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Auto merge of #77597 - simonvandel:uninhabited-hashset, r=jonas-schievink

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perf: UninhabitedEnumBranching avoid n^2

Avoid n² complexity. This showed up in a profile for match-stress-enum that has 8192 variants

I have only profiled locally against `match-stress-enum`, so we should have it perf tested to make sure it does not regress other crates.
This commit is contained in:
bors 2020-10-07 23:44:57 +00:00
commit e055f87cdf
1 changed files with 7 additions and 2 deletions
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9
compiler/rustc_mir/src/transform/uninhabited_enum_branching.rs
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@ -1,6 +1,7 @@
//! A pass that eliminates branches on uninhabited enum variants. //! A pass that eliminates branches on uninhabited enum variants.
use crate::transform::MirPass; use crate::transform::MirPass;
use rustc_data_structures::stable_set::FxHashSet;
use rustc_middle::mir::{ use rustc_middle::mir::{
BasicBlock, BasicBlockData, Body, Local, Operand, Rvalue, StatementKind, TerminatorKind, BasicBlock, BasicBlockData, Body, Local, Operand, Rvalue, StatementKind, TerminatorKind,
}; };
@ -52,9 +53,13 @@ fn variant_discriminants<'tcx>(
layout: &TyAndLayout<'tcx>, layout: &TyAndLayout<'tcx>,
ty: Ty<'tcx>, ty: Ty<'tcx>,
tcx: TyCtxt<'tcx>, tcx: TyCtxt<'tcx>,
) -> Vec<u128> { ) -> FxHashSet<u128> {
match &layout.variants { match &layout.variants {
Variants::Single { index } => vec![index.as_u32() as u128], Variants::Single { index } => {
let mut res = FxHashSet::default();
res.insert(index.as_u32() as u128);
res
}
Variants::Multiple { variants, .. } => variants Variants::Multiple { variants, .. } => variants
.iter_enumerated() .iter_enumerated()
.filter_map(|(idx, layout)| { .filter_map(|(idx, layout)| {