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Remove ConstGoto and SeparateConstSwitch.

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This commit is contained in:
Camille GILLOT 2023-02-26 13:42:28 +00:00
parent e132cac3c4
commit 014b29eecf
15 changed files with 125 additions and 733 deletions
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128
compiler/rustc_mir_transform/src/const_goto.rs
View file

@ -1,128 +0,0 @@
//! This pass optimizes the following sequence
//! ```rust,ignore (example)
//! bb2: {
//! _2 = const true;
//! goto -> bb3;
//! }
//!
//! bb3: {
//! switchInt(_2) -> [false: bb4, otherwise: bb5];
//! }
//! ```
//! into
//! ```rust,ignore (example)
//! bb2: {
//! _2 = const true;
//! goto -> bb5;
//! }
//! ```
use rustc_middle::mir::*;
use rustc_middle::ty::TyCtxt;
use rustc_middle::{mir::visit::Visitor, ty::ParamEnv};
use super::simplify::{simplify_cfg, simplify_locals};
pub struct ConstGoto;
impl<'tcx> MirPass<'tcx> for ConstGoto {
fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
// This pass participates in some as-of-yet untested unsoundness found
// in https://github.com/rust-lang/rust/issues/112460
sess.mir_opt_level() >= 2 && sess.opts.unstable_opts.unsound_mir_opts
}
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
trace!("Running ConstGoto on {:?}", body.source);
let param_env = tcx.param_env_reveal_all_normalized(body.source.def_id());
let mut opt_finder =
ConstGotoOptimizationFinder { tcx, body, optimizations: vec![], param_env };
opt_finder.visit_body(body);
let should_simplify = !opt_finder.optimizations.is_empty();
for opt in opt_finder.optimizations {
let block = &mut body.basic_blocks_mut()[opt.bb_with_goto];
block.statements.extend(opt.stmts_move_up);
let terminator = block.terminator_mut();
let new_goto = TerminatorKind::Goto { target: opt.target_to_use_in_goto };
debug!("SUCCESS: replacing `{:?}` with `{:?}`", terminator.kind, new_goto);
terminator.kind = new_goto;
}
// if we applied optimizations, we potentially have some cfg to cleanup to
// make it easier for further passes
if should_simplify {
simplify_cfg(body);
simplify_locals(body, tcx);
}
}
}
impl<'tcx> Visitor<'tcx> for ConstGotoOptimizationFinder<'_, 'tcx> {
fn visit_basic_block_data(&mut self, block: BasicBlock, data: &BasicBlockData<'tcx>) {
if data.is_cleanup {
// Because of the restrictions around control flow in cleanup blocks, we don't perform
// this optimization at all in such blocks.
return;
}
self.super_basic_block_data(block, data);
}
fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
let _: Option<_> = try {
let target = terminator.kind.as_goto()?;
// We only apply this optimization if the last statement is a const assignment
let last_statement = self.body.basic_blocks[location.block].statements.last()?;
if let (place, Rvalue::Use(Operand::Constant(_const))) =
last_statement.kind.as_assign()?
{
// We found a constant being assigned to `place`.
// Now check that the target of this Goto switches on this place.
let target_bb = &self.body.basic_blocks[target];
// The `StorageDead(..)` statement does not affect the functionality of mir.
// We can move this part of the statement up to the predecessor.
let mut stmts_move_up = Vec::new();
for stmt in &target_bb.statements {
if let StatementKind::StorageDead(..) = stmt.kind {
stmts_move_up.push(stmt.clone())
} else {
None?;
}
}
let target_bb_terminator = target_bb.terminator();
let (discr, targets) = target_bb_terminator.kind.as_switch()?;
if discr.place() == Some(*place) {
let switch_ty = place.ty(self.body.local_decls(), self.tcx).ty;
debug_assert_eq!(switch_ty, _const.ty());
// We now know that the Switch matches on the const place, and it is statementless
// Now find which value in the Switch matches the const value.
