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Skip Drop terminators for enum variants without drop glue #68943

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Mar 1, 2020
Merged

Skip Drop terminators for enum variants without drop glue #68943

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45 changes: 30 additions & 15 deletions src/librustc_mir/util/elaborate_drops.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -153,9 +153,9 @@ where
// FIXME: I think we should just control the flags externally,
// and then we do not need this machinery.
pub fn elaborate_drop(&mut self, bb: BasicBlock) {
debug!("elaborate_drop({:?})", self);
debug!("elaborate_drop({:?}, {:?})", bb, self);
let style = self.elaborator.drop_style(self.path, DropFlagMode::Deep);
debug!("elaborate_drop({:?}): live - {:?}", self, style);
debug!("elaborate_drop({:?}, {:?}): live - {:?}", bb, self, style);
match style {
DropStyle::Dead => {
self.elaborator
Expand Down Expand Up @@ -426,25 +426,21 @@ where
let mut unwind_blocks =
if unwind.is_cleanup() { None } else { Some(Vec::with_capacity(adt.variants.len())) };

let mut have_otherwise_with_drop_glue = false;
let mut have_otherwise = false;
let tcx = self.tcx();

for (variant_index, discr) in adt.discriminants(tcx) {
let variant = &adt.variants[variant_index];
let subpath = self.elaborator.downcast_subpath(self.path, variant_index);

if let Some(variant_path) = subpath {
let base_place = tcx.mk_place_elem(
self.place.clone(),
ProjectionElem::Downcast(
Some(adt.variants[variant_index].ident.name),
variant_index,
),
);
let fields = self.move_paths_for_fields(
&base_place,
variant_path,
&adt.variants[variant_index],
substs,
ProjectionElem::Downcast(Some(variant.ident.name), variant_index),
);
let fields =
self.move_paths_for_fields(&base_place, variant_path, &variant, substs);
values.push(discr.val);
if let Unwind::To(unwind) = unwind {
// We can't use the half-ladder from the original
Expand Down Expand Up @@ -474,16 +470,30 @@ where
normal_blocks.push(normal);
} else {
have_otherwise = true;

let param_env = self.elaborator.param_env();
let have_field_with_drop_glue = variant
.fields
.iter()
.any(|field| field.ty(tcx, substs).needs_drop(tcx, param_env));
if have_field_with_drop_glue {
have_otherwise_with_drop_glue = true;
}
}
}

if have_otherwise {
if !have_otherwise {
values.pop();
} else if !have_otherwise_with_drop_glue {
normal_blocks.push(self.goto_block(succ, unwind));
if let Unwind::To(unwind) = unwind {
unwind_blocks.as_mut().unwrap().push(self.goto_block(unwind, Unwind::InCleanup));
}
} else {
normal_blocks.push(self.drop_block(succ, unwind));
if let Unwind::To(unwind) = unwind {
unwind_blocks.as_mut().unwrap().push(self.drop_block(unwind, Unwind::InCleanup));
}
} else {
values.pop();
}

(
Expand Down Expand Up @@ -929,6 +939,11 @@ where
self.new_block(unwind, block)
}

fn goto_block(&mut self, target: BasicBlock, unwind: Unwind) -> BasicBlock {
let block = TerminatorKind::Goto { target };
self.new_block(unwind, block)
}

fn drop_flag_test_block(
&mut self,
on_set: BasicBlock,
Expand Down