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Less cloning in early otherwise branch #77472

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e7811f9
Do not clone vec and bail early
simonvandel Oct 2, 2020
7a8c277
less clone
simonvandel Oct 2, 2020
62b6896
Remove duplicated check
simonvandel Oct 2, 2020
4784a5c
reorder checks from (intuitively) cheapest to most expensive
simonvandel Oct 2, 2020
586ba9d
Restructure data such that data about the first switch is not duplicated
simonvandel Oct 2, 2020
b4e144e
Move helper creation out of loop
simonvandel Oct 3, 2020
bd56b11
Use SwitchTargets instead of storing in Vec
simonvandel Oct 14, 2020
1a8d28a
Use otherwise from switchTargets
simonvandel Oct 14, 2020
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115 changes: 49 additions & 66 deletions compiler/rustc_mir/src/transform/early_otherwise_branch.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -34,12 +34,12 @@ impl<'tcx> MirPass<'tcx> for EarlyOtherwiseBranch {
let bbs_with_switch =
body.basic_blocks().iter_enumerated().filter(|(_, bb)| is_switch(bb.terminator()));

let helper = Helper { body, tcx };
let opts_to_apply: Vec<OptimizationToApply<'tcx>> = bbs_with_switch
.flat_map(|(bb_idx, bb)| {
let switch = bb.terminator();
let helper = Helper { body, tcx };
let infos = helper.go(bb, switch)?;
Some(OptimizationToApply { infos, basic_block_first_switch: bb_idx })
let info = helper.go(bb, switch)?;
Some(OptimizationToApply { info, basic_block_first_switch: bb_idx })
})
.collect();

Expand All @@ -48,6 +48,8 @@ impl<'tcx> MirPass<'tcx> for EarlyOtherwiseBranch {
for opt_to_apply in opts_to_apply {
trace!("SUCCESS: found optimization possibility to apply: {:?}", &opt_to_apply);

let first_switch_info = opt_to_apply.info.first_switch_info;
let second_switch_info = &opt_to_apply.info.second_switch_infos[0];
let statements_before =
body.basic_blocks()[opt_to_apply.basic_block_first_switch].statements.len();
let end_of_block_location = Location {
Expand All @@ -58,8 +60,8 @@ impl<'tcx> MirPass<'tcx> for EarlyOtherwiseBranch {
let mut patch = MirPatch::new(body);

// create temp to store second discriminant in
let discr_type = opt_to_apply.infos[0].second_switch_info.discr_ty;
let discr_span = opt_to_apply.infos[0].second_switch_info.discr_source_info.span;
let discr_type = second_switch_info.discr_ty;
let discr_span = second_switch_info.discr_source_info.span;
let second_discriminant_temp = patch.new_temp(discr_type, discr_span);

patch.add_statement(
Expand All @@ -68,8 +70,7 @@ impl<'tcx> MirPass<'tcx> for EarlyOtherwiseBranch {
);

// create assignment of discriminant
let place_of_adt_to_get_discriminant_of =
opt_to_apply.infos[0].second_switch_info.place_of_adt_discr_read;
let place_of_adt_to_get_discriminant_of = second_switch_info.place_of_adt_discr_read;
patch.add_assign(
end_of_block_location,
Place::from(second_discriminant_temp),
Expand All @@ -83,8 +84,7 @@ impl<'tcx> MirPass<'tcx> for EarlyOtherwiseBranch {
patch.add_statement(end_of_block_location, StatementKind::StorageLive(not_equal_temp));

// create NotEqual comparison between the two discriminants
let first_descriminant_place =
opt_to_apply.infos[0].first_switch_info.discr_used_in_switch;
let first_descriminant_place = first_switch_info.discr_used_in_switch;
let not_equal_rvalue = Rvalue::BinaryOp(
not_equal,
Operand::Copy(Place::from(second_discriminant_temp)),
Expand All @@ -96,19 +96,17 @@ impl<'tcx> MirPass<'tcx> for EarlyOtherwiseBranch {
);

let new_targets = opt_to_apply
.infos
.info
.second_switch_infos
.iter()
.flat_map(|x| x.second_switch_info.targets_with_values.iter())
.cloned();
.flat_map(|x| x.switch_targets.iter().map(|x| (x.0, x.1)));

let targets = SwitchTargets::new(
new_targets,
opt_to_apply.infos[0].first_switch_info.otherwise_bb,
);
let targets =
SwitchTargets::new(new_targets, first_switch_info.switch_targets.otherwise());

