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…=jonas-schievink

Replace `discriminant_switch_effect` with more general version

rust-lang#68528 added a new edge-specific effect for `SwitchInt` terminators, `discriminant_switch_effect`, to the dataflow framework. While this accomplished the short-term goal of making drop elaboration more precise, it wasn't really useful in other contexts: It only supported `SwitchInt`s on the discriminant of an `enum` and did not allow effects to be applied along the "otherwise" branch. In const-propagation, for example, arbitrary edge-specific effects for the targets of a `SwitchInt` can be used to remember the value a `match` scrutinee must have in each arm.

This PR replaces `discriminant_switch_effect` with a more general `switch_int_edge_effects` method. The new method has a slightly different interface from the other edge-specific effect methods (e.g. `call_return_effect`). This divergence is explained in the new method's documentation, and reading the changes to the various dataflow impls as well as `direction.rs` should further clarify things. This PR should not change behavior.
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bors committed Sep 27, 2020
2 parents 7f7a1cb + c0cd1b0 commit c0b15cc
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146 changes: 68 additions & 78 deletions compiler/rustc_mir/src/dataflow/framework/direction.rs
View file
Original file line number Diff line number Diff line change
@@ -1,10 +1,10 @@
use rustc_index::bit_set::BitSet;
use rustc_middle::mir::{self, BasicBlock, Location};
use rustc_middle::ty::{self, TyCtxt};
use rustc_middle::ty::TyCtxt;
use std::ops::RangeInclusive;

use super::visitor::{ResultsVisitable, ResultsVisitor};
use super::{Analysis, Effect, EffectIndex, GenKillAnalysis, GenKillSet};
use super::{Analysis, Effect, EffectIndex, GenKillAnalysis, GenKillSet, SwitchIntTarget};

pub trait Direction {
fn is_forward() -> bool;
Expand Down Expand Up @@ -425,8 +425,8 @@ impl Direction for Forward {

fn join_state_into_successors_of<A>(
analysis: &A,
tcx: TyCtxt<'tcx>,
body: &mir::Body<'tcx>,
_tcx: TyCtxt<'tcx>,
_body: &mir::Body<'tcx>,
dead_unwinds: Option<&BitSet<BasicBlock>>,
exit_state: &mut A::Domain,
(bb, bb_data): (BasicBlock, &'_ mir::BasicBlockData<'tcx>),
Expand Down Expand Up @@ -489,88 +489,78 @@ impl Direction for Forward {
}

SwitchInt { ref targets, ref values, ref discr, switch_ty: _ } => {
let enum_ = discr
.place()
.and_then(|discr| switch_on_enum_discriminant(tcx, &body, bb_data, discr));
match enum_ {
// If this is a switch on an enum discriminant, a custom effect may be applied
// along each outgoing edge.
Some((enum_place, enum_def)) => {
// MIR building adds discriminants to the `values` array in the same order as they
// are yielded by `AdtDef::discriminants`. We rely on this to match each
// discriminant in `values` to its corresponding variant in linear time.
let mut tmp = analysis.bottom_value(body);
let mut discriminants = enum_def.discriminants(tcx);
for (value, target) in values.iter().zip(targets.iter().copied()) {
let (variant_idx, _) =
discriminants.find(|&(_, discr)| discr.val == *value).expect(
"Order of `AdtDef::discriminants` differed \
from that of `SwitchInt::values`",
);

tmp.clone_from(exit_state);
analysis.apply_discriminant_switch_effect(
&mut tmp,
bb,
enum_place,
enum_def,
variant_idx,
);
propagate(target, &tmp);
}

// Move out of `tmp` so we don't accidentally use it below.
std::mem::drop(tmp);

// Propagate dataflow state along the "otherwise" edge.
let otherwise = targets.last().copied().unwrap();
propagate(otherwise, exit_state)
}

// Otherwise, it's just a normal `SwitchInt`, and every successor sees the same
// exit state.
None => {
for target in targets.iter().copied() {
propagate(target, exit_state);
}
let mut applier = SwitchIntEdgeEffectApplier {
exit_state,
targets: targets.as_ref(),
values: values.as_ref(),
propagate,
effects_applied: false,
};

analysis.apply_switch_int_edge_effects(bb, discr, &mut applier);

let SwitchIntEdgeEffectApplier {
exit_state, mut propagate, effects_applied, ..
} = applier;

if !effects_applied {
for &target in targets.iter() {
propagate(target, exit_state);
}
}
}
}
}
}