let const_value = _const.const_.try_eval_bits(self.tcx, self.param_env)?;
let target_to_use_in_goto = targets.target_for_value(const_value);
self.optimizations.push(OptimizationToApply {
bb_with_goto: location.block,
target_to_use_in_goto,
stmts_move_up,
});
}
}
Some(())
};
self.super_terminator(terminator, location);
}
}
struct OptimizationToApply<'tcx> {
bb_with_goto: BasicBlock,
target_to_use_in_goto: BasicBlock,
stmts_move_up: Vec<Statement<'tcx>>,
}
pub struct ConstGotoOptimizationFinder<'a, 'tcx> {
tcx: TyCtxt<'tcx>,
body: &'a Body<'tcx>,
param_env: ParamEnv<'tcx>,
optimizations: Vec<OptimizationToApply<'tcx>>,
}

7
compiler/rustc_mir_transform/src/lib.rs
View file

@ -59,7 +59,6 @@ mod remove_place_mention;
mod add_subtyping_projections;
pub mod cleanup_post_borrowck;
mod const_debuginfo;
mod const_goto;
mod const_prop;
mod const_prop_lint;
mod copy_prop;
@ -103,7 +102,6 @@ mod remove_unneeded_drops;
mod remove_zsts;
mod required_consts;
mod reveal_all;
mod separate_const_switch;
mod shim;
mod ssa;
// This pass is public to allow external drivers to perform MIR cleanup
@ -590,7 +588,6 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
// Has to run after `slice::len` lowering
&normalize_array_len::NormalizeArrayLen,
&const_goto::ConstGoto,
&ref_prop::ReferencePropagation,
&sroa::ScalarReplacementOfAggregates,
&match_branches::MatchBranchSimplification,
@ -601,10 +598,6 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
&dead_store_elimination::DeadStoreElimination::Initial,
&gvn::GVN,
&simplify::SimplifyLocals::AfterGVN,
// Perform `SeparateConstSwitch` after SSA-based analyses, as cloning blocks may
// destroy the SSA property. It should still happen before const-propagation, so the
// latter pass will leverage the created opportunities.
&separate_const_switch::SeparateConstSwitch,
&dataflow_const_prop::DataflowConstProp,
&const_debuginfo::ConstDebugInfo,
&o1(simplify_branches::SimplifyConstCondition::AfterConstProp),

343
compiler/rustc_mir_transform/src/separate_const_switch.rs
View file

@ -1,343 +0,0 @@
//! A pass that duplicates switch-terminated blocks
//! into a new copy for each predecessor, provided
//! the predecessor sets the value being switched
//! over to a constant.
//!
//! The purpose of this pass is to help constant
//! propagation passes to simplify the switch terminator
//! of the copied blocks into gotos when some predecessors
//! statically determine the output of switches.
//!
//! ```text
//! x = 12 --- ---> something
//! \ / 12
//! --> switch x
//! / \ otherwise
//! x = y --- ---> something else
//! ```
//! becomes
//! ```text
//! x = 12 ---> switch x ------> something
//! \ / 12
//! X
//! / \ otherwise
//! x = y ---> switch x ------> something else
//! ```
//! so it can hopefully later be turned by another pass into
//! ```text
//! x = 12 --------------------> something
//! / 12
//! /
//! / otherwise
//! x = y ---- switch x ------> something else
//! ```
//!
//! This optimization is meant to cover simple cases
//! like `?` desugaring. For now, it thus focuses on
//! simplicity rather than completeness (it notably
//! sometimes duplicates abusively).
use rustc_middle::mir::*;
use rustc_middle::ty::TyCtxt;
use smallvec::SmallVec;
pub struct SeparateConstSwitch;
impl<'tcx> MirPass<'tcx> for SeparateConstSwitch {
fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
// This pass participates in some as-of-yet untested unsoundness found
// in https://github.com/rust-lang/rust/issues/112460
sess.mir_opt_level() >= 2 && sess.opts.unstable_opts.unsound_mir_opts
}
fn run_pass(&self, _: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
// If execution did something, applying a simplification layer
// helps later passes optimize the copy away.
if separate_const_switch(body) > 0 {
super::simplify::simplify_cfg(body);
}
}
}
/// Returns the amount of blocks that were duplicated
pub fn separate_const_switch(body: &mut Body<'_>) -> usize {
let mut new_blocks: SmallVec<[(BasicBlock, BasicBlock); 6]> = SmallVec::new();
let predecessors = body.basic_blocks.predecessors();
'block_iter: for (block_id, block) in body.basic_blocks.iter_enumerated() {
if let TerminatorKind::SwitchInt {
discr: Operand::Copy(switch_place) | Operand::Move(switch_place),
..
