, proj));
}
if let Some(local) = self.try_as_local(value, location)
&& local != place.local // in case we had no projection to begin with.
{
*place = local.into();
self.reused_locals.insert(local);
self.any_replacement = true;
} else if place_ref.local != place.local
|| place_ref.projection.len() < place.projection.len()
{
// By the invariant on `place_ref`.
*place = place_ref.project_deeper(&[], self.tcx);
self.reused_locals.insert(place_ref.local);
self.any_replacement = true;
}
Some(value)
}
#[instrument(level = "trace", skip(self), ret)]
fn simplify_operand(
&mut self,
operand: &mut Operand<'tcx>,
location: Location,
) -> Option<VnIndex> {
match *operand {
Operand::Constant(ref constant) => Some(self.insert(Value::Constant(constant.const_))),
Operand::Copy(ref mut place) | Operand::Move(ref mut place) => {
let value = self.simplify_place_value(place, location)?;
if let Some(const_) = self.try_as_constant(value) {
*operand = Operand::Constant(Box::new(const_));
self.any_replacement = true;
}
Some(value)
}
}
}
#[instrument(level = "trace", skip(self), ret)]
fn simplify_rvalue(
&mut self,
rvalue: &mut Rvalue<'tcx>,
location: Location,
) -> Option<VnIndex> {
let value = match *rvalue {
// Forward values.
Rvalue::Use(ref mut operand) => return self.simplify_operand(operand, location),
Rvalue::CopyForDeref(place) => {
let mut operand = Operand::Copy(place);
let val = self.simplify_operand(&mut operand, location);
*rvalue = Rvalue::Use(operand);
return val;
}
// Roots.
Rvalue::Repeat(ref mut op, amount) => {
let op = self.simplify_operand(op, location)?;
Value::Repeat(op, amount)
}
Rvalue::NullaryOp(op, ty) => Value::NullaryOp(op, ty),
Rvalue::Aggregate(box ref kind, ref mut fields) => {
let variant_index = match *kind {
AggregateKind::Array(..)
| AggregateKind::Tuple
| AggregateKind::Closure(..)
| AggregateKind::Generator(..) => FIRST_VARIANT,
AggregateKind::Adt(_, variant_index, _, _, None) => variant_index,
// Do not track unions.
AggregateKind::Adt(_, _, _, _, Some(_)) => return None,
};
let fields: Option<Vec<_>> = fields
.iter_mut()
.map(|op| self.simplify_operand(op, location).or_else(|| self.new_opaque()))
.collect();
let ty = rvalue.ty(self.local_decls, self.tcx);
Value::Aggregate(ty, variant_index, fields?)
}
Rvalue::Ref(.., place) | Rvalue::AddressOf(_, place) => return self.new_pointer(place),
// Operations.
Rvalue::Len(ref mut place) => {
let place = self.simplify_place_value(place, location)?;
Value::Len(place)
}
Rvalue::Cast(kind, ref mut value, to) => {
let from = value.ty(self.local_decls, self.tcx);
let value = self.simplify_operand(value, location)?;
Value::Cast { kind, value, from, to }
}
Rvalue::BinaryOp(op, box (ref mut lhs, ref mut rhs)) => {
let lhs = self.simplify_operand(lhs, location);
let rhs = self.simplify_operand(rhs, location);
Value::BinaryOp(op, lhs?, rhs?)
}
Rvalue::CheckedBinaryOp(op, box (ref mut lhs, ref mut rhs)) => {
let lhs = self.simplify_operand(lhs, location);
let rhs = self.simplify_operand(rhs, location);
Value::CheckedBinaryOp(op, lhs?, rhs?)
}
Rvalue::UnaryOp(op, ref mut arg) => {
let arg = self.simplify_operand(arg, location)?;
Value::UnaryOp(op, arg)
}
Rvalue::Discriminant(ref mut place) => {
let place = self.simplify_place_value(place, location)?;
Value::Discriminant(place)
}
// Unsupported values.
