bjoernager/rust
bjoernager/rust
1
Fork 0
Code Activity

Auto merge of #107009 - cjgillot:jump-threading, r=pnkfelix

Browse source 
Implement jump threading MIR opt

This pass is an attempt to generalize `ConstGoto` and `SeparateConstSwitch` passes into a more complete jump threading pass.

This pass is rather heavy, as it performs a truncated backwards DFS on MIR starting from each `SwitchInt` terminator. This backwards DFS remains very limited, as it only walks through `Goto` terminators.

It is build to support constants and discriminants, and a propagating through a very limited set of operations.

The pass successfully manages to disentangle the `Some(x?)` use case and the DFA use case. It still needs a few tests before being ready.
This commit is contained in:
bors 2023-10-23 18:05:44 +00:00
commit 1322f92634
31 changed files with 2799 additions and 139 deletions
Download patch file Download diff file Expand all files Collapse all files

98
compiler/rustc_mir_transform/src/cost_checker.rs Normal file
View file

@ -0,0 +1,98 @@
use rustc_middle::mir::visit::*;
use rustc_middle::mir::*;
use rustc_middle::ty::{self, ParamEnv, Ty, TyCtxt};
const INSTR_COST: usize = 5;
const CALL_PENALTY: usize = 25;
const LANDINGPAD_PENALTY: usize = 50;
const RESUME_PENALTY: usize = 45;
/// Verify that the callee body is compatible with the caller.
#[derive(Clone)]
pub(crate) struct CostChecker<'b, 'tcx> {
tcx: TyCtxt<'tcx>,
param_env: ParamEnv<'tcx>,
cost: usize,
callee_body: &'b Body<'tcx>,
instance: Option<ty::Instance<'tcx>>,
}
impl<'b, 'tcx> CostChecker<'b, 'tcx> {
pub fn new(
tcx: TyCtxt<'tcx>,
param_env: ParamEnv<'tcx>,
instance: Option<ty::Instance<'tcx>>,
callee_body: &'b Body<'tcx>,
) -> CostChecker<'b, 'tcx> {
CostChecker { tcx, param_env, callee_body, instance, cost: 0 }
}
pub fn cost(&self) -> usize {
self.cost
}
fn instantiate_ty(&self, v: Ty<'tcx>) -> Ty<'tcx> {
if let Some(instance) = self.instance {
instance.instantiate_mir(self.tcx, ty::EarlyBinder::bind(&v))
} else {
v
}
}
}
impl<'tcx> Visitor<'tcx> for CostChecker<'_, 'tcx> {
fn visit_statement(&mut self, statement: &Statement<'tcx>, _: Location) {
// Don't count StorageLive/StorageDead in the inlining cost.
match statement.kind {
StatementKind::StorageLive(_)
| StatementKind::StorageDead(_)
| StatementKind::Deinit(_)
| StatementKind::Nop => {}
_ => self.cost += INSTR_COST,
}
}
fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, _: Location) {
let tcx = self.tcx;
match terminator.kind {
TerminatorKind::Drop { ref place, unwind, .. } => {
// If the place doesn't actually need dropping, treat it like a regular goto.
let ty = self.instantiate_ty(place.ty(self.callee_body, tcx).ty);
if ty.needs_drop(tcx, self.param_env) {
self.cost += CALL_PENALTY;
if let UnwindAction::Cleanup(_) = unwind {
self.cost += LANDINGPAD_PENALTY;
}
} else {
self.cost += INSTR_COST;
}
}
TerminatorKind::Call { func: Operand::Constant(ref f), unwind, .. } => {
let fn_ty = self.instantiate_ty(f.const_.ty());
self.cost += if let ty::FnDef(def_id, _) = *fn_ty.kind() && tcx.is_intrinsic(def_id) {
// Don't give intrinsics the extra penalty for calls
INSTR_COST
} else {
CALL_PENALTY
};
if let UnwindAction::Cleanup(_) = unwind {
self.cost += LANDINGPAD_PENALTY;
}
}
TerminatorKind::Assert { unwind, .. } => {
self.cost += CALL_PENALTY;
if let UnwindAction::Cleanup(_) = unwind {
self.cost += LANDINGPAD_PENALTY;
}
}
TerminatorKind::UnwindResume => self.cost += RESUME_PENALTY,
TerminatorKind::InlineAsm { unwind, .. } => {
self.cost += INSTR_COST;
if let UnwindAction::Cleanup(_) = unwind {
self.cost += LANDINGPAD_PENALTY;
}
}
_ => self.cost += INSTR_COST,
}
}
}

92
compiler/rustc_mir_transform/src/inline.rs
View file

@ -14,6 +14,7 @@ use rustc_session::config::OptLevel;
use rustc_target::abi::FieldIdx;
use rustc_target::spec::abi::Abi;
use crate::cost_checker::CostChecker;
use crate::simplify::{remove_dead_blocks, CfgSimplifier};
use crate::util;
use crate::MirPass;
@ -22,11 +23,6 @@ use std::ops::{Range, RangeFrom};
pub(crate) mod cycle;
const INSTR_COST: usize = 5;
const CALL_PENALTY: usize = 25;
const LANDINGPAD_PENALTY: usize = 50;
const RESUME_PENALTY: usize = 45;
const TOP_DOWN_DEPTH_LIMIT: usize = 5;
pub struct Inline;
@ -479,13 +475,8 @@ impl<'tcx> Inliner<'tcx> {
// FIXME: Give a bonus to functions with only a single caller
let mut checker = CostChecker {
tcx: self.tcx,
param_env: self.param_env,
instance: callsite.callee,
callee_body,
cost: 0,
};
let mut checker =
CostChecker::new(self.tcx, self.param_env, Some(callsite.callee), callee_body);
// Traverse the MIR manually so we can account for the effects of inlining on the CFG.
let mut work_list = vec![START_BLOCK];
@ -530,7 +521,7 @@ impl<'tcx> Inliner<'tcx> {
// That attribute is often applied to very large functions that exceed LLVM's (very
// generous) inlining threshold. Such functions are very poor MIR inlining candidates.
// Always inlining #[inline(always)] functions in MIR, on net, slows down the compiler.
let cost = checker.cost;
let cost = checker.cost();
if cost <= threshold {
debug!("INLINING {:?} [cost={} <= threshold={}]", callsite, cost, threshold);
Ok(())
@ -803,81 +794,6 @@ impl<'tcx> Inliner<'tcx> {
}
}
/// Verify that the callee body is compatible with the caller.
///
/// This visitor mostly computes the inlining cost,
/// but also needs to verify that types match because of normalization failure.
struct CostChecker<'b, 'tcx> {
tcx: TyCtxt<'tcx>,
param_env: ParamEnv<'tcx>,
cost: usize,
callee_body: &'b Body<'tcx>,
instance: ty::Instance<'tcx>,
}
impl<'tcx> Visitor<'tcx> for CostChecker<'_, 'tcx> {
fn visit_statement(&mut self, statement: &Statement<'tcx>, _: Location) {
// Don't count StorageLive/StorageDead in the inlining cost.
match statement.kind {
StatementKind::StorageLive(_)
| StatementKind::StorageDead(_)
| StatementKind::Deinit(_)
| StatementKind::Nop => {}
_ => self.cost += INSTR_COST,
}
}
fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, _: Location) {
let tcx = self.tcx;
match terminator.kind {
TerminatorKind::Drop { ref place, unwind, .. } => {
// If the place doesn't actually need dropping, treat it like a regular goto.
let ty = self.instance.instantiate_mir(
tcx,
ty::EarlyBinder::bind(&place.ty(self.callee_body, tcx).ty),
);
if ty.needs_drop(tcx, self.param_env) {
self.cost += CALL_PENALTY;
if let UnwindAction::Cleanup(_) = unwind {
self.cost += LANDINGPAD_PENALTY;
}
} else {
self.cost += INSTR_COST;
}
}
TerminatorKind::Call { func: Operand::Constant(ref f), unwind, .. } => {
let fn_ty =
self.instance.instantiate_mir(tcx, ty::EarlyBinder::bind(&f.const_.ty()));
self.cost += if let ty::FnDef(def_id, _) = *fn_ty.kind()
&& tcx.is_intrinsic(def_id)
{
// Don't give intrinsics the extra penalty for calls
INSTR_COST
} else {
CALL_PENALTY
};
if let UnwindAction::Cleanup(_) = unwind {
self.cost += LANDINGPAD_PENALTY;
}
}
TerminatorKind::Assert { unwind, .. } => {
self.cost += CALL_PENALTY;
if let UnwindAction::Cleanup(_) = unwind {
self.cost += LANDINGPAD_PENALTY;
}
}
TerminatorKind::UnwindResume => self.cost += RESUME_PENALTY,
TerminatorKind::InlineAsm { unwind, .. } => {
self.cost += INSTR_COST;
if let UnwindAction::Cleanup(_) = unwind {
self.cost += LANDINGPAD_PENALTY;
}
}
_ => self.cost += INSTR_COST,
}
}
}
/**
* Integrator.
*

759
compiler/rustc_mir_transform/src/jump_threading.rs Normal file
View file

@ -0,0 +1,759 @@
//! A jump threading optimization.
//!
//! This optimization seeks to replace join-then-switch control flow patterns by straight jumps
//! X = 0 X = 0
//! ------------\ /-------- ------------
//! X = 1 X----X SwitchInt(X) => X = 1
//! ------------/ \-------- ------------
//!
//!
//! We proceed by walking the cfg backwards starting from each `SwitchInt` terminator,
//! looking for assignments that will turn the `SwitchInt` into a simple `Goto`.
//!
//! The algorithm maintains a set of replacement conditions:
//! - `conditions[place]` contains `Condition { value, polarity: Eq, target }`
//! if assigning `value` to `place` turns the `SwitchInt` into `Goto { target }`.
//! - `conditions[place]` contains `Condition { value, polarity: Ne, target }`
//! if assigning anything different from `value` to `place` turns the `SwitchInt`
//! into `Goto { target }`.
//!
//! In this file, we denote as `place ?= value` the existence of a replacement condition
//! on `place` with given `value`, irrespective of the polarity and target of that
//! replacement condition.
//!
//! We then walk the CFG backwards transforming the set of conditions.
//! When we find a fulfilling assignment, we record a `ThreadingOpportunity`.
//! All `ThreadingOpportunity`s are applied to the body, by duplicating blocks if required.
//!
//! The optimization search can be very heavy, as it performs a DFS on MIR starting from
//! each `SwitchInt` terminator. To manage the complexity, we:
//! - bound the maximum depth by a constant `MAX_BACKTRACK`;
//! - we only traverse `Goto` terminators.
//!
//! We try to avoid creating irreducible control-flow by not threading through a loop header.
//!
//! Likewise, applying the optimisation can create a lot of new MIR, so we bound the instruction
//! cost by `MAX_COST`.
use rustc_arena::DroplessArena;
use rustc_data_structures::fx::FxHashSet;
use rustc_index::bit_set::BitSet;
use rustc_index::IndexVec;
use rustc_middle::mir::visit::Visitor;
use rustc_middle::mir::*;
use rustc_middle::ty::{self, ScalarInt, Ty, TyCtxt};
use rustc_mir_dataflow::value_analysis::{Map, PlaceIndex, State, TrackElem};
use crate::cost_checker::CostChecker;
use crate::MirPass;
pub struct JumpThreading;
const MAX_BACKTRACK: usize = 5;
const MAX_COST: usize = 100;
const MAX_PLACES: usize = 100;
impl<'tcx> MirPass<'tcx> for JumpThreading {
fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
sess.mir_opt_level() >= 4
}
#[instrument(skip_all level = "debug")]
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
let def_id = body.source.def_id();
debug!(?def_id);
let param_env = tcx.param_env_reveal_all_normalized(def_id);
let map = Map::new(tcx, body, Some(MAX_PLACES));
let loop_headers = loop_headers(body);
let arena = DroplessArena::default();
let mut finder = TOFinder {
tcx,
param_env,
body,
arena: &arena,
map: &map,
loop_headers: &loop_headers,
opportunities: Vec::new(),
};
for (bb, bbdata) in body.basic_blocks.iter_enumerated() {
debug!(?bb, term = ?bbdata.terminator());
if bbdata.is_cleanup || loop_headers.contains(bb) {
continue;
}
let Some((discr, targets)) = bbdata.terminator().kind.as_switch() else { continue };
let Some(discr) = discr.place() else { continue };
debug!(?discr, ?bb);
let discr_ty = discr.ty(body, tcx).ty;
let Ok(discr_layout) = tcx.layout_of(param_env.and(discr_ty)) else { continue };
let Some(discr) = finder.map.find(discr.as_ref()) else { continue };
debug!(?discr);
let cost = CostChecker::new(tcx, param_env, None, body);
let mut state = State::new(ConditionSet::default(), &finder.map);
let conds = if let Some((value, then, else_)) = targets.as_static_if() {
let Some(value) = ScalarInt::try_from_uint(value, discr_layout.size) else {
continue;
};
arena.alloc_from_iter([
Condition { value, polarity: Polarity::Eq, target: then },
Condition { value, polarity: Polarity::Ne, target: else_ },
])
} else {
arena.alloc_from_iter(targets.iter().filter_map(|(value, target)| {
let value = ScalarInt::try_from_uint(value, discr_layout.size)?;
Some(Condition { value, polarity: Polarity::Eq, target })
}))
};
let conds = ConditionSet(conds);
state.insert_value_idx(discr, conds, &finder.map);
finder.find_opportunity(bb, state, cost, 0);
}
let opportunities = finder.opportunities;
debug!(?opportunities);
if opportunities.is_empty() {
return;
}
// Verify that we do not thread through a loop header.
for to in opportunities.iter() {
assert!(to.chain.iter().all(|&block| !loop_headers.contains(block)));
}
OpportunitySet::new(body, opportunities).apply(body);
}
}
#[derive(Debug)]
struct ThreadingOpportunity {
/// The list of `BasicBlock`s from the one that found the opportunity to the `SwitchInt`.
chain: Vec<BasicBlock>,
/// The `SwitchInt` will be replaced by `Goto { target }`.
target: BasicBlock,
}
struct TOFinder<'tcx, 'a> {
tcx: TyCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
body: &'a Body<'tcx>,
map: &'a Map,
loop_headers: &'a BitSet<BasicBlock>,
/// We use an arena to avoid cloning the slices when cloning `state`.
arena: &'a DroplessArena,
opportunities: Vec<ThreadingOpportunity>,
}
/// Represent the following statement. If we can prove that the current local is equal/not-equal
/// to `value`, jump to `target`.
#[derive(Copy, Clone, Debug)]
struct Condition {
value: ScalarInt,
polarity: Polarity,
target: BasicBlock,
}
#[derive(Copy, Clone, Debug, Eq, PartialEq)]
enum Polarity {
Ne,
Eq,
}
impl Condition {
fn matches(&self, value: ScalarInt) -> bool {
(self.value == value) == (self.polarity == Polarity::Eq)
}
fn inv(mut self) -> Self {
self.polarity = match self.polarity {
Polarity::Eq => Polarity::Ne,
Polarity::Ne => Polarity::Eq,
};
self
}
}
#[derive(Copy, Clone, Debug, Default)]
struct ConditionSet<'a>(&'a [Condition]);
impl<'a> ConditionSet<'a> {
fn iter(self) -> impl Iterator<Item = Condition> + 'a {
self.0.iter().copied()
}
fn iter_matches(self, value: ScalarInt) -> impl Iterator<Item = Condition> + 'a {
self.iter().filter(move |c| c.matches(value))
}
fn map(self, arena: &'a DroplessArena, f: impl Fn(Condition) -> Condition) -> ConditionSet<'a> {
ConditionSet(arena.alloc_from_iter(self.iter().map(f)))
}
}
impl<'tcx, 'a> TOFinder<'tcx, 'a> {
fn is_empty(&self, state: &State<ConditionSet<'a>>) -> bool {
state.all(|cs| cs.0.is_empty())
}
/// Recursion entry point to find threading opportunities.
#[instrument(level = "trace", skip(self, cost), ret)]
fn find_opportunity(
&mut self,
bb: BasicBlock,
mut state: State<ConditionSet<'a>>,
mut cost: CostChecker<'_, 'tcx>,
depth: usize,
) {
// Do not thread through loop headers.
if self.loop_headers.contains(bb) {
return;
}
debug!(cost = ?cost.cost());
for (statement_index, stmt) in
self.body.basic_blocks[bb].statements.iter().enumerate().rev()
{
if self.is_empty(&state) {
return;
}
cost.visit_statement(stmt, Location { block: bb, statement_index });
if cost.cost() > MAX_COST {
return;
}
// Attempt to turn the `current_condition` on `lhs` into a condition on another place.
self.process_statement(bb, stmt, &mut state);
// When a statement mutates a place, assignments to that place that happen
// above the mutation cannot fulfill a condition.
// _1 = 5 // Whatever happens here, it won't change the result of a `SwitchInt`.
// _1 = 6
if let Some((lhs, tail)) = self.mutated_statement(stmt) {
state.flood_with_tail_elem(lhs.as_ref(), tail, self.map, ConditionSet::default());
}
}
if self.is_empty(&state) || depth >= MAX_BACKTRACK {
return;
}
let last_non_rec = self.opportunities.len();
let predecessors = &self.body.basic_blocks.predecessors()[bb];
if let &[pred] = &predecessors[..] && bb != START_BLOCK {
let term = self.body.basic_blocks[pred].terminator();
match term.kind {
TerminatorKind::SwitchInt { ref discr, ref targets } => {
self.process_switch_int(discr, targets, bb, &mut state);
self.find_opportunity(pred, state, cost, depth + 1);
}
_ => self.recurse_through_terminator(pred, &state, &cost, depth),
}
} else {
for &pred in predecessors {
self.recurse_through_terminator(pred, &state, &cost, depth);
}
}
let new_tos = &mut self.opportunities[last_non_rec..];
debug!(?new_tos);
// Try to deduplicate threading opportunities.
if new_tos.len() > 1
&& new_tos.len() == predecessors.len()
&& predecessors
.iter()
.zip(new_tos.iter())
.all(|(&pred, to)| to.chain == &[pred] && to.target == new_tos[0].target)
{
// All predecessors have a threading opportunity, and they all point to the same block.
debug!(?new_tos, "dedup");
let first = &mut new_tos[0];
*first = ThreadingOpportunity { chain: vec![bb], target: first.target };
self.opportunities.truncate(last_non_rec + 1);
return;
}
for op in self.opportunities[last_non_rec..].iter_mut() {
op.chain.push(bb);
}
}
/// Extract the mutated place from a statement.
