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[EXPERIMENTAL] Lower constant pattern subslices into valtrees #121540

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[EXPERIMENTAL] Lower constant pattern subslices into valtrees #121540

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1 change: 1 addition & 0 deletions compiler/rustc_middle/src/arena.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -115,6 +115,7 @@ macro_rules! arena_types {
[] features: rustc_feature::Features,
[decode] specialization_graph: rustc_middle::traits::specialization_graph::Graph,
[] crate_inherent_impls: rustc_middle::ty::CrateInherentImpls,
[] pats: rustc_middle::thir::Pat<'tcx>,
]);
)
}
Expand Down
158 changes: 141 additions & 17 deletions compiler/rustc_mir_build/src/build/matches/util.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -5,7 +5,8 @@ use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKi
use rustc_middle::mir::*;
use rustc_middle::thir::{self, *};
use rustc_middle::ty;
use rustc_middle::ty::TypeVisitableExt;
use rustc_middle::ty::{Ty, TypeVisitableExt};
use rustc_span::Span;

impl<'a, 'tcx> Builder<'a, 'tcx> {
pub(crate) fn field_match_pairs<'pat>(
Expand All @@ -25,6 +26,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {

pub(crate) fn prefix_slice_suffix<'pat>(
&mut self,
base_pat: &'pat Pat<'tcx>,
match_pairs: &mut Vec<MatchPair<'pat, 'tcx>>,
place: &PlaceBuilder<'tcx>,
prefix: &'pat [Box<Pat<'tcx>>],
Expand All @@ -41,11 +43,34 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
((prefix.len() + suffix.len()).try_into().unwrap(), false)
};

match_pairs.extend(prefix.iter().enumerate().map(|(idx, subpattern)| {
let elem =
ProjectionElem::ConstantIndex { offset: idx as u64, min_length, from_end: false };
MatchPair::new(place.clone_project(elem), subpattern, self)
}));
// Here we try to special-case constant pattern subslices into valtrees to generate
// nicer MIR.

// Try to perform simplification of a constant pattern slice `prefix` sequence into
// a valtree.
if self.is_constant_pattern_subslice(prefix) {
let elem_ty = prefix[0].ty;
let prefix_valtree = self.simplify_const_pattern_slice_into_valtree(prefix);
// FIXME(jieyouxu): triple check these place calculations!
let match_pair = self.valtree_to_match_pair(
base_pat.ty,
base_pat.span,
prefix.len() as u64,
prefix_valtree,
place.base().into(),
elem_ty,
);
match_pairs.push(match_pair);
} else {
match_pairs.extend(prefix.iter().enumerate().map(|(idx, subpattern)| {
let elem = ProjectionElem::ConstantIndex {
offset: idx as u64,
min_length,
from_end: false,
};
MatchPair::new(place.clone_project(elem), subpattern, self)
}));
}

if let Some(subslice_pat) = opt_slice {
let suffix_len = suffix.len() as u64;
Expand All @@ -57,16 +82,115 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
match_pairs.push(MatchPair::new(subslice, subslice_pat, self));
}

match_pairs.extend(suffix.iter().rev().enumerate().map(|(idx, subpattern)| {
let end_offset = (idx + 1) as u64;
let elem = ProjectionElem::ConstantIndex {
offset: if exact_size { min_length - end_offset } else { end_offset },
min_length,
// Try to perform simplification of a constant pattern slice `suffix` sequence into
// a valtree.
if self.is_constant_pattern_subslice(suffix) {
let elem_ty = suffix[0].ty;
let suffix_valtree = self.simplify_const_pattern_slice_into_valtree(suffix);
let suffix_len = suffix.len() as u64;
// FIXME(jieyouxu): triple check these place calculations.
let place = place.clone_project(PlaceElem::Subslice {
from: prefix.len() as u64,
to: if exact_size { min_length } else { suffix_len },
from_end: !exact_size,
};
let place = place.clone_project(elem);
MatchPair::new(place, subpattern, self)
}));
});
let match_pair = self.valtree_to_match_pair(
base_pat.ty,
base_pat.span,
suffix.len() as u64,
suffix_valtree,
place,
elem_ty,
);
match_pairs.push(match_pair);
} else {
match_pairs.extend(suffix.iter().rev().enumerate().map(|(idx, subpattern)| {
let end_offset = (idx + 1) as u64;
let elem = ProjectionElem::ConstantIndex {
offset: if exact_size { min_length - end_offset } else { end_offset },
min_length,
from_end: !exact_size,
};
let place = place.clone_project(elem);
MatchPair::new(place, subpattern, self)
}));
}
}

