rust/compiler/rustc_mir_transform/src/simplify.rs

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//! A number of passes which remove various redundancies in the CFG.
//!
//! The `SimplifyCfg` pass gets rid of unnecessary blocks in the CFG, whereas the `SimplifyLocals`
//! gets rid of all the unnecessary local variable declarations.
//!
//! The `SimplifyLocals` pass is kinda expensive and therefore not very suitable to be run often.
//! Most of the passes should not care or be impacted in meaningful ways due to extra locals
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//! either, so running the pass once, right before codegen, should suffice.
//!
//! On the other side of the spectrum, the `SimplifyCfg` pass is considerably cheap to run, thus
//! one should run it after every pass which may modify CFG in significant ways. This pass must
//! also be run before any analysis passes because it removes dead blocks, and some of these can be
//! ill-typed.
//!
//! The cause of this typing issue is typeck allowing most blocks whose end is not reachable have
//! an arbitrary return type, rather than having the usual () return type (as a note, typeck's
//! notion of reachability is in fact slightly weaker than MIR CFG reachability - see #31617). A
//! standard example of the situation is:
//!
//! ```rust
//! fn example() {
//! let _a: char = { return; };
//! }
//! ```
//!
//! Here the block (`{ return; }`) has the return type `char`, rather than `()`, but the MIR we
//! naively generate still contains the `_a = ()` write in the unreachable block "after" the
//! return.
use crate::MirPass;
use rustc_data_structures::fx::FxHashSet;
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use rustc_index::vec::{Idx, IndexVec};
use rustc_middle::mir::coverage::*;
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use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor};
use rustc_middle::mir::*;
use rustc_middle::ty::TyCtxt;
use smallvec::SmallVec;
use std::borrow::Cow;
use std::convert::TryInto;
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pub struct SimplifyCfg {
label: String,
}
impl SimplifyCfg {
pub fn new(label: &str) -> Self {
SimplifyCfg { label: format!("SimplifyCfg-{}", label) }
}
}
pub fn simplify_cfg<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
CfgSimplifier::new(body).simplify();
remove_dead_blocks(tcx, body);
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// FIXME: Should probably be moved into some kind of pass manager
body.basic_blocks_mut().raw.shrink_to_fit();
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}
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impl<'tcx> MirPass<'tcx> for SimplifyCfg {
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fn name(&self) -> Cow<'_, str> {
Cow::Borrowed(&self.label)
}
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
debug!("SimplifyCfg({:?}) - simplifying {:?}", self.label, body.source);
simplify_cfg(tcx, body);
}
}
pub struct CfgSimplifier<'a, 'tcx> {
basic_blocks: &'a mut IndexVec<BasicBlock, BasicBlockData<'tcx>>,
pred_count: IndexVec<BasicBlock, u32>,
}
impl<'a, 'tcx> CfgSimplifier<'a, 'tcx> {
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pub fn new(body: &'a mut Body<'tcx>) -> Self {
let mut pred_count = IndexVec::from_elem(0u32, body.basic_blocks());
// we can't use mir.predecessors() here because that counts
// dead blocks, which we don't want to.
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pred_count[START_BLOCK] = 1;
for (_, data) in traversal::preorder(body) {
if let Some(ref term) = data.terminator {
for tgt in term.successors() {
pred_count[tgt] += 1;
}
}
}
let basic_blocks = body.basic_blocks_mut();
CfgSimplifier { basic_blocks, pred_count }
}
pub fn simplify(mut self) {
self.strip_nops();
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// Vec of the blocks that should be merged. We store the indices here, instead of the
// statements itself to avoid moving the (relatively) large statements twice.
