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Auto merge of #109597 - cjgillot:gvn, r=oli-obk

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Implement a global value numbering MIR optimization

The aim of this pass is to avoid repeated computations by reusing past assignments. It is based on an analysis of SSA locals, in order to perform a restricted form of common subexpression elimination.

By opportunity, this pass allows for some simplifications by combining assignments. For instance, this pass could be able to see through projections of aggregates to directly reuse the aggregate field (not in this PR).

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:
```rust
_a = *_b // _b is &Freeze
_c = *_b // replaced by _c = _a
```
This commit is contained in:
bors 2023-09-27 21:06:30 +00:00
commit e7c502d930
32 changed files with 7204 additions and 23 deletions
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2
compiler/rustc_middle/src/mir/syntax.rs
View file

@ -1333,7 +1333,7 @@ pub enum AggregateKind<'tcx> {
Generator(DefId, GenericArgsRef<'tcx>, hir::Movability),
}
#[derive(Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, Hash, HashStable)]
#[derive(Copy, Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, Hash, HashStable)]
pub enum NullOp<'tcx> {
/// Returns the size of a value of that type
SizeOf,

538
compiler/rustc_mir_transform/src/gvn.rs Normal file
View file

@ -0,0 +1,538 @@
//! 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),
};
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),
}
}
}

2
compiler/rustc_mir_transform/src/lib.rs
View file

@ -76,6 +76,7 @@ mod errors;
mod ffi_unwind_calls;
mod function_item_references;
mod generator;
mod gvn;
pub mod inline;
mod instsimplify;
mod large_enums;
@ -549,6 +550,7 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
// latter pass will leverage the created opportunities.
&separate_const_switch::SeparateConstSwitch,
&const_prop::ConstProp,
&gvn::GVN,
&dataflow_const_prop::DataflowConstProp,
//
// Const-prop runs unconditionally, but doesn't mutate the MIR at mir-opt-level=0.

2
compiler/rustc_mir_transform/src/ref_prop.rs
View file

@ -108,7 +108,7 @@ enum Value<'tcx> {
}
/// For each local, save the place corresponding to `*local`.
#[instrument(level = "trace", skip(tcx, body))]
#[instrument(level = "trace", skip(tcx, body, ssa))]
fn compute_replacement<'tcx>(
tcx: TyCtxt<'tcx>,
body: &Body<'tcx>,

38
compiler/rustc_mir_transform/src/ssa.rs
View file

@ -13,7 +13,6 @@ use rustc_middle::middle::resolve_bound_vars::Set1;
use rustc_middle::mir::visit::*;
use rustc_middle::mir::*;
#[derive(Debug)]
pub struct SsaLocals {
/// Assignments to each local. This defines whether the local is SSA.
assignments: IndexVec<Local, Set1<LocationExtended>>,
@ -129,6 +128,25 @@ impl SsaLocals {
self.direct_uses[local]
}
pub fn assignment_dominates(
&self,
dominators: &Dominators<BasicBlock>,
local: Local,
location: Location,
) -> bool {
match self.assignments[local] {
Set1::One(LocationExtended::Arg) => true,
Set1::One(LocationExtended::Plain(ass)) => {
if ass.block == location.block {
ass.statement_index < location.statement_index
} else {
dominators.dominates(ass.block, location.block)
}
}
_ => false,
}
}
pub fn assignments<'a, 'tcx>(
&'a self,
body: &'a Body<'tcx>,
@ -146,6 +164,24 @@ impl SsaLocals {
})
}
pub fn for_each_assignment_mut<'tcx>(
&self,
basic_blocks: &mut BasicBlocks<'tcx>,
mut f: impl FnMut(Local, &mut Rvalue<'tcx>, Location),
) {
for &local in &self.assignment_order {
if let Set1::One(LocationExtended::Plain(loc)) = self.assignments[local] {
// `loc` must point to a direct assignment to `local`.
let bbs = basic_blocks.as_mut_preserves_cfg();
let bb = &mut bbs[loc.block];
let stmt = &mut bb.statements[loc.statement_index];
let StatementKind::Assign(box (target, ref mut rvalue)) = stmt.kind else { bug!() };
assert_eq!(target.as_local(), Some(local));
f(local, rvalue, loc)
}
}
}
/// Compute the equivalence classes for locals, based on copy statements.
///
/// The returned vector maps each local to the one it copies. In the following case: