Move rustc_mir::transform to rustc_mir_transform.
This commit is contained in:
Camille GILLOT
parent
31a61ccc38
commit
bba4be681d
64 changed files with 775 additions and 698 deletions
592
compiler/rustc_mir_transform/src/simplify.rs
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592
compiler/rustc_mir_transform/src/simplify.rs
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//! A number of passes which remove various redundancies in the CFG.
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//!
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//! The `SimplifyCfg` pass gets rid of unnecessary blocks in the CFG, whereas the `SimplifyLocals`
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//! gets rid of all the unnecessary local variable declarations.
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//!
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//! The `SimplifyLocals` pass is kinda expensive and therefore not very suitable to be run often.
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//! 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.
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//!
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//! On the other side of the spectrum, the `SimplifyCfg` pass is considerably cheap to run, thus
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//! one should run it after every pass which may modify CFG in significant ways. This pass must
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//! also be run before any analysis passes because it removes dead blocks, and some of these can be
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//! ill-typed.
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//!
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//! The cause of this typing issue is typeck allowing most blocks whose end is not reachable have
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//! an arbitrary return type, rather than having the usual () return type (as a note, typeck's
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//! notion of reachability is in fact slightly weaker than MIR CFG reachability - see #31617). A
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//! standard example of the situation is:
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//!
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//! ```rust
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//! fn example() {
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//! let _a: char = { return; };
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//! }
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//! ```
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//!
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//! Here the block (`{ return; }`) has the return type `char`, rather than `()`, but the MIR we
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//! naively generate still contains the `_a = ()` write in the unreachable block "after" the
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//! return.
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use crate::MirPass;
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use rustc_index::vec::{Idx, IndexVec};
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use rustc_middle::mir::coverage::*;
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use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor};
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use rustc_middle::mir::*;
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use rustc_middle::ty::TyCtxt;
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use smallvec::SmallVec;
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use std::borrow::Cow;
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use std::convert::TryInto;
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pub struct SimplifyCfg {
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label: String,
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}
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impl SimplifyCfg {
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pub fn new(label: &str) -> Self {
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SimplifyCfg { label: format!("SimplifyCfg-{}", label) }
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}
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}
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pub fn simplify_cfg(tcx: TyCtxt<'tcx>, body: &mut Body<'_>) {
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CfgSimplifier::new(body).simplify();
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remove_dead_blocks(tcx, body);
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// FIXME: Should probably be moved into some kind of pass manager
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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> {
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Cow::Borrowed(&self.label)
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}
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fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
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debug!("SimplifyCfg({:?}) - simplifying {:?}", self.label, body.source);
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simplify_cfg(tcx, body);
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}
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}
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pub struct CfgSimplifier<'a, 'tcx> {
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basic_blocks: &'a mut IndexVec<BasicBlock, BasicBlockData<'tcx>>,
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pred_count: IndexVec<BasicBlock, u32>,
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}
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impl<'a, 'tcx> CfgSimplifier<'a, 'tcx> {
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pub fn new(body: &'a mut Body<'tcx>) -> Self {
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let mut pred_count = IndexVec::from_elem(0u32, body.basic_blocks());
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// we can't use mir.predecessors() here because that counts
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// dead blocks, which we don't want to.
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pred_count[START_BLOCK] = 1;
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for (_, data) in traversal::preorder(body) {
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if let Some(ref term) = data.terminator {
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for &tgt in term.successors() {
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pred_count[tgt] += 1;
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}
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}
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}
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let basic_blocks = body.basic_blocks_mut();
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CfgSimplifier { basic_blocks, pred_count }
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}
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pub fn simplify(mut self) {
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self.strip_nops();
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let mut start = START_BLOCK;
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// Vec of the blocks that should be merged. We store the indices here, instead of the
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// statements itself to avoid moving the (relatively) large statements twice.
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// We do not push the statements directly into the target block (`bb`) as that is slower
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// due to additional reallocations
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let mut merged_blocks = Vec::new();
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loop {
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let mut changed = false;
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self.collapse_goto_chain(&mut start, &mut changed);
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for bb in self.basic_blocks.indices() {
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if self.pred_count[bb] == 0 {
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continue;
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}
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debug!("simplifying {:?}", bb);
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let mut terminator =
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self.basic_blocks[bb].terminator.take().expect("invalid terminator state");
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for successor in terminator.successors_mut() {
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self.collapse_goto_chain(successor, &mut changed);
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}
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let mut inner_changed = true;
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merged_blocks.clear();
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while inner_changed {
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inner_changed = false;
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inner_changed |= self.simplify_branch(&mut terminator);
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inner_changed |= self.merge_successor(&mut merged_blocks, &mut terminator);
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changed |= inner_changed;
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}
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let statements_to_merge =
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merged_blocks.iter().map(|&i| self.basic_blocks[i].statements.len()).sum();
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if statements_to_merge > 0 {
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let mut statements = std::mem::take(&mut self.basic_blocks[bb].statements);
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statements.reserve(statements_to_merge);
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for &from in &merged_blocks {
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statements.append(&mut self.basic_blocks[from].statements);
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}
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self.basic_blocks[bb].statements = statements;
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}
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self.basic_blocks[bb].terminator = Some(terminator);
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}
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if !changed {
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break;
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}
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}
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if start != START_BLOCK {
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debug_assert!(self.pred_count[START_BLOCK] == 0);
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self.basic_blocks.swap(START_BLOCK, start);
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self.pred_count.swap(START_BLOCK, start);
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// pred_count == 1 if the start block has no predecessor _blocks_.
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if self.pred_count[START_BLOCK] > 1 {
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for (bb, data) in self.basic_blocks.iter_enumerated_mut() {
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if self.pred_count[bb] == 0 {
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continue;
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}
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for target in data.terminator_mut().successors_mut() {
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if *target == start {
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*target = START_BLOCK;
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}
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}
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}
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}
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}
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}
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/// This function will return `None` if
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/// * the block has statements
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/// * the block has a terminator other than `goto`
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/// * the block has no terminator (meaning some other part of the current optimization stole it)
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fn take_terminator_if_simple_goto(&mut self, bb: BasicBlock) -> Option<Terminator<'tcx>> {
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match self.basic_blocks[bb] {
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BasicBlockData {
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ref statements,
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terminator:
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ref mut terminator @ Some(Terminator { kind: TerminatorKind::Goto { .. }, .. }),
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..
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} if statements.is_empty() => terminator.take(),
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// if `terminator` is None, this means we are in a loop. In that
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// case, let all the loop collapse to its entry.
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_ => None,
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}
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}
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/// Collapse a goto chain starting from `start`
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fn collapse_goto_chain(&mut self, start: &mut BasicBlock, changed: &mut bool) {
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// Using `SmallVec` here, because in some logs on libcore oli-obk saw many single-element
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// goto chains. We should probably benchmark different sizes.
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let mut terminators: SmallVec<[_; 1]> = Default::default();
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let mut current = *start;
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while let Some(terminator) = self.take_terminator_if_simple_goto(current) {
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let target = match terminator {
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Terminator { kind: TerminatorKind::Goto { target }, .. } => target,
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_ => unreachable!(),
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};
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terminators.push((current, terminator));
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current = target;
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}
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let last = current;
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*start = last;
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while let Some((current, mut terminator)) = terminators.pop() {
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let target = match terminator {
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Terminator { kind: TerminatorKind::Goto { ref mut target }, .. } => target,
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_ => unreachable!(),
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};
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*changed |= *target != last;
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*target = last;
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debug!("collapsing goto chain from {:?} to {:?}", current, target);
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if self.pred_count[current] == 1 {
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// This is the last reference to current, so the pred-count to
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// to target is moved into the current block.
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self.pred_count[current] = 0;
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} else {
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self.pred_count[*target] += 1;
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self.pred_count[current] -= 1;
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}
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self.basic_blocks[current].terminator = Some(terminator);
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}
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}
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// merge a block with 1 `goto` predecessor to its parent
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fn merge_successor(
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&mut self,
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merged_blocks: &mut Vec<BasicBlock>,
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terminator: &mut Terminator<'tcx>,
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) -> bool {
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let target = match terminator.kind {
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TerminatorKind::Goto { target } if self.pred_count[target] == 1 => target,
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_ => return false,
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};
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debug!("merging block {:?} into {:?}", target, terminator);
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*terminator = match self.basic_blocks[target].terminator.take() {
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Some(terminator) => terminator,
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None => {
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// unreachable loop - this should not be possible, as we
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// don't strand blocks, but handle it correctly.