// We do not push the statements directly into the target block (`bb`) as that is slower
// due to additional reallocations
let mut merged_blocks = Vec::new();
loop {
let mut changed = false;
for bb in self.basic_blocks.indices() {
if self.pred_count[bb] == 0 {
continue;
}
debug!("simplifying {:?}", bb);
let mut terminator =
self.basic_blocks[bb].terminator.take().expect("invalid terminator state");
for successor in terminator.successors_mut() {
self.collapse_goto_chain(successor, &mut changed);
}
let mut inner_changed = true;
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merged_blocks.clear();
while inner_changed {
inner_changed = false;
inner_changed |= self.simplify_branch(&mut terminator);
inner_changed |= self.merge_successor(&mut merged_blocks, &mut terminator);
changed |= inner_changed;
}
let statements_to_merge =
merged_blocks.iter().map(|&i| self.basic_blocks[i].statements.len()).sum();
if statements_to_merge > 0 {
let mut statements = std::mem::take(&mut self.basic_blocks[bb].statements);
statements.reserve(statements_to_merge);
for &from in &merged_blocks {
statements.append(&mut self.basic_blocks[from].statements);
}
self.basic_blocks[bb].statements = statements;
}
self.basic_blocks[bb].terminator = Some(terminator);
}
if !changed {
break;
}
}
}
/// This function will return `None` if
/// * the block has statements
/// * the block has a terminator other than `goto`
/// * the block has no terminator (meaning some other part of the current optimization stole it)
fn take_terminator_if_simple_goto(&mut self, bb: BasicBlock) -> Option<Terminator<'tcx>> {
match self.basic_blocks[bb] {
BasicBlockData {
ref statements,
terminator:
ref mut terminator @ Some(Terminator { kind: TerminatorKind::Goto { .. }, .. }),
..
} if statements.is_empty() => terminator.take(),
// if `terminator` is None, this means we are in a loop. In that
// case, let all the loop collapse to its entry.
_ => None,
}
}
/// Collapse a goto chain starting from `start`
fn collapse_goto_chain(&mut self, start: &mut BasicBlock, changed: &mut bool) {
// Using `SmallVec` here, because in some logs on libcore oli-obk saw many single-element
// goto chains. We should probably benchmark different sizes.
let mut terminators: SmallVec<[_; 1]> = Default::default();
let mut current = *start;
while let Some(terminator) = self.take_terminator_if_simple_goto(current) {
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let Terminator { kind: TerminatorKind::Goto { target }, .. } = terminator else {
unreachable!();
};
terminators.push((current, terminator));
current = target;
}
let last = current;
*start = last;
while let Some((current, mut terminator)) = terminators.pop() {
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let Terminator { kind: TerminatorKind::Goto { ref mut target }, .. } = terminator else {
unreachable!();
};
*changed |= *target != last;
*target = last;
debug!("collapsing goto chain from {:?} to {:?}", current, target);
if self.pred_count[current] == 1 {
// This is the last reference to current, so the pred-count to
// to target is moved into the current block.
self.pred_count[current] = 0;
} else {
self.pred_count[*target] += 1;
self.pred_count[current] -= 1;
}
self.basic_blocks[current].terminator = Some(terminator);
}
}
// merge a block with 1 `goto` predecessor to its parent
fn merge_successor(
&mut self,
merged_blocks: &mut Vec<BasicBlock>,
terminator: &mut Terminator<'tcx>,
) -> bool {
let target = match terminator.kind {
TerminatorKind::Goto { target } if self.pred_count[target] == 1 => target,
_ => return false,
};
debug!("merging block {:?} into {:?}", target, terminator);
*terminator = match self.basic_blocks[target].terminator.take() {
Some(terminator) => terminator,
None => {
// unreachable loop - this should not be possible, as we
// don't strand blocks, but handle it correctly.
return false;
}
};
merged_blocks.push(target);
self.pred_count[target] = 0;
true
}
// turn a branch with all successors identical to a goto
fn simplify_branch(&mut self, terminator: &mut Terminator<'tcx>) -> bool {
match terminator.kind {
TerminatorKind::SwitchInt { .. } => {}
_ => return false,
};
let first_succ = {
if let Some(first_succ) = terminator.successors().next() {
if terminator.successors().all(|s| s == first_succ) {
let count = terminator.successors().count();
self.pred_count[first_succ] -= (count - 1) as u32;
first_succ
} else {
return false;
}
} else {
return false;
}
};
debug!("simplifying branch {:?}", terminator);
terminator.kind = TerminatorKind::Goto { target: first_succ };
true
}
fn strip_nops(&mut self) {
for blk in self.basic_blocks.iter_mut() {
blk.statements.retain(|stmt| !matches!(stmt.kind, StatementKind::Nop))
}
}
}
pub fn remove_dead_blocks<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
let reachable = traversal::reachable_as_bitset(body);
let num_blocks = body.basic_blocks().len();
if num_blocks == reachable.count() {
return;
}
let basic_blocks = body.basic_blocks.as_mut();
let source_scopes = &body.source_scopes;
let mut replacements: Vec<_> = (0..num_blocks).map(BasicBlock::new).collect();
let mut used_blocks = 0;
for alive_index in reachable.iter() {
let alive_index = alive_index.index();
replacements[alive_index] = BasicBlock::new(used_blocks);
if alive_index != used_blocks {
// Swap the next alive block data with the current available slot. Since
// alive_index is non-decreasing this is a valid operation.
basic_blocks.raw.swap(alive_index, used_blocks);
}
used_blocks += 1;
}
if tcx.sess.instrument_coverage() {
save_unreachable_coverage(basic_blocks, source_scopes, used_blocks);
}
basic_blocks.raw.truncate(used_blocks);
for block in basic_blocks {
for target in block.terminator_mut().successors_mut() {
*target = replacements[target.index()];
}
}
}
/// Some MIR transforms can determine at compile time that a sequences of
/// statements will never be executed, so they can be dropped from the MIR.
/// For example, an `if` or `else` block that is guaranteed to never be executed
/// because its condition can be evaluated at compile time, such as by const
/// evaluation: `if false { ... }`.
///
/// Those statements are bypassed by redirecting paths in the CFG around the
/// `dead blocks`; but with `-C instrument-coverage`, the dead blocks usually
/// include `Coverage` statements representing the Rust source code regions to
/// be counted at runtime. Without these `Coverage` statements, the regions are
/// lost, and the Rust source code will show no coverage information.
///
/// What we want to show in a coverage report is the dead code with coverage
/// counts of `0`. To do this, we need to save the code regions, by injecting
/// `Unreachable` coverage statements. These are non-executable statements whose
/// code regions are still recorded in the coverage map, representing regions
/// with `0` executions.
///
/// If there are no live `Counter` `Coverage` statements remaining, we remove
/// `Coverage` statements along with the dead blocks. Since at least one
/// counter per function is required by LLVM (and necessary, to add the
/// `function_hash` to the counter's call to the LLVM intrinsic
/// `instrprof.increment()`).
///
/// The `generator::StateTransform` MIR pass and MIR inlining can create
/// atypical conditions, where all live `Counter`s are dropped from the MIR.
///
/// With MIR inlining we can have coverage counters belonging to different
/// instances in a single body, so the strategy described above is applied to
/// coverage counters from each instance individually.
fn save_unreachable_coverage(
basic_blocks: &mut IndexVec<BasicBlock, BasicBlockData<'_>>,
source_scopes: &IndexVec<SourceScope, SourceScopeData<'_>>,
first_dead_block: usize,
) {
// Identify instances that still have some live coverage counters left.
let mut live = FxHashSet::default();
for basic_block in &basic_blocks.raw[0..first_dead_block] {
for statement in &basic_block.statements {
let StatementKind::Coverage(coverage) = &statement.kind else { continue };
let CoverageKind::Counter { .. } = coverage.kind else { continue };
let instance = statement.source_info.scope.inlined_instance(source_scopes);
live.insert(instance);
}
}
for block in &mut basic_blocks.raw[..first_dead_block] {
for statement in &mut block.statements {
let StatementKind::Coverage(_) = &statement.kind else { continue };
let instance = statement.source_info.scope.inlined_instance(source_scopes);
if !live.contains(&instance) {
statement.make_nop();
}
}
}
if live.is_empty() {
return;
}
// Retain coverage for instances that still have some live counters left.
let mut retained_coverage = Vec::new();
for dead_block in &basic_blocks.raw[first_dead_block..] {
for statement in &dead_block.statements {
let StatementKind::Coverage(coverage) = &statement.kind else { continue };
let Some(code_region) = &coverage.code_region else { continue };
let instance = statement.source_info.scope.inlined_instance(source_scopes);
if live.contains(&instance) {
retained_coverage.push((statement.source_info, code_region.clone()));
}
}
}
let start_block = &mut basic_blocks[START_BLOCK];
start_block.statements.extend(retained_coverage.into_iter().map(
|(source_info, code_region)| Statement {
source_info,
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kind: StatementKind::Coverage(Box::new(Coverage {
kind: CoverageKind::Unreachable,
code_region: Some(code_region),
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})),
},
));
}
pub struct SimplifyLocals;
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impl<'tcx> MirPass<'tcx> for SimplifyLocals {
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fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
sess.mir_opt_level() > 0
}
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
trace!("running SimplifyLocals on {:?}", body.source);
simplify_locals(body, tcx);
}
}
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);
// 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.
