Rollup merge of rust-lang#155184 - scottmcm:intercept-array-drop-shim, r=WaffleLapkin

 Have arrays' `drop_glue` just unsize and call the slice version

It's silly to emit two loops (because of the drop ladder -- just one in panic=abort) for every array length that's dropped when we can just polymorphize to the slice version.

Built atop rust-lang#154327 to avoid conflicts later, so draft for now.

r? @WaffleLapkin

Author: JonathanBrouwer

compiler/rustc_middle/src/ty/instance.rs

@@ -292,7 +292,7 @@ impl<'tcx> InstanceKind<'tcx> {
         use rustc_hir::definitions::DefPathData;
         let def_id = match *self {
             ty::InstanceKind::Item(def) => def,
-            ty::InstanceKind::DropGlue(_, Some(_)) => return false,
+            ty::InstanceKind::DropGlue(_, Some(ty)) => return ty.is_array(),
             ty::InstanceKind::AsyncDropGlueCtorShim(_, ty) => return ty.is_coroutine(),
             ty::InstanceKind::FutureDropPollShim(_, _, _) => return false,
             ty::InstanceKind::AsyncDropGlue(_, _) => return false,

compiler/rustc_mir_transform/src/shim.rs

@@ -317,16 +317,56 @@ fn build_drop_shim<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId, ty: Option<Ty<'tcx>>)
     let block = |blocks: &mut IndexVec<_, _>, kind| {
         blocks.push(BasicBlockData::new(Some(Terminator { source_info, kind }), false))
     };
-    block(&mut blocks, TerminatorKind::Goto { target: return_block });
+    if ty.is_some() {
+        block(&mut blocks, TerminatorKind::Goto { target: return_block });
+    }
     block(&mut blocks, TerminatorKind::Return);
 
     let source = MirSource::from_instance(ty::InstanceKind::DropGlue(def_id, ty));
     let mut body =
         new_body(source, blocks, local_decls_for_sig(&sig, span), sig.inputs().len(), span);
 
+    let Some(ty) = ty else {
+        return body;
+    };
+
     let dropee_ptr = Place::from(Local::arg(0));
 
-    if ty.is_some() {
+    if let ty::Array(ety, _len) = *ty.kind() {
+        // Don't write out the elaboration for each array type.
+        // Instead, just delegate to the slice version.
+        let slice_ty = Ty::new_slice(tcx, ety);
+        let mut_slice_ty = Ty::new_ref(tcx, tcx.lifetimes.re_erased, slice_ty, ty::Mutability::Mut);
+        let erased_local = body.local_decls.push(LocalDecl::new(mut_slice_ty, span));
+
+        let start = &mut body.basic_blocks_mut()[START_BLOCK];
+        start.statements.push(Statement::new(
+            source_info,
+            StatementKind::Assign(Box::new((
+                Place::from(erased_local),
+                Rvalue::Cast(
+                    CastKind::PointerCoercion(
+                        ty::adjustment::PointerCoercion::Unsize,
+                        CoercionSource::Implicit,
+                    ),
+                    Operand::Move(dropee_ptr),
+                    mut_slice_ty,
+                ),
+            ))),
+        ));
+        start.terminator = Some(Terminator {
+            source_info,
+            kind: TerminatorKind::Call {
+                func: Operand::function_handle(tcx, def_id, [ty::GenericArg::from(slice_ty)], span),
+                args: Box::new([Spanned { span, node: Operand::Move(Place::from(erased_local)) }]),
+                destination: Place::from(RETURN_PLACE),
+                target: Some(return_block),
+                unwind: UnwindAction::Continue,
+                call_source: CallSource::Misc,
+                fn_span: span,
+            },
+        });
+    } else {
         let patch = {
             let typing_env = ty::TypingEnv::post_analysis(tcx, def_id);
             let mut elaborator = DropShimElaborator {

tests/codegen-llvm/array-drop-glue.rs

@@ -0,0 +1,67 @@
+//@ revisions: RAW OPT
+//@ compile-flags: -C opt-level=z -C panic=abort
+//@[RAW] compile-flags: -C no-prepopulate-passes -Z inline-mir
+
+#![crate_type = "lib"]
+
+// Ensure all the different array drop_glue functions just delegate to the slice one,
+// rather than emitting two loops in each of the three.
+
+// When this test was first written, the array drop glues came out in the
+// seemingly-arbitrary order of 42, then 7, then 13, so to avoid potential
+// fragility from that changing we don't check any particular order.
+
+// RAW: ; core::ptr::drop_glue::<[array_drop_glue::NeedsDrop; [[N:7|13|42]]]>
+// RAW-NEXT: inlinehint
+// RAW: call core::ptr::drop_glue::<[array_drop_glue::NeedsDrop]>
+// RAW-NEXT: noundef [[N]])
+// RAW: }
+
+// RAW: ; core::ptr::drop_glue::<[array_drop_glue::NeedsDrop; [[N:7|13|42]]]>
+// RAW-NEXT: inlinehint
+// RAW: call core::ptr::drop_glue::<[array_drop_glue::NeedsDrop]>
+// RAW-NEXT: noundef [[N]])
+// RAW: }
+
+// RAW: ; core::ptr::drop_glue::<[array_drop_glue::NeedsDrop; [[N:7|13|42]]]>
+// RAW-NEXT: inlinehint
+// RAW: call core::ptr::drop_glue::<[array_drop_glue::NeedsDrop]>
+// RAW-NEXT: noundef [[N]])
+// RAW: }
+
+// CHECK-LABEL: ; core::ptr::drop_glue::<[array_drop_glue::NeedsDrop]>
+// CHECK-NOT: inlinehint
+// OPT: add nuw nsw {{.+}} 1
+// CHECK: }
+
+#[no_mangle]
+// CHECK-LABEL: @drop_arrays
+pub fn drop_arrays(x: [NeedsDrop; 7], y: [NeedsDrop; 13], z: [NeedsDrop; 42]) {
+    // I don't remember the parameter drop order, so write out the order the test expects.
+
+    // RAW: call core::ptr::drop_glue::<[array_drop_glue::NeedsDrop; 7]>
+    // OPT: call core::ptr::drop_glue::<[array_drop_glue::NeedsDrop]>
+    // OPT-NEXT: noundef 7)
+    drop(x);
+    // RAW: call core::ptr::drop_glue::<[array_drop_glue::NeedsDrop; 13]>
+    // OPT: call core::ptr::drop_glue::<[array_drop_glue::NeedsDrop]>
+    // OPT-NEXT: noundef 13)
+    drop(y);
+    // RAW: call core::ptr::drop_glue::<[array_drop_glue::NeedsDrop; 42]>
+    // OPT: call core::ptr::drop_glue::<[array_drop_glue::NeedsDrop]>
+    // OPT-NEXT: noundef 42)
+    drop(z);
+}
+
+struct NeedsDrop(u32);
+
+impl Drop for NeedsDrop {
+    #[inline]
+    fn drop(&mut self) {
+        do_the_drop(self);
+    }
+}
+
+unsafe extern "Rust" {
+    safe fn do_the_drop(_: &mut NeedsDrop);
+}