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return false;
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}
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};
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merged_blocks.push(target);
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self.pred_count[target] = 0;
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true
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}
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// turn a branch with all successors identical to a goto
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fn simplify_branch(&mut self, terminator: &mut Terminator<'tcx>) -> bool {
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match terminator.kind {
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TerminatorKind::SwitchInt { .. } => {}
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_ => return false,
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};
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let first_succ = {
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if let Some(&first_succ) = terminator.successors().next() {
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if terminator.successors().all(|s| *s == first_succ) {
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let count = terminator.successors().count();
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self.pred_count[first_succ] -= (count - 1) as u32;
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first_succ
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} else {
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return false;
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}
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} else {
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return false;
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}
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};
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debug!("simplifying branch {:?}", terminator);
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terminator.kind = TerminatorKind::Goto { target: first_succ };
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true
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}
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fn strip_nops(&mut self) {
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for blk in self.basic_blocks.iter_mut() {
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blk.statements.retain(|stmt| !matches!(stmt.kind, StatementKind::Nop))
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}
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}
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}
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pub fn remove_dead_blocks(tcx: TyCtxt<'tcx>, body: &mut Body<'_>) {
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let reachable = traversal::reachable_as_bitset(body);
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let num_blocks = body.basic_blocks().len();
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if num_blocks == reachable.count() {
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return;
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}
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let basic_blocks = body.basic_blocks_mut();
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let mut replacements: Vec<_> = (0..num_blocks).map(BasicBlock::new).collect();
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let mut used_blocks = 0;
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for alive_index in reachable.iter() {
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let alive_index = alive_index.index();
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replacements[alive_index] = BasicBlock::new(used_blocks);
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if alive_index != used_blocks {
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// Swap the next alive block data with the current available slot. Since
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// alive_index is non-decreasing this is a valid operation.
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basic_blocks.raw.swap(alive_index, used_blocks);
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}
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used_blocks += 1;
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}
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if tcx.sess.instrument_coverage() {
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save_unreachable_coverage(basic_blocks, used_blocks);
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}
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basic_blocks.raw.truncate(used_blocks);
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for block in basic_blocks {
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for target in block.terminator_mut().successors_mut() {
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*target = replacements[target.index()];
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}
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}
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}
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/// Some MIR transforms can determine at compile time that a sequences of
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/// statements will never be executed, so they can be dropped from the MIR.
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/// For example, an `if` or `else` block that is guaranteed to never be executed
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/// because its condition can be evaluated at compile time, such as by const
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/// evaluation: `if false { ... }`.
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///
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/// Those statements are bypassed by redirecting paths in the CFG around the
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/// `dead blocks`; but with `-Z instrument-coverage`, the dead blocks usually
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/// include `Coverage` statements representing the Rust source code regions to
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/// be counted at runtime. Without these `Coverage` statements, the regions are
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/// lost, and the Rust source code will show no coverage information.
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///
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/// What we want to show in a coverage report is the dead code with coverage
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/// counts of `0`. To do this, we need to save the code regions, by injecting
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/// `Unreachable` coverage statements. These are non-executable statements whose
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/// code regions are still recorded in the coverage map, representing regions
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/// with `0` executions.
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fn save_unreachable_coverage(
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basic_blocks: &mut IndexVec<BasicBlock, BasicBlockData<'_>>,
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first_dead_block: usize,
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) {
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let has_live_counters = basic_blocks.raw[0..first_dead_block].iter().any(|live_block| {
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live_block.statements.iter().any(|statement| {
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if let StatementKind::Coverage(coverage) = &statement.kind {
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matches!(coverage.kind, CoverageKind::Counter { .. })
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} else {
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false
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}
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})
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});
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if !has_live_counters {
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// If there are no live `Counter` `Coverage` statements anymore, don't
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// move dead coverage to the `START_BLOCK`. Just allow the dead
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// `Coverage` statements to be dropped with the dead blocks.
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//
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// The `generator::StateTransform` MIR pass can create atypical
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// conditions, where all live `Counter`s are dropped from the MIR.
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//
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// At least one Counter per function is required by LLVM (and necessary,
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// to add the `function_hash` to the counter's call to the LLVM
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// intrinsic `instrprof.increment()`).