// We don't consider an empty subslice as a constant pattern subslice.
fn is_constant_pattern_subslice(&self, subslice: &[Box<Pat<'tcx>>]) -> bool {
!subslice.is_empty() && subslice.iter().all(|p| self.is_constant_pattern(p))
}

fn is_constant_pattern(&self, pat: &Pat<'tcx>) -> bool {
if let PatKind::Constant { value } = pat.kind
&& let Const::Ty(const_) = value
&& let ty::ConstKind::Value(valtree) = const_.kind()
&& matches!(valtree, ty::ValTree::Leaf(_))
{
true
} else {
false
}
}

// This must only be called after ensuring that the subslice consists of only constant
// patterns, or it will panic.
fn simplify_const_pattern_slice_into_valtree(
&self,
subslice: &[Box<Pat<'tcx>>],
) -> ty::ValTree<'tcx> {
let leaves = subslice.iter().map(|p| self.extract_leaf(p));
let interned = self.tcx.arena.alloc_from_iter(leaves);
ty::ValTree::Branch(interned)
}

// Must only be called by `try_simplify_const_pattern_slice_into_valtree`.
fn extract_leaf(&self, pat: &Pat<'tcx>) -> ty::ValTree<'tcx> {
if let PatKind::Constant { value } = pat.kind
&& let Const::Ty(const_) = value
&& let valtree = const_.to_valtree()
&& matches!(valtree, ty::ValTree::Leaf(_))
{
valtree
} else {
unreachable!()
}
}

fn valtree_to_match_pair<'pat>(
&mut self,
base_pat_ty: Ty<'tcx>,
span: Span,
subslice_len: u64,
valtree: ty::ValTree<'tcx>,
place: PlaceBuilder<'tcx>,
elem_ty: Ty<'tcx>,
) -> MatchPair<'pat, 'tcx> {
let tcx = self.tcx;
let (const_ty, pat_ty) = if base_pat_ty.is_slice() {
(
Ty::new_imm_ref(
tcx,
tcx.lifetimes.re_erased,
Ty::new_array(tcx, elem_ty, subslice_len),
),
Ty::new_imm_ref(tcx, tcx.lifetimes.re_erased, Ty::new_slice(tcx, elem_ty)),
)
} else {
let arr_ty = Ty::new_array(tcx, elem_ty, subslice_len);
(arr_ty, arr_ty)
};

let r#const = ty::Const::new(tcx, ty::ConstKind::Value(valtree), const_ty);

let replacement_pat = tcx.arena.alloc(Pat {
ty: pat_ty,
span,
kind: PatKind::Constant { value: Const::Ty(r#const) },
});

MatchPair::new(place, replacement_pat, self)
}

/// Creates a false edge to `imaginary_target` and a real edge to
Expand Down Expand Up @@ -205,11 +329,11 @@ impl<'pat, 'tcx> MatchPair<'pat, 'tcx> {
}

PatKind::Array { ref prefix, ref slice, ref suffix } => {
cx.prefix_slice_suffix(&mut subpairs, &place, prefix, slice, suffix);
cx.prefix_slice_suffix(pattern, &mut subpairs, &place, prefix, slice, suffix);
default_irrefutable()
}
PatKind::Slice { ref prefix, ref slice, ref suffix } => {
cx.prefix_slice_suffix(&mut subpairs, &place, prefix, slice, suffix);
cx.prefix_slice_suffix(pattern, &mut subpairs, &place, prefix, slice, suffix);

if prefix.is_empty() && slice.is_some() && suffix.is_empty() {
default_irrefutable()
Expand Down