///
/// This method returns the `Place` so we can flood the state in case of a partial assignment.
/// (_1 as Ok).0 = _5;
/// (_1 as Err).0 = _6;
/// We want to ensure that a `SwitchInt((_1 as Ok).0)` does not see the first assignment, as
/// the value may have been mangled by the second assignment.
///
/// In case we assign to a discriminant, we return `Some(TrackElem::Discriminant)`, so we can
/// stop at flooding the discriminant, and preserve the variant fields.
/// (_1 as Some).0 = _6;
/// SetDiscriminant(_1, 1);
/// switchInt((_1 as Some).0)
#[instrument(level = "trace", skip(self), ret)]
fn mutated_statement(
&self,
stmt: &Statement<'tcx>,
) -> Option<(Place<'tcx>, Option<TrackElem>)> {
match stmt.kind {
StatementKind::Assign(box (place, _))
| StatementKind::Deinit(box place) => Some((place, None)),
StatementKind::SetDiscriminant { box place, variant_index: _ } => {
Some((place, Some(TrackElem::Discriminant)))
}
StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
Some((Place::from(local), None))
}
StatementKind::Retag(..)
| StatementKind::Intrinsic(box NonDivergingIntrinsic::Assume(..))
// copy_nonoverlapping takes pointers and mutated the pointed-to value.
| StatementKind::Intrinsic(box NonDivergingIntrinsic::CopyNonOverlapping(..))
| StatementKind::AscribeUserType(..)
| StatementKind::Coverage(..)
| StatementKind::FakeRead(..)
| StatementKind::ConstEvalCounter
| StatementKind::PlaceMention(..)
| StatementKind::Nop => None,
}
}
#[instrument(level = "trace", skip(self))]
fn process_operand(
&mut self,
bb: BasicBlock,
lhs: PlaceIndex,
rhs: &Operand<'tcx>,
state: &mut State<ConditionSet<'a>>,
) -> Option<!> {
let register_opportunity = |c: Condition| {
debug!(?bb, ?c.target, "register");
self.opportunities.push(ThreadingOpportunity { chain: vec![bb], target: c.target })
};
match rhs {
// If we expect `lhs ?= A`, we have an opportunity if we assume `constant == A`.
Operand::Constant(constant) => {
let conditions = state.try_get_idx(lhs, self.map)?;
let constant =
constant.const_.normalize(self.tcx, self.param_env).try_to_scalar_int()?;
conditions.iter_matches(constant).for_each(register_opportunity);
}
// Transfer the conditions on the copied rhs.
Operand::Move(rhs) | Operand::Copy(rhs) => {
let rhs = self.map.find(rhs.as_ref())?;
state.insert_place_idx(rhs, lhs, self.map);
}
}
None
}
#[instrument(level = "trace", skip(self))]
fn process_statement(
&mut self,
bb: BasicBlock,
stmt: &Statement<'tcx>,
state: &mut State<ConditionSet<'a>>,
) -> Option<!> {
let register_opportunity = |c: Condition| {
debug!(?bb, ?c.target, "register");
self.opportunities.push(ThreadingOpportunity { chain: vec![bb], target: c.target })
};
// Below, `lhs` is the return value of `mutated_statement`,
// the place to which `conditions` apply.
let discriminant_for_variant = |enum_ty: Ty<'tcx>, variant_index| {
let discr = enum_ty.discriminant_for_variant(self.tcx, variant_index)?;
let discr_layout = self.tcx.layout_of(self.param_env.and(discr.ty)).ok()?;
let scalar = ScalarInt::try_from_uint(discr.val, discr_layout.size)?;
Some(Operand::const_from_scalar(
self.tcx,
discr.ty,
scalar.into(),
rustc_span::DUMMY_SP,
))
};
match &stmt.kind {
// If we expect `discriminant(place) ?= A`,
// we have an opportunity if `variant_index ?= A`.
StatementKind::SetDiscriminant { box place, variant_index } => {
let discr_target = self.map.find_discr(place.as_ref())?;
let enum_ty = place.ty(self.body, self.tcx).ty;
let discr = discriminant_for_variant(enum_ty, *variant_index)?;
self.process_operand(bb, discr_target, &discr, state)?;
}
// If we expect `lhs ?= true`, we have an opportunity if we assume `lhs == true`.