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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);
// Only bother running the `LocalUpdater` if we actually found locals to remove.
if map.iter().any(Option::is_none) {
// Update references to all vars and tmps now
let mut updater = LocalUpdater { map, tcx };
updater.visit_body_preserves_cfg(body);
body.local_decls.shrink_to_fit();
}
}
/// Construct the mapping while swapping out unused stuff out from the `vec`.
fn make_local_map<V>(
local_decls: &mut IndexVec<Local, V>,
used_locals: &UsedLocals,
) -> IndexVec<Local, Option<Local>> {
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;
}
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 {
increment: bool,
arg_count: u32,
use_count: IndexVec<Local, u32>,
}
impl UsedLocals {
/// Determines which locals are used & unused in the given body.
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),
};
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
}
/// Updates the use counts to reflect the removal of given statement.
fn statement_removed(&mut self, statement: &Statement<'_>) {
self.increment = false;
// The location of the statement is irrelevant.
let location = Location { block: START_BLOCK, statement_index: 0 };
self.visit_statement(statement, location);
}
/// Visits a left-hand side of an assignment.
fn visit_lhs(&mut self, place: &Place<'_>, location: Location) {
if place.is_indirect() {
// A use, not a definition.
self.visit_place(place, PlaceContext::MutatingUse(MutatingUseContext::Store), location);
} else {
// A definition. The base local itself is not visited, so this occurrence is not counted
// toward its use count. There might be other locals still, used in an indexing
// projection.
self.super_projection(
place.as_ref(),
PlaceContext::MutatingUse(MutatingUseContext::Projection),
location,
);
}
}
}
impl<'tcx> Visitor<'tcx> for UsedLocals {
fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
match statement.kind {
StatementKind::CopyNonOverlapping(..)
| StatementKind::Retag(..)
| StatementKind::Coverage(..)
| StatementKind::FakeRead(..)
| StatementKind::AscribeUserType(..) => {
self.super_statement(statement, location);
}
StatementKind::Nop => {}
StatementKind::StorageLive(_local) | StatementKind::StorageDead(_local) => {}
StatementKind::Assign(box (ref place, ref rvalue)) => {
if rvalue.is_safe_to_remove() {
self.visit_lhs(place, location);
self.visit_rvalue(rvalue, location);
} else {
self.super_statement(statement, location);
}
}
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StatementKind::SetDiscriminant { ref place, variant_index: _ }
| StatementKind::Deinit(ref place) => {
self.visit_lhs(place, location);
}
}
}
fn visit_local(&mut self, local: Local, _ctx: PlaceContext, _location: Location) {
if self.increment {
self.use_count[local] += 1;
} else {
assert_ne!(self.use_count[local], 0);
self.use_count[local] -= 1;
}
}
}
/// Removes unused definitions. Updates the used locals to reflect the changes made.
fn remove_unused_definitions(used_locals: &mut UsedLocals, body: &mut Body<'_>) {
// 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 mut modified = true;
while modified {
modified = false;
for data in body.basic_blocks.as_mut_preserves_cfg() {
// Remove unnecessary StorageLive and StorageDead annotations.
data.statements.retain(|statement| {
let keep = match &statement.kind {
StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
used_locals.is_used(*local)
}
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StatementKind::Assign(box (place, _)) => used_locals.is_used(place.local),
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StatementKind::SetDiscriminant { ref place, .. }
| StatementKind::Deinit(ref place) => used_locals.is_used(place.local),
_ => true,
};
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if !keep {
trace!("removing statement {:?}", statement);
modified = true;
used_locals.statement_removed(statement);
}
keep
});
}
}
}
struct LocalUpdater<'tcx> {
map: IndexVec<Local, Option<Local>>,
tcx: TyCtxt<'tcx>,
}
impl<'tcx> MutVisitor<'tcx> for LocalUpdater<'tcx> {
fn tcx(&self) -> TyCtxt<'tcx> {
self.tcx
}
fn visit_local(&mut self, l: &mut Local, _: PlaceContext, _: Location) {
*l = self.map[*l].unwrap();
}
}