/// Inspect a `SwitchInt`-terminated basic block to see if the condition of that `SwitchInt` is
/// an enum discriminant.
///
/// We expect such blocks to have a call to `discriminant` as their last statement like so:
/// _42 = discriminant(_1)
/// SwitchInt(_42, ..)
///
/// If the basic block matches this pattern, this function returns the place corresponding to the
/// enum (`_1` in the example above) as well as the `AdtDef` of that enum.
fn switch_on_enum_discriminant(
tcx: TyCtxt<'tcx>,
body: &'mir mir::Body<'tcx>,
block: &'mir mir::BasicBlockData<'tcx>,
switch_on: mir::Place<'tcx>,
) -> Option<(mir::Place<'tcx>, &'tcx ty::AdtDef)> {
match block.statements.last().map(|stmt| &stmt.kind) {
Some(mir::StatementKind::Assign(box (lhs, mir::Rvalue::Discriminant(discriminated))))
if *lhs == switch_on =>
{
match &discriminated.ty(body, tcx).ty.kind() {
ty::Adt(def, _) => Some((*discriminated, def)),

// `Rvalue::Discriminant` is also used to get the active yield point for a
// generator, but we do not need edge-specific effects in that case. This may
// change in the future.
ty::Generator(..) => None,

t => bug!("`discriminant` called on unexpected type {:?}", t),
}
struct SwitchIntEdgeEffectApplier<'a, D, F> {
exit_state: &'a mut D,
values: &'a [u128],
targets: &'a [BasicBlock],
propagate: F,

effects_applied: bool,
}

impl<D, F> super::SwitchIntEdgeEffects<D> for SwitchIntEdgeEffectApplier<'_, D, F>
where
D: Clone,
F: FnMut(BasicBlock, &D),
{
fn apply(&mut self, mut apply_edge_effect: impl FnMut(&mut D, SwitchIntTarget)) {
assert!(!self.effects_applied);

let mut tmp = None;
for (&value, &target) in self.values.iter().zip(self.targets.iter()) {
let tmp = opt_clone_from_or_clone(&mut tmp, self.exit_state);
apply_edge_effect(tmp, SwitchIntTarget { value: Some(value), target });
(self.propagate)(target, tmp);
}

_ => None,
// Once we get to the final, "otherwise" branch, there is no need to preserve `exit_state`,
// so pass it directly to `apply_edge_effect` to save a clone of the dataflow state.
let otherwise = self.targets.last().copied().unwrap();
apply_edge_effect(self.exit_state, SwitchIntTarget { value: None, target: otherwise });
(self.propagate)(otherwise, self.exit_state);

self.effects_applied = true;
}
}

/// An analogue of `Option::get_or_insert_with` that stores a clone of `val` into `opt`, but uses
/// the more efficient `clone_from` if `opt` was `Some`.
///
/// Returns a mutable reference to the new clone that resides in `opt`.
//
// FIXME: Figure out how to express this using `Option::clone_from`, or maybe lift it into the
// standard library?
fn opt_clone_from_or_clone<T: Clone>(opt: &'a mut Option<T>, val: &T) -> &'a mut T {
if opt.is_some() {
let ret = opt.as_mut().unwrap();
ret.clone_from(val);
ret
} else {
*opt = Some(val.clone());
opt.as_mut().unwrap()
}
}
65 changes: 44 additions & 21 deletions compiler/rustc_mir/src/dataflow/framework/mod.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -37,8 +37,7 @@ use rustc_hir::def_id::DefId;
use rustc_index::bit_set::{BitSet, HybridBitSet};
use rustc_index::vec::Idx;
use rustc_middle::mir::{self, BasicBlock, Location};
use rustc_middle::ty::{self, TyCtxt};
use rustc_target::abi::VariantIdx;
use rustc_middle::ty::TyCtxt;

mod cursor;
mod direction;
Expand Down Expand Up @@ -152,6 +151,8 @@ pub trait Analysis<'tcx>: AnalysisDomain<'tcx> {
) {
}