StatementKind::Intrinsic(box NonDivergingIntrinsic::Assume(
Operand::Copy(place) | Operand::Move(place),
)) => {
let conditions = state.try_get(place.as_ref(), self.map)?;
conditions.iter_matches(ScalarInt::TRUE).for_each(register_opportunity);
}
StatementKind::Assign(box (lhs_place, rhs)) => {
if let Some(lhs) = self.map.find(lhs_place.as_ref()) {
match rhs {
Rvalue::Use(operand) => self.process_operand(bb, lhs, operand, state)?,
// Transfer the conditions on the copy rhs.
Rvalue::CopyForDeref(rhs) => {
self.process_operand(bb, lhs, &Operand::Copy(*rhs), state)?
}
Rvalue::Discriminant(rhs) => {
let rhs = self.map.find_discr(rhs.as_ref())?;
state.insert_place_idx(rhs, lhs, self.map);
}
// If we expect `lhs ?= A`, we have an opportunity if we assume `constant == A`.
Rvalue::Aggregate(box ref kind, ref operands) => {
let agg_ty = lhs_place.ty(self.body, self.tcx).ty;
let lhs = match kind {
// Do not support unions.
AggregateKind::Adt(.., Some(_)) => return None,
AggregateKind::Adt(_, variant_index, ..) if agg_ty.is_enum() => {
if let Some(discr_target) = self.map.apply(lhs, TrackElem::Discriminant)
&& let Some(discr_value) = discriminant_for_variant(agg_ty, *variant_index)
{
self.process_operand(bb, discr_target, &discr_value, state);
}
self.map.apply(lhs, TrackElem::Variant(*variant_index))?
}
_ => lhs,
};
for (field_index, operand) in operands.iter_enumerated() {
if let Some(field) =
self.map.apply(lhs, TrackElem::Field(field_index))
{
self.process_operand(bb, field, operand, state);
}
}
}
// Transfer the conditions on the copy rhs, after inversing polarity.
Rvalue::UnaryOp(UnOp::Not, Operand::Move(place) | Operand::Copy(place)) => {
let conditions = state.try_get_idx(lhs, self.map)?;
let place = self.map.find(place.as_ref())?;
let conds = conditions.map(self.arena, Condition::inv);
state.insert_value_idx(place, conds, self.map);
}
// We expect `lhs ?= A`. We found `lhs = Eq(rhs, B)`.
// Create a condition on `rhs ?= B`.
Rvalue::BinaryOp(
op,
box (
Operand::Move(place) | Operand::Copy(place),
Operand::Constant(value),
)
| box (
Operand::Constant(value),
Operand::Move(place) | Operand::Copy(place),
),
) => {
let conditions = state.try_get_idx(lhs, self.map)?;
let place = self.map.find(place.as_ref())?;
let equals = match op {
BinOp::Eq => ScalarInt::TRUE,
BinOp::Ne => ScalarInt::FALSE,
_ => return None,
};
let value = value
.const_
.normalize(self.tcx, self.param_env)
.try_to_scalar_int()?;
let conds = conditions.map(self.arena, |c| Condition {
value,
polarity: if c.matches(equals) {
Polarity::Eq
} else {
Polarity::Ne
},
..c
});
state.insert_value_idx(place, conds, self.map);
}
_ => {}
}
}
}
_ => {}
}
None
}
#[instrument(level = "trace", skip(self, cost))]
fn recurse_through_terminator(
&mut self,
bb: BasicBlock,
state: &State<ConditionSet<'a>>,
cost: &CostChecker<'_, 'tcx>,
depth: usize,
) {
let register_opportunity = |c: Condition| {
debug!(?bb, ?c.target, "register");
self.opportunities.push(ThreadingOpportunity { chain: vec![bb], target: c.target })
};
let term = self.body.basic_blocks[bb].terminator();
let place_to_flood = match term.kind {
// We come from a target, so those are not possible.
TerminatorKind::UnwindResume
| TerminatorKind::UnwindTerminate(_)
| TerminatorKind::Return
| TerminatorKind::Unreachable
| TerminatorKind::CoroutineDrop => bug!("{term:?} has no terminators"),
// Disallowed during optimizations.
TerminatorKind::FalseEdge { .. }
| TerminatorKind::FalseUnwind { .. }
| TerminatorKind::Yield { .. } => bug!("{term:?} invalid"),
// Cannot reason about inline asm.
TerminatorKind::InlineAsm { .. } => return,
// `SwitchInt` is handled specially.
TerminatorKind::SwitchInt { .. } => return,
// We can recurse, no thing particular to do.
TerminatorKind::Goto { .. } => None,
// Flood the overwritten place, and progress through.
TerminatorKind::Drop { place: destination, .. }
| TerminatorKind::Call { destination, .. } => Some(destination),
// Treat as an `assume(cond == expected)`.