} = block.terminator().kind
{
// If the block is on an unwind path, do not
// apply the optimization as unwind paths
// rely on a unique parent invariant
if block.is_cleanup {
continue 'block_iter;
}
// If the block has fewer than 2 predecessors, ignore it
// we could maybe chain blocks that have exactly one
// predecessor, but for now we ignore that
if predecessors[block_id].len() < 2 {
continue 'block_iter;
}
// First, let's find a non-const place
// that determines the result of the switch
if let Some(switch_place) = find_determining_place(switch_place, block) {
// We now have an input place for which it would
// be interesting if predecessors assigned it from a const
let mut predecessors_left = predecessors[block_id].len();
'predec_iter: for predecessor_id in predecessors[block_id].iter().copied() {
let predecessor = &body.basic_blocks[predecessor_id];
// First we make sure the predecessor jumps
// in a reasonable way
match &predecessor.terminator().kind {
// The following terminators are
// unconditionally valid
TerminatorKind::Goto { .. } | TerminatorKind::SwitchInt { .. } => {}
TerminatorKind::FalseEdge { real_target, .. } => {
if *real_target != block_id {
continue 'predec_iter;
}
}
// The following terminators are not allowed
TerminatorKind::UnwindResume
| TerminatorKind::Drop { .. }
| TerminatorKind::Call { .. }
| TerminatorKind::Assert { .. }
| TerminatorKind::FalseUnwind { .. }
| TerminatorKind::Yield { .. }
| TerminatorKind::UnwindTerminate(_)
| TerminatorKind::Return
| TerminatorKind::Unreachable
| TerminatorKind::InlineAsm { .. }
| TerminatorKind::CoroutineDrop => {
continue 'predec_iter;
}
}
if is_likely_const(switch_place, predecessor) {
new_blocks.push((predecessor_id, block_id));
predecessors_left -= 1;
if predecessors_left < 2 {
// If the original block only has one predecessor left,
// we have nothing left to do
break 'predec_iter;
}
}
}
}
}
}
// Once the analysis is done, perform the duplication
let body_span = body.span;
let copied_blocks = new_blocks.len();
let blocks = body.basic_blocks_mut();
for (pred_id, target_id) in new_blocks {
let new_block = blocks[target_id].clone();
let new_block_id = blocks.push(new_block);
let terminator = blocks[pred_id].terminator_mut();
match terminator.kind {
TerminatorKind::Goto { ref mut target } => {
*target = new_block_id;
}
TerminatorKind::FalseEdge { ref mut real_target, .. } => {
if *real_target == target_id {
*real_target = new_block_id;
}
}
TerminatorKind::SwitchInt { ref mut targets, .. } => {
targets.all_targets_mut().iter_mut().for_each(|x| {
if *x == target_id {
*x = new_block_id;
}
});
}
TerminatorKind::UnwindResume
| TerminatorKind::UnwindTerminate(_)
| TerminatorKind::Return
| TerminatorKind::Unreachable
| TerminatorKind::CoroutineDrop
| TerminatorKind::Assert { .. }
| TerminatorKind::FalseUnwind { .. }
| TerminatorKind::Drop { .. }
| TerminatorKind::Call { .. }
| TerminatorKind::InlineAsm { .. }
| TerminatorKind::Yield { .. } => {
span_bug!(
body_span,
"basic block terminator had unexpected kind {:?}",
&terminator.kind
)
}
}
}
copied_blocks
}
/// This function describes a rough heuristic guessing
/// whether a place is last set with a const within the block.
/// Notably, it will be overly pessimistic in cases that are already
/// not handled by `separate_const_switch`.
fn is_likely_const<'tcx>(mut tracked_place: Place<'tcx>, block: &BasicBlockData<'tcx>) -> bool {
for statement in block.statements.iter().rev() {
match &statement.kind {
StatementKind::Assign(assign) => {
if assign.0 == tracked_place {
match assign.1 {
// These rvalues are definitely constant
Rvalue::Use(Operand::Constant(_))
| Rvalue::Ref(_, _, _)
| Rvalue::AddressOf(_, _)
| Rvalue::Cast(_, Operand::Constant(_), _)
| Rvalue::NullaryOp(_, _)
| Rvalue::ShallowInitBox(_, _)
| Rvalue::UnaryOp(_, Operand::Constant(_)) => return true,
// These rvalues make things ambiguous
Rvalue::Repeat(_, _)
| Rvalue::ThreadLocalRef(_)
| Rvalue::Len(_)
| Rvalue::BinaryOp(_, _)
| Rvalue::CheckedBinaryOp(_, _)
| Rvalue::Aggregate(_, _) => return false,
// These rvalues move the place to track
Rvalue::Cast(_, Operand::Copy(place) | Operand::Move(place), _)
| Rvalue::Use(Operand::Copy(place) | Operand::Move(place))
| Rvalue::CopyForDeref(place)
| Rvalue::UnaryOp(_, Operand::Copy(place) | Operand::Move(place))
| Rvalue::Discriminant(place) => tracked_place = place,
}
}
}
// If the discriminant is set, it is always set
// as a constant, so the job is done.
