diff options
3 files changed, 33 insertions, 12 deletions
diff --git a/compiler/rustc_mir_transform/src/remove_unneeded_drops.rs b/compiler/rustc_mir_transform/src/remove_unneeded_drops.rs index 43f80508e4a..6c2dfc59da2 100644 --- a/compiler/rustc_mir_transform/src/remove_unneeded_drops.rs +++ b/compiler/rustc_mir_transform/src/remove_unneeded_drops.rs @@ -4,7 +4,13 @@ //! useful because (unlike MIR building) it runs after type checking, so it can make use of //! `TypingMode::PostAnalysis` to provide more precise type information, especially about opaque //! types. +//! +//! When we're optimizing, we also remove calls to `drop_in_place<T>` when `T` isn't `needs_drop`, +//! as those are essentially equivalent to `Drop` terminators. While the compiler doesn't insert +//! them automatically, preferring the built-in instead, they're common in generic code (such as +//! `Vec::truncate`) so removing them from things like inlined `Vec<u8>` is helpful. +use rustc_hir::LangItem; use rustc_middle::mir::*; use rustc_middle::ty::TyCtxt; use tracing::{debug, trace}; @@ -21,15 +27,26 @@ impl<'tcx> crate::MirPass<'tcx> for RemoveUnneededDrops { let mut should_simplify = false; for block in body.basic_blocks.as_mut() { let terminator = block.terminator_mut(); - if let TerminatorKind::Drop { place, target, .. } = terminator.kind { - let ty = place.ty(&body.local_decls, tcx); - if ty.ty.needs_drop(tcx, typing_env) { - continue; + let (ty, target) = match terminator.kind { + TerminatorKind::Drop { place, target, .. } => { + (place.ty(&body.local_decls, tcx).ty, target) + } + TerminatorKind::Call { ref func, target: Some(target), .. } + if tcx.sess.mir_opt_level() > 0 + && let Some((def_id, generics)) = func.const_fn_def() + && tcx.is_lang_item(def_id, LangItem::DropInPlace) => + { + (generics.type_at(0), target) } - debug!("SUCCESS: replacing `drop` with goto({:?})", target); - terminator.kind = TerminatorKind::Goto { target }; - should_simplify = true; + _ => continue, + }; + + if ty.needs_drop(tcx, typing_env) { + continue; } + debug!("SUCCESS: replacing `drop` with goto({:?})", target); + terminator.kind = TerminatorKind::Goto { target }; + should_simplify = true; } // if we applied optimizations, we potentially have some cfg to cleanup to diff --git a/tests/mir-opt/remove_unneeded_drop_in_place.rs b/tests/mir-opt/remove_unneeded_drop_in_place.rs index 7fb7b1e78de..470c6499d16 100644 --- a/tests/mir-opt/remove_unneeded_drop_in_place.rs +++ b/tests/mir-opt/remove_unneeded_drop_in_place.rs @@ -1,13 +1,17 @@ //@ test-mir-pass: RemoveUnneededDrops //@ needs-unwind +//@ compile-flags: -Z mir-opt-level=1 // EMIT_MIR remove_unneeded_drop_in_place.slice_in_place.RemoveUnneededDrops.diff unsafe fn slice_in_place(ptr: *mut [char]) { + // CHECK-LABEL: fn slice_in_place(_1: *mut [char]) + // CHECK: bb0: { + // CHECK-NEXT: return; + // CHECK-NEXT: } std::ptr::drop_in_place(ptr) } fn main() { - // CHECK-LABEL: fn main( let mut a = ['o', 'k']; unsafe { slice_in_place(&raw mut a) }; } diff --git a/tests/mir-opt/remove_unneeded_drop_in_place.slice_in_place.RemoveUnneededDrops.diff b/tests/mir-opt/remove_unneeded_drop_in_place.slice_in_place.RemoveUnneededDrops.diff index 4068e7940bc..4d70e7151c3 100644 --- a/tests/mir-opt/remove_unneeded_drop_in_place.slice_in_place.RemoveUnneededDrops.diff +++ b/tests/mir-opt/remove_unneeded_drop_in_place.slice_in_place.RemoveUnneededDrops.diff @@ -9,10 +9,10 @@ bb0: { StorageLive(_2); _2 = copy _1; - _0 = drop_in_place::<[char]>(move _2) -> [return: bb1, unwind continue]; - } - - bb1: { +- _0 = drop_in_place::<[char]>(move _2) -> [return: bb1, unwind continue]; +- } +- +- bb1: { StorageDead(_2); return; } |