TerminatorKind::Assert { ref cond, expected, .. } => {
if let Some(place) = cond.place()
&& let Some(conditions) = state.try_get(place.as_ref(), self.map)
{
let expected = if expected { ScalarInt::TRUE } else { ScalarInt::FALSE };
conditions.iter_matches(expected).for_each(register_opportunity);
}
None
}
};
// We can recurse through this terminator.
let mut state = state.clone();
if let Some(place_to_flood) = place_to_flood {
state.flood_with(place_to_flood.as_ref(), self.map, ConditionSet::default());
}
self.find_opportunity(bb, state, cost.clone(), depth + 1);
}
#[instrument(level = "trace", skip(self))]
fn process_switch_int(
&mut self,
discr: &Operand<'tcx>,
targets: &SwitchTargets,
target_bb: BasicBlock,
state: &mut State<ConditionSet<'a>>,
) -> Option<!> {
debug_assert_ne!(target_bb, START_BLOCK);
debug_assert_eq!(self.body.basic_blocks.predecessors()[target_bb].len(), 1);
let discr = discr.place()?;
let discr_ty = discr.ty(self.body, self.tcx).ty;
let discr_layout = self.tcx.layout_of(self.param_env.and(discr_ty)).ok()?;
let conditions = state.try_get(discr.as_ref(), self.map)?;
if let Some((value, _)) = targets.iter().find(|&(_, target)| target == target_bb) {
let value = ScalarInt::try_from_uint(value, discr_layout.size)?;
debug_assert_eq!(targets.iter().filter(|&(_, target)| target == target_bb).count(), 1);
// We are inside `target_bb`. Since we have a single predecessor, we know we passed
// through the `SwitchInt` before arriving here. Therefore, we know that
// `discr == value`. If one condition can be fulfilled by `discr == value`,
// that's an opportunity.
for c in conditions.iter_matches(value) {
debug!(?target_bb, ?c.target, "register");
self.opportunities.push(ThreadingOpportunity { chain: vec![], target: c.target });
}
} else if let Some((value, _, else_bb)) = targets.as_static_if()
&& target_bb == else_bb
{
let value = ScalarInt::try_from_uint(value, discr_layout.size)?;
// We only know that `discr != value`. That's much weaker information than
// the equality we had in the previous arm. All we can conclude is that
// the replacement condition `discr != value` can be threaded, and nothing else.
for c in conditions.iter() {
if c.value == value && c.polarity == Polarity::Ne {
debug!(?target_bb, ?c.target, "register");
self.opportunities
.push(ThreadingOpportunity { chain: vec![], target: c.target });
}
}
}
None
}
}
struct OpportunitySet {
opportunities: Vec<ThreadingOpportunity>,
/// For each bb, give the TOs in which it appears. The pair corresponds to the index
/// in `opportunities` and the index in `ThreadingOpportunity::chain`.
involving_tos: IndexVec<BasicBlock, Vec<(usize, usize)>>,
/// Cache the number of predecessors for each block, as we clear the basic block cache..
predecessors: IndexVec<BasicBlock, usize>,
}
impl OpportunitySet {
fn new(body: &Body<'_>, opportunities: Vec<ThreadingOpportunity>) -> OpportunitySet {
let mut involving_tos = IndexVec::from_elem(Vec::new(), &body.basic_blocks);
for (index, to) in opportunities.iter().enumerate() {
for (ibb, &bb) in to.chain.iter().enumerate() {
involving_tos[bb].push((index, ibb));
}
involving_tos[to.target].push((index, to.chain.len()));
}
let predecessors = predecessor_count(body);
OpportunitySet { opportunities, involving_tos, predecessors }
}
/// Apply the opportunities on the graph.
fn apply(&mut self, body: &mut Body<'_>) {
for i in 0..self.opportunities.len() {
self.apply_once(i, body);
}
}
#[instrument(level = "trace", skip(self, body))]
fn apply_once(&mut self, index: usize, body: &mut Body<'_>) {
debug!(?self.predecessors);
debug!(?self.involving_tos);
// Check that `predecessors` satisfies its invariant.
debug_assert_eq!(self.predecessors, predecessor_count(body));
// Remove the TO from the vector to allow modifying the other ones later.
let op = &mut self.opportunities[index];
debug!(?op);
let op_chain = std::mem::take(&mut op.chain);
let op_target = op.target;
debug_assert_eq!(op_chain.len(), op_chain.iter().collect::<FxHashSet<_>>().len());
let Some((current, chain)) = op_chain.split_first() else { return };
let basic_blocks = body.basic_blocks.as_mut();
// Invariant: the control-flow is well-formed at the end of each iteration.