// As we are **ignoring projections**, if the place
// we are tracking sees its discriminant be set,
// that means we had to be tracking the discriminant
// specifically (as it is impossible to switch over
// an enum directly, and if we were switching over
// its content, we would have had to at least cast it to
// some variant first)
StatementKind::SetDiscriminant { place, .. } => {
if **place == tracked_place {
return true;
}
}
// These statements have no influence on the place
// we are interested in
StatementKind::FakeRead(_)
| StatementKind::Deinit(_)
| StatementKind::StorageLive(_)
| StatementKind::Retag(_, _)
| StatementKind::AscribeUserType(_, _)
| StatementKind::PlaceMention(..)
| StatementKind::Coverage(_)
| StatementKind::StorageDead(_)
| StatementKind::Intrinsic(_)
| StatementKind::ConstEvalCounter
| StatementKind::Nop => {}
}
}
// If no good reason for the place to be const is found,
// give up. We could maybe go up predecessors, but in
// most cases giving up now should be sufficient.
false
}
/// Finds a unique place that entirely determines the value
/// of `switch_place`, if it exists. This is only a heuristic.
/// Ideally we would like to track multiple determining places
/// for some edge cases, but one is enough for a lot of situations.
fn find_determining_place<'tcx>(
mut switch_place: Place<'tcx>,
block: &BasicBlockData<'tcx>,
) -> Option<Place<'tcx>> {
for statement in block.statements.iter().rev() {
match &statement.kind {
StatementKind::Assign(op) => {
if op.0 != switch_place {
continue;
}
match op.1 {
// The following rvalues move the place
// that may be const in the predecessor
Rvalue::Use(Operand::Move(new) | Operand::Copy(new))
| Rvalue::UnaryOp(_, Operand::Copy(new) | Operand::Move(new))
| Rvalue::CopyForDeref(new)
| Rvalue::Cast(_, Operand::Move(new) | Operand::Copy(new), _)
| Rvalue::Repeat(Operand::Move(new) | Operand::Copy(new), _)
| Rvalue::Discriminant(new)
=> switch_place = new,
// The following rvalues might still make the block
// be valid but for now we reject them
Rvalue::Len(_)
| Rvalue::Ref(_, _, _)
| Rvalue::BinaryOp(_, _)
| Rvalue::CheckedBinaryOp(_, _)
| Rvalue::Aggregate(_, _)
// The following rvalues definitely mean we cannot
// or should not apply this optimization
| Rvalue::Use(Operand::Constant(_))
| Rvalue::Repeat(Operand::Constant(_), _)
| Rvalue::ThreadLocalRef(_)
| Rvalue::AddressOf(_, _)
| Rvalue::NullaryOp(_, _)
| Rvalue::ShallowInitBox(_, _)
| Rvalue::UnaryOp(_, Operand::Constant(_))
| Rvalue::Cast(_, Operand::Constant(_), _) => return None,
}
}
// These statements have no influence on the place
// we are interested in
StatementKind::FakeRead(_)
| StatementKind::Deinit(_)
| StatementKind::StorageLive(_)
| StatementKind::StorageDead(_)
| StatementKind::Retag(_, _)
| StatementKind::AscribeUserType(_, _)
| StatementKind::PlaceMention(..)
| StatementKind::Coverage(_)
| StatementKind::Intrinsic(_)
| StatementKind::ConstEvalCounter
| StatementKind::Nop => {}
// If the discriminant is set, it is always set
// as a constant, so the job is already done.
// As we are **ignoring projections**, if the place
// we are tracking sees its discriminant be set,
// that means we had to be tracking the discriminant
// specifically (as it is impossible to switch over
// an enum directly, and if we were switching over
// its content, we would have had to at least cast it to
// some variant first)
StatementKind::SetDiscriminant { place, .. } => {
if **place == switch_place {
return None;
}
}
}
}
Some(switch_place)
}