// new block that jumps to the correct discriminant case. This block is switched to if the discriminants are equal
let new_switch_data = BasicBlockData::new(Some(Terminator {
source_info: opt_to_apply.infos[0].second_switch_info.discr_source_info,
source_info: second_switch_info.discr_source_info,
kind: TerminatorKind::SwitchInt {
// the first and second discriminants are equal, so just pick one
discr: Operand::Copy(first_descriminant_place),
Expand All @@ -121,7 +119,7 @@ impl<'tcx> MirPass<'tcx> for EarlyOtherwiseBranch {

// switch on the NotEqual. If true, then jump to the `otherwise` case.
// If false, then jump to a basic block that then jumps to the correct disciminant case
let true_case = opt_to_apply.infos[0].first_switch_info.otherwise_bb;
let true_case = first_switch_info.switch_targets.otherwise();
let false_case = new_switch_bb;
patch.patch_terminator(
opt_to_apply.basic_block_first_switch,
Expand Down Expand Up @@ -174,10 +172,8 @@ struct Helper<'a, 'tcx> {
struct SwitchDiscriminantInfo<'tcx> {
/// Type of the discriminant being switched on
discr_ty: Ty<'tcx>,
/// The basic block that the otherwise branch points to
otherwise_bb: BasicBlock,
/// Target along with the value being branched from. Otherwise is not included
targets_with_values: Vec<(u128, BasicBlock)>,
/// Targets and values for the switch
switch_targets: SwitchTargets,
discr_source_info: SourceInfo,
/// The place of the discriminant used in the switch
discr_used_in_switch: Place<'tcx>,
Expand All @@ -189,7 +185,7 @@ struct SwitchDiscriminantInfo<'tcx> {

#[derive(Debug)]
struct OptimizationToApply<'tcx> {
infos: Vec<OptimizationInfo<'tcx>>,
info: OptimizationInfo<'tcx>,
/// Basic block of the original first switch
basic_block_first_switch: BasicBlock,
}
Expand All @@ -198,68 +194,51 @@ struct OptimizationToApply<'tcx> {
struct OptimizationInfo<'tcx> {
/// Info about the first switch and discriminant
first_switch_info: SwitchDiscriminantInfo<'tcx>,
/// Info about the second switch and discriminant
second_switch_info: SwitchDiscriminantInfo<'tcx>,
/// Info about all swtiches that are successors of the first switch
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Suggested change
/// Info about all swtiches that are successors of the first switch
/// Info about all switches that are successors of the first switch

second_switch_infos: Vec<SwitchDiscriminantInfo<'tcx>>,
}

impl<'a, 'tcx> Helper<'a, 'tcx> {
pub fn go(
&self,
bb: &BasicBlockData<'tcx>,
switch: &Terminator<'tcx>,
) -> Option<Vec<OptimizationInfo<'tcx>>> {
) -> Option<OptimizationInfo<'tcx>> {
// try to find the statement that defines the discriminant that is used for the switch
let discr = self.find_switch_discriminant_info(bb, switch)?;

// go through each target, finding a discriminant read, and a switch
let results = discr.targets_with_values.iter().map(|(value, target)| {
self.find_discriminant_switch_pairing(&discr, target.clone(), value.clone())
});

// if the optimization did not apply for one of the targets, then abort
if results.clone().any(|x| x.is_none()) || results.len() == 0 {
trace!("NO: not all of the targets matched the pattern for optimization");
return None;
let mut second_switch_infos = vec![];

for (value, target) in discr.switch_targets.iter() {
let info = self.find_discriminant_switch_pairing(&discr, target, value)?;
second_switch_infos.push(info);
}

Some(results.flatten().collect())
Some(OptimizationInfo { first_switch_info: discr, second_switch_infos })
}

fn find_discriminant_switch_pairing(
&self,
discr_info: &SwitchDiscriminantInfo<'tcx>,
target: BasicBlock,
value: u128,
) -> Option<OptimizationInfo<'tcx>> {
) -> Option<SwitchDiscriminantInfo<'tcx>> {
let bb = &self.body.basic_blocks()[target];
// find switch
let terminator = bb.terminator();
if is_switch(terminator) {
let this_bb_discr_info = self.find_switch_discriminant_info(bb, terminator)?;

// the types of the two adts matched on have to be equalfor this optimization to apply
if discr_info.type_adt_matched_on != this_bb_discr_info.type_adt_matched_on {
trace!(
"NO: types do not match. LHS: {:?}, RHS: {:?}",
discr_info.type_adt_matched_on,
this_bb_discr_info.type_adt_matched_on
);
return None;
}

// the otherwise branch of the two switches have to point to the same bb
if discr_info.otherwise_bb != this_bb_discr_info.otherwise_bb {
// The otherwise branch of the two switches have to point to the same bb
if discr_info.switch_targets.otherwise()
!= this_bb_discr_info.switch_targets.otherwise()
{
trace!("NO: otherwise target is not the same");
return None;
}

// check that the value being matched on is the same. The
if this_bb_discr_info.targets_with_values.iter().find(|x| x.0 == value).is_none() {
Comment on lines -256 to -257
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Where is this check duplicated? The check above does not look at targets_with_values at all.

trace!("NO: values being matched on are not the same");
return None;
}

// only allow optimization if the left and right of the tuple being matched are the same variants.
// Only allow optimization if the left and right of the tuple being matched are the same variants.
// so the following should not optimize
// ```rust
// let x: Option<()>;
Expand All @@ -270,22 +249,29 @@ impl<'a, 'tcx> Helper<'a, 'tcx> {
// }
// ```
// We check this by seeing that the value of the first discriminant is the only other discriminant value being used as a target in the second switch
if !(this_bb_discr_info.targets_with_values.len() == 1
&& this_bb_discr_info.targets_with_values[0].0 == value)
if !(this_bb_discr_info.switch_targets.iter().len() == 1
&& this_bb_discr_info.switch_targets.iter().next()?.0 == value)
{
trace!(
"NO: The second switch did not have only 1 target (besides otherwise) that had the same value as the value from the first switch that got us here"
);
return None;
}

// The types of the two adts matched on have to be equal for this optimization to apply
if discr_info.type_adt_matched_on != this_bb_discr_info.type_adt_matched_on {
trace!(
"NO: types do not match. LHS: {:?}, RHS: {:?}",
discr_info.type_adt_matched_on,
this_bb_discr_info.type_adt_matched_on
);
return None;
}
Comment on lines +261 to +269
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There doesn't seem to be much point to this change without benchmarks. Personally I would expect a simple != to be faster than two conditionals.


// if we reach this point, the optimization applies, and we should be able to optimize this case
// store the info that is needed to apply the optimization

Some(OptimizationInfo {
first_switch_info: discr_info.clone(),
second_switch_info: this_bb_discr_info,
})
Some(this_bb_discr_info)
} else {
None
}
Expand All @@ -302,9 +288,7 @@ impl<'a, 'tcx> Helper<'a, 'tcx> {
// the declaration of the discriminant read. Place of this read is being used in the switch
let discr_decl = &self.body.local_decls()[discr_local];
let discr_ty = discr_decl.ty;
// the otherwise target lies as the last element
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I don't think you meant to remove this comment.

let otherwise_bb = targets.otherwise();
let targets_with_values = targets.iter().collect();
let targets_with_values = targets.clone();

// find the place of the adt where the discriminant is being read from
// assume this is the last statement of the block
Expand All @@ -320,8 +304,7 @@ impl<'a, 'tcx> Helper<'a, 'tcx> {
Some(SwitchDiscriminantInfo {
discr_used_in_switch: discr.place()?,
discr_ty,
otherwise_bb,
targets_with_values,
switch_targets: targets_with_values,
discr_source_info: discr_decl.source_info,
place_of_adt_discr_read,
type_adt_matched_on,
Expand Down