let mut current = *current;
for &succ in chain {
debug!(?current, ?succ);
// `succ` must be a successor of `current`. If it is not, this means this TO is not
// satisfiable and a previous TO erased this edge, so we bail out.
if basic_blocks[current].terminator().successors().find(|s| *s == succ).is_none() {
debug!("impossible");
return;
}
// Fast path: `succ` is only used once, so we can reuse it directly.
if self.predecessors[succ] == 1 {
debug!("single");
current = succ;
continue;
}
let new_succ = basic_blocks.push(basic_blocks[succ].clone());
debug!(?new_succ);
// Replace `succ` by `new_succ` where it appears.
let mut num_edges = 0;
for s in basic_blocks[current].terminator_mut().successors_mut() {
if *s == succ {
*s = new_succ;
num_edges += 1;
}
}
// Update predecessors with the new block.
let _new_succ = self.predecessors.push(num_edges);
debug_assert_eq!(new_succ, _new_succ);
self.predecessors[succ] -= num_edges;
self.update_predecessor_count(basic_blocks[new_succ].terminator(), Update::Incr);
// Replace the `current -> succ` edge by `current -> new_succ` in all the following
// TOs. This is necessary to avoid trying to thread through a non-existing edge. We
// use `involving_tos` here to avoid traversing the full set of TOs on each iteration.
let mut new_involved = Vec::new();
for &(to_index, in_to_index) in &self.involving_tos[current] {
// That TO has already been applied, do nothing.
if to_index <= index {
continue;
}
let other_to = &mut self.opportunities[to_index];
if other_to.chain.get(in_to_index) != Some(&current) {
continue;
}
let s = other_to.chain.get_mut(in_to_index + 1).unwrap_or(&mut other_to.target);
if *s == succ {
// `other_to` references the `current -> succ` edge, so replace `succ`.
*s = new_succ;
new_involved.push((to_index, in_to_index + 1));
}
}
// The TOs that we just updated now reference `new_succ`. Update `involving_tos`
// in case we need to duplicate an edge starting at `new_succ` later.
let _new_succ = self.involving_tos.push(new_involved);
debug_assert_eq!(new_succ, _new_succ);
current = new_succ;
}
let current = &mut basic_blocks[current];
self.update_predecessor_count(current.terminator(), Update::Decr);
current.terminator_mut().kind = TerminatorKind::Goto { target: op_target };
self.predecessors[op_target] += 1;
}
fn update_predecessor_count(&mut self, terminator: &Terminator<'_>, incr: Update) {
match incr {
Update::Incr => {
for s in terminator.successors() {
self.predecessors[s] += 1;
}
}
Update::Decr => {
for s in terminator.successors() {
self.predecessors[s] -= 1;
}
}
}
}
}
fn predecessor_count(body: &Body<'_>) -> IndexVec<BasicBlock, usize> {
let mut predecessors: IndexVec<_, _> =
body.basic_blocks.predecessors().iter().map(|ps| ps.len()).collect();
predecessors[START_BLOCK] += 1; // Account for the implicit entry edge.
predecessors
}
enum Update {
Incr,
Decr,
}
/// Compute the set of loop headers in the given body. We define a loop header as a block which has
/// at least a predecessor which it dominates. This definition is only correct for reducible CFGs.
/// But if the CFG is already irreducible, there is no point in trying much harder.
/// is already irreducible.
fn loop_headers(body: &Body<'_>) -> BitSet<BasicBlock> {
let mut loop_headers = BitSet::new_empty(body.basic_blocks.len());
let dominators = body.basic_blocks.dominators();
// Only visit reachable blocks.
for (bb, bbdata) in traversal::preorder(body) {
for succ in bbdata.terminator().successors() {
if dominators.dominates(succ, bb) {
loop_headers.insert(succ);
}
}
}
loop_headers
}

3
compiler/rustc_mir_transform/src/lib.rs
View file

@ -62,6 +62,7 @@ mod const_prop;
mod const_prop_lint;
mod copy_prop;
mod coroutine;
mod cost_checker;
mod coverage;
mod cross_crate_inline;
mod ctfe_limit;
@ -81,6 +82,7 @@ mod function_item_references;
mod gvn;
pub mod inline;
mod instsimplify;
mod jump_threading;
mod large_enums;
mod lower_intrinsics;
mod lower_slice_len;
@ -571,6 +573,7 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
&dataflow_const_prop::DataflowConstProp,
&const_debuginfo::ConstDebugInfo,
&o1(simplify_branches::SimplifyConstCondition::AfterConstProp),
&jump_threading::JumpThreading,
&early_otherwise_branch::EarlyOtherwiseBranch,
&simplify_comparison_integral::SimplifyComparisonIntegral,
&dead_store_elimination::DeadStoreElimination,