/* Edge-specific effects */

/// Updates the current dataflow state with the effect of a successful return from a `Call`
/// terminator.
///
Expand Down Expand Up @@ -183,20 +184,28 @@ pub trait Analysis<'tcx>: AnalysisDomain<'tcx> {
/// Updates the current dataflow state with the effect of taking a particular branch in a
/// `SwitchInt` terminator.
///
/// Much like `apply_call_return_effect`, this effect is only propagated along a single
/// outgoing edge from this basic block.
/// Unlike the other edge-specific effects, which are allowed to mutate `Self::Domain`
/// directly, overriders of this method must pass a callback to
/// `SwitchIntEdgeEffects::apply`. The callback will be run once for each outgoing edge and
/// will have access to the dataflow state that will be propagated along that edge.
///
/// This interface is somewhat more complex than the other visitor-like "effect" methods.
/// However, it is both more ergonomic—callers don't need to recompute or cache information
/// about a given `SwitchInt` terminator for each one of its edges—and more efficient—the
/// engine doesn't need to clone the exit state for a block unless
/// `SwitchIntEdgeEffects::apply` is actually called.
///
/// FIXME: This class of effects is not supported for backward dataflow analyses.
fn apply_discriminant_switch_effect(
fn apply_switch_int_edge_effects(
&self,
_state: &mut Self::Domain,
_block: BasicBlock,
_enum_place: mir::Place<'tcx>,
_adt: &ty::AdtDef,
_variant: VariantIdx,
_discr: &mir::Operand<'tcx>,
_apply_edge_effects: &mut impl SwitchIntEdgeEffects<Self::Domain>,
) {
}

/* Extension methods */

/// Creates an `Engine` to find the fixpoint for this dataflow problem.
///
/// You shouldn't need to override this outside this module, since the combination of the
Expand Down Expand Up @@ -267,6 +276,8 @@ pub trait GenKillAnalysis<'tcx>: Analysis<'tcx> {
) {
}

/* Edge-specific effects */

/// See `Analysis::apply_call_return_effect`.
fn call_return_effect(
&self,
Expand All @@ -286,14 +297,12 @@ pub trait GenKillAnalysis<'tcx>: Analysis<'tcx> {
) {
}

/// See `Analysis::apply_discriminant_switch_effect`.
fn discriminant_switch_effect(
/// See `Analysis::apply_switch_int_edge_effects`.
fn switch_int_edge_effects<G: GenKill<Self::Idx>>(
&self,
_state: &mut impl GenKill<Self::Idx>,
_block: BasicBlock,
_enum_place: mir::Place<'tcx>,
_adt: &ty::AdtDef,
_variant: VariantIdx,
_discr: &mir::Operand<'tcx>,
_edge_effects: &mut impl SwitchIntEdgeEffects<G>,
) {
}
}
Expand Down Expand Up @@ -339,6 +348,8 @@ where
self.before_terminator_effect(state, terminator, location);
}

/* Edge-specific effects */

fn apply_call_return_effect(
&self,
state: &mut A::Domain,
Expand All @@ -359,17 +370,17 @@ where
self.yield_resume_effect(state, resume_block, resume_place);
}

fn apply_discriminant_switch_effect(
fn apply_switch_int_edge_effects(
&self,
state: &mut A::Domain,
block: BasicBlock,
enum_place: mir::Place<'tcx>,
adt: &ty::AdtDef,
variant: VariantIdx,
discr: &mir::Operand<'tcx>,
edge_effects: &mut impl SwitchIntEdgeEffects<A::Domain>,
) {
self.discriminant_switch_effect(state, block, enum_place, adt, variant);
self.switch_int_edge_effects(block, discr, edge_effects);
}

/* Extension methods */

fn into_engine(
self,
tcx: TyCtxt<'tcx>,
Expand Down Expand Up @@ -531,5 +542,17 @@ impl EffectIndex {
}
}

pub struct SwitchIntTarget {
pub value: Option<u128>,
pub target: BasicBlock,
}

/// A type that records the edge-specific effects for a `SwitchInt` terminator.
pub trait SwitchIntEdgeEffects<D> {
/// Calls `apply_edge_effect` for each outgoing edge from a `SwitchInt` terminator and
/// records the results.
fn apply(&mut self, apply_edge_effect: impl FnMut(&mut D, SwitchIntTarget));
}

#[cfg(test)]
mod tests;
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