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|
//! A pass that eliminates branches on uninhabited enum variants.
use crate::MirPass;
use rustc_data_structures::fx::FxHashSet;
use rustc_middle::mir::{
BasicBlockData, Body, Local, Operand, Rvalue, StatementKind, Terminator, TerminatorKind,
};
use rustc_middle::ty::layout::TyAndLayout;
use rustc_middle::ty::{Ty, TyCtxt};
use rustc_target::abi::{Abi, Variants};
pub struct UninhabitedEnumBranching;
fn get_discriminant_local(terminator: &TerminatorKind<'_>) -> Option<Local> {
if let TerminatorKind::SwitchInt { discr: Operand::Move(p), .. } = terminator {
p.as_local()
} else {
None
}
}
/// If the basic block terminates by switching on a discriminant, this returns the `Ty` the
/// discriminant is read from. Otherwise, returns None.
fn get_switched_on_type<'tcx>(
block_data: &BasicBlockData<'tcx>,
tcx: TyCtxt<'tcx>,
body: &Body<'tcx>,
) -> Option<Ty<'tcx>> {
let terminator = block_data.terminator();
// Only bother checking blocks which terminate by switching on a local.
let local = get_discriminant_local(&terminator.kind)?;
let stmt_before_term = block_data.statements.last()?;
if let StatementKind::Assign(box (l, Rvalue::Discriminant(place))) = stmt_before_term.kind
&& l.as_local() == Some(local)
{
let ty = place.ty(body, tcx).ty;
if ty.is_enum() {
return Some(ty);
}
}
None
}
fn variant_discriminants<'tcx>(
layout: &TyAndLayout<'tcx>,
ty: Ty<'tcx>,
tcx: TyCtxt<'tcx>,
) -> FxHashSet<u128> {
match &layout.variants {
Variants::Single { index } => {
let mut res = FxHashSet::default();
res.insert(
ty.discriminant_for_variant(tcx, *index)
.map_or(index.as_u32() as u128, |discr| discr.val),
);
res
}
Variants::Multiple { variants, .. } => variants
.iter_enumerated()
.filter_map(|(idx, layout)| {
(layout.abi != Abi::Uninhabited)
.then(|| ty.discriminant_for_variant(tcx, idx).unwrap().val)
})
.collect(),
}
}
impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
sess.mir_opt_level() > 0
}
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
trace!("UninhabitedEnumBranching starting for {:?}", body.source);
let mut removable_switchs = Vec::new();
for (bb, bb_data) in body.basic_blocks.iter_enumerated() {
trace!("processing block {:?}", bb);
if bb_data.is_cleanup {
continue;
}
let Some(discriminant_ty) = get_switched_on_type(bb_data, tcx, body) else { continue };
let layout = tcx.layout_of(
tcx.param_env_reveal_all_normalized(body.source.def_id()).and(discriminant_ty),
);
let allowed_variants = if let Ok(layout) = layout {
variant_discriminants(&layout, discriminant_ty, tcx)
} else {
continue;
};
trace!("allowed_variants = {:?}", allowed_variants);
let terminator = bb_data.terminator();
let TerminatorKind::SwitchInt { targets, .. } = &terminator.kind else { bug!() };
let mut reachable_count = 0;
for (index, (val, _)) in targets.iter().enumerate() {
if allowed_variants.contains(&val) {
reachable_count += 1;
} else {
removable_switchs.push((bb, index));
}
}
if reachable_count == allowed_variants.len() {
removable_switchs.push((bb, targets.iter().count()));
}
}
if removable_switchs.is_empty() {
return;
}
let new_block = BasicBlockData::new(Some(Terminator {
source_info: body.basic_blocks[removable_switchs[0].0].terminator().source_info,
kind: TerminatorKind::Unreachable,
}));
let unreachable_block = body.basic_blocks.as_mut().push(new_block);
for (bb, index) in removable_switchs {
let bb = &mut body.basic_blocks.as_mut()[bb];
let terminator = bb.terminator_mut();
let TerminatorKind::SwitchInt { targets, .. } = &mut terminator.kind else { bug!() };
targets.all_targets_mut()[index] = unreachable_block;
}
}
}
|
