bjoernager/rust
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Auto merge of #93387 - JakobDegen:improve_partialeq, r=tmiasko

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Extend uninhabited enum variant branch elimination to also affect fallthrough

The `uninhabited_enum_branching` mir opt eliminates branches on variants where the data is uninhabited. This change extends this pass to also ensure that the `otherwise` case points to a trivially unreachable bb if all inhabited variants are present in the non-otherwise branches.

I believe it was `@scottmcm` who said that LLVM eliminates some of this information in its SimplifyCFG pass. This is unfortunate, but this change should still be at least a small improvement in principle (I don't think it will show up on any benchmarks)
This commit is contained in:
bors 2022-02-20 05:24:52 +00:00
commit a6fe969541
7 changed files with 167 additions and 20 deletions
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42
compiler/rustc_mir_transform/src/uninhabited_enum_branching.rs
View file

@ -3,7 +3,8 @@
use crate::MirPass;
use rustc_data_structures::stable_set::FxHashSet;
use rustc_middle::mir::{
BasicBlockData, Body, Local, Operand, Rvalue, StatementKind, SwitchTargets, TerminatorKind,
BasicBlockData, Body, Local, Operand, Rvalue, StatementKind, SwitchTargets, Terminator,
TerminatorKind,
};
use rustc_middle::ty::layout::TyAndLayout;
use rustc_middle::ty::{Ty, TyCtxt};
@ -71,6 +72,28 @@ fn variant_discriminants<'tcx>(
}
}
/// Ensures that the `otherwise` branch leads to an unreachable bb, returning `None` if so and a new
/// bb to use as the new target if not.
fn ensure_otherwise_unreachable<'tcx>(
body: &Body<'tcx>,
targets: &SwitchTargets,
) -> Option<BasicBlockData<'tcx>> {
let otherwise = targets.otherwise();
let bb = &body.basic_blocks()[otherwise];
if bb.terminator().kind == TerminatorKind::Unreachable
&& bb.statements.iter().all(|s| matches!(&s.kind, StatementKind::StorageDead(_)))
{
return None;
}
let mut new_block = BasicBlockData::new(Some(Terminator {
source_info: bb.terminator().source_info,
kind: TerminatorKind::Unreachable,
}));
new_block.is_cleanup = bb.is_cleanup;
Some(new_block)
}
impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
sess.mir_opt_level() > 0
@ -99,12 +122,25 @@ impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
if let TerminatorKind::SwitchInt { targets, .. } =
&mut body.basic_blocks_mut()[bb].terminator_mut().kind
{
let new_targets = SwitchTargets::new(
let mut new_targets = SwitchTargets::new(
targets.iter().filter(|(val, _)| allowed_variants.contains(val)),
targets.otherwise(),
);
*targets = new_targets;
if new_targets.iter().count() == allowed_variants.len() {
if let Some(updated) = ensure_otherwise_unreachable(body, &new_targets) {
let new_otherwise = body.basic_blocks_mut().push(updated);
*new_targets.all_targets_mut().last_mut().unwrap() = new_otherwise;
}
}
if let TerminatorKind::SwitchInt { targets, .. } =
&mut body.basic_blocks_mut()[bb].terminator_mut().kind
{
*targets = new_targets;
} else {
unreachable!()
}
} else {
unreachable!()
}