Rvalue::ThreadLocalRef(..) | Rvalue::ShallowInitBox(..) => return None,
};
debug!(?value);
Some(self.insert(value))
}
}
impl<'tcx> VnState<'_, 'tcx> {
/// If `index` is a `Value::Constant`, return the `Constant` to be put in the MIR.
fn try_as_constant(&mut self, index: VnIndex) -> Option<ConstOperand<'tcx>> {
if let Value::Constant(const_) = *self.get(index) {
// Some constants may contain pointers. We need to preserve the provenance of these
// pointers, but not all constants guarantee this:
// - valtrees purposefully do not;
// - ConstValue::Slice does not either.
match const_ {
Const::Ty(c) => match c.kind() {
ty::ConstKind::Value(valtree) => match valtree {
// This is just an integer, keep it.
ty::ValTree::Leaf(_) => {}
ty::ValTree::Branch(_) => return None,
},
ty::ConstKind::Param(..)
| ty::ConstKind::Unevaluated(..)
| ty::ConstKind::Expr(..) => {}
// Should not appear in runtime MIR.
ty::ConstKind::Infer(..)
| ty::ConstKind::Bound(..)
| ty::ConstKind::Placeholder(..)
| ty::ConstKind::Error(..) => bug!(),
},
Const::Unevaluated(..) => {}
// If the same slice appears twice in the MIR, we cannot guarantee that we will
// give the same `AllocId` to the data.
Const::Val(ConstValue::Slice { .. }, _) => return None,
Const::Val(
ConstValue::ZeroSized | ConstValue::Scalar(_) | ConstValue::Indirect { .. },
_,
) => {}
}
Some(ConstOperand { span: rustc_span::DUMMY_SP, user_ty: None, const_ })
} else {
None
}
}
/// If there is a local which is assigned `index`, and its assignment strictly dominates `loc`,
/// return it.
fn try_as_local(&mut self, index: VnIndex, loc: Location) -> Option<Local> {
let other = self.rev_locals.get(&index)?;
other
.iter()
.copied()
.find(|&other| self.ssa.assignment_dominates(self.dominators, other, loc))
}
}
impl<'tcx> MutVisitor<'tcx> for VnState<'_, 'tcx> {
fn tcx(&self) -> TyCtxt<'tcx> {
self.tcx
}
fn visit_operand(&mut self, operand: &mut Operand<'tcx>, location: Location) {
self.simplify_operand(operand, location);
}
fn visit_statement(&mut self, stmt: &mut Statement<'tcx>, location: Location) {
self.super_statement(stmt, location);
if let StatementKind::Assign(box (_, ref mut rvalue)) = stmt.kind
// Do not try to simplify a constant, it's already in canonical shape.
&& !matches!(rvalue, Rvalue::Use(Operand::Constant(_)))
&& let Some(value) = self.simplify_rvalue(rvalue, location)
{
if let Some(const_) = self.try_as_constant(value) {
*rvalue = Rvalue::Use(Operand::Constant(Box::new(const_)));
self.any_replacement = true;
} else if let Some(local) = self.try_as_local(value, location)
&& *rvalue != Rvalue::Use(Operand::Move(local.into()))
{
*rvalue = Rvalue::Use(Operand::Copy(local.into()));
self.reused_locals.insert(local);
self.any_replacement = true;
}
}
}
}
struct StorageRemover<'tcx> {
tcx: TyCtxt<'tcx>,
reused_locals: BitSet<Local>,
}
impl<'tcx> MutVisitor<'tcx> for StorageRemover<'tcx> {
fn tcx(&self) -> TyCtxt<'tcx> {
self.tcx
}
fn visit_operand(&mut self, operand: &mut Operand<'tcx>, _: Location) {
if let Operand::Move(place) = *operand
&& let Some(local) = place.as_local()
&& self.reused_locals.contains(local)
{
*operand = Operand::Copy(place);
}
}
fn visit_statement(&mut self, stmt: &mut Statement<'tcx>, loc: Location) {
match stmt.kind {
// When removing storage statements, we need to remove both (#107511).
StatementKind::StorageLive(l) | StatementKind::StorageDead(l)
if self.reused_locals.contains(l) =>
{
stmt.make_nop()
}
_ => self.super_statement(stmt, loc),
}
}
}