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return;
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}
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// Retain coverage info for dead blocks, so coverage reports will still
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// report `0` executions for the uncovered code regions.
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let mut dropped_coverage = Vec::new();
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for dead_block in basic_blocks.raw[first_dead_block..].iter() {
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for statement in dead_block.statements.iter() {
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if let StatementKind::Coverage(coverage) = &statement.kind {
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if let Some(code_region) = &coverage.code_region {
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dropped_coverage.push((statement.source_info, code_region.clone()));
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}
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}
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}
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}
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let start_block = &mut basic_blocks[START_BLOCK];
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for (source_info, code_region) in dropped_coverage {
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start_block.statements.push(Statement {
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source_info,
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kind: StatementKind::Coverage(Box::new(Coverage {
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kind: CoverageKind::Unreachable,
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code_region: Some(code_region),
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})),
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})
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}
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}
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pub struct SimplifyLocals;
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impl<'tcx> MirPass<'tcx> for SimplifyLocals {
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fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
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trace!("running SimplifyLocals on {:?}", body.source);
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simplify_locals(body, tcx);
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}
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}
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pub fn simplify_locals<'tcx>(body: &mut Body<'tcx>, tcx: TyCtxt<'tcx>) {
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// First, we're going to get a count of *actual* uses for every `Local`.
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let mut used_locals = UsedLocals::new(body);
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// Next, we're going to remove any `Local` with zero actual uses. When we remove those
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// `Locals`, we're also going to subtract any uses of other `Locals` from the `used_locals`
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// count. For example, if we removed `_2 = discriminant(_1)`, then we'll subtract one from
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// `use_counts[_1]`. That in turn might make `_1` unused, so we loop until we hit a
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// fixedpoint where there are no more unused locals.
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remove_unused_definitions(&mut used_locals, body);
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// Finally, we'll actually do the work of shrinking `body.local_decls` and remapping the `Local`s.
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let map = make_local_map(&mut body.local_decls, &used_locals);
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// Only bother running the `LocalUpdater` if we actually found locals to remove.
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if map.iter().any(Option::is_none) {
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// Update references to all vars and tmps now
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let mut updater = LocalUpdater { map, tcx };
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updater.visit_body(body);
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body.local_decls.shrink_to_fit();
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}
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}
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/// Construct the mapping while swapping out unused stuff out from the `vec`.
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fn make_local_map<V>(
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local_decls: &mut IndexVec<Local, V>,
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used_locals: &UsedLocals,
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) -> IndexVec<Local, Option<Local>> {
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let mut map: IndexVec<Local, Option<Local>> = IndexVec::from_elem(None, &*local_decls);
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let mut used = Local::new(0);
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for alive_index in local_decls.indices() {
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// `is_used` treats the `RETURN_PLACE` and arguments as used.
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if !used_locals.is_used(alive_index) {
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continue;
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}
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map[alive_index] = Some(used);
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if alive_index != used {
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local_decls.swap(alive_index, used);
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}
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used.increment_by(1);
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}
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local_decls.truncate(used.index());
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map
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}
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/// Keeps track of used & unused locals.
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struct UsedLocals {
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increment: bool,
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arg_count: u32,
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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<'tcx>) {
|
||||
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<'tcx>, 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 Visitor<'_> for UsedLocals {
|
||||
fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
|
||||
match statement.kind {
|
||||
StatementKind::LlvmInlineAsm(..)
|
||||
| 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)) => {
|
||||
self.visit_lhs(place, location);
|
||||
self.visit_rvalue(rvalue, location);
|
||||
}
|
||||
|
||||
StatementKind::SetDiscriminant { ref place, variant_index: _ } => {
|
||||
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<'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
|
||||
// computation reaches a fixed point.
|
||||
|
||||
let mut modified = true;
|
||||
while modified {
|
||||
modified = false;
|
||||
|
||||
for data in body.basic_blocks_mut() {
|
||||
// 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)
|
||||
}
|
||||
StatementKind::Assign(box (place, _)) => used_locals.is_used(place.local),
|
||||
|
||||
StatementKind::SetDiscriminant { ref place, .. } => {
|
||||
used_locals.is_used(place.local)
|
||||
}
|
||||
_ => true,
|
||||
};
|
||||
|
||||
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();
|
||||
}
|
||||
}
|
||||
Loading…
Add table
Add a link
Reference in a new issue