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Rollup merge of #97077 - ouz-a:Optimize-backend, r=oli-obk

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Simplify some code that depend on Deref

Now that we can assume #97025 works, it's safe to expect Deref is always in the first place of projections. With this, I was able to simplify some code that depended on Deref's place in projections. When we are able to move Derefer before `ElaborateDrops` successfully we will be able to optimize more places.

r? `@oli-obk`
This commit is contained in:
Yuki Okushi 2022-07-26 07:14:44 +09:00 • committed by GitHub
commit 29892759f6
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3 changed files with 22 additions and 30 deletions
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26
compiler/rustc_mir_transform/src/add_retag.rs
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@ -15,22 +15,9 @@ pub struct AddRetag;
/// (Concurrent accesses by other threads are no problem as these are anyway non-atomic
/// copies. Data races are UB.)
fn is_stable(place: PlaceRef<'_>) -> bool {
place.projection.iter().all(|elem| {
match elem {
// Which place this evaluates to can change with any memory write,
// so cannot assume this to be stable.
ProjectionElem::Deref => false,
// Array indices are interesting, but MIR building generates a *fresh*
// temporary for every array access, so the index cannot be changed as
// a side-effect.
ProjectionElem::Index { .. } |
// The rest is completely boring, they just offset by a constant.
ProjectionElem::Field { .. } |
ProjectionElem::ConstantIndex { .. } |
ProjectionElem::Subslice { .. } |
ProjectionElem::Downcast { .. } => true,
}
})
// Which place this evaluates to can change with any memory write,
// so cannot assume deref to be stable.
!place.has_deref()
}
/// Determine whether this type may contain a reference (or box), and thus needs retagging.
@ -91,11 +78,8 @@ impl<'tcx> MirPass<'tcx> for AddRetag {
};
let place_base_raw = |place: &Place<'tcx>| {
// If this is a `Deref`, get the type of what we are deref'ing.
let deref_base =
place.projection.iter().rposition(|p| matches!(p, ProjectionElem::Deref));
if let Some(deref_base) = deref_base {
let base_proj = &place.projection[..deref_base];
let ty = Place::ty_from(place.local, base_proj, &*local_decls, tcx).ty;
if place.has_deref() {
let ty = &local_decls[place.local].ty;
ty.is_unsafe_ptr()
} else {
// Not a deref, and thus not raw.