bjoernager/rust
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avoid calling for types which can't be ZSTs

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This commit is contained in:
Erik Desjardins 2021-03-19 22:25:56 -04:00
parent 93c1380e0b
commit 46fd49cdc3
1 changed files with 30 additions and 12 deletions
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42
compiler/rustc_mir/src/transform/remove_zsts.rs
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@ -2,7 +2,7 @@
use crate::transform::MirPass;
use rustc_middle::mir::{Body, StatementKind};
use rustc_middle::ty::TyCtxt;
use rustc_middle::ty::{self, Ty, TyCtxt};
pub struct RemoveZsts;
@ -15,17 +15,23 @@ impl<'tcx> MirPass<'tcx> for RemoveZsts {
match statement.kind {
StatementKind::Assign(box (place, _)) => {
let place_ty = place.ty(local_decls, tcx).ty;
if let Ok(layout) = tcx.layout_of(param_env.and(place_ty)) {
if layout.is_zst() {
if tcx.consider_optimizing(|| {
format!(
"RemoveZsts - Place: {:?} SourceInfo: {:?}",
place, statement.source_info
)
}) {
statement.make_nop();
}
}
if !maybe_zst(place_ty) {
continue;
}
let layout = match tcx.layout_of(param_env.and(place_ty)) {
Ok(layout) => layout,
Err(_) => continue,
};
if !layout.is_zst() {
continue;
}
if tcx.consider_optimizing(|| {
format!(
"RemoveZsts - Place: {:?} SourceInfo: {:?}",
place, statement.source_info
)
}) {
statement.make_nop();
}
}
_ => {}
@ -34,3 +40,15 @@ impl<'tcx> MirPass<'tcx> for RemoveZsts {
}
}
}
/// A cheap, approximate check to avoid unnecessary `layout_of` calls.
fn maybe_zst(ty: Ty<'_>) -> bool {
match ty.kind() {
// maybe ZST (could be more precise)
ty::Adt(..) | ty::Array(..) | ty::Closure(..) | ty::Tuple(..) | ty::Opaque(..) => true,
// definitely ZST
ty::FnDef(..) | ty::Never => true,
// unreachable or can't be ZST
_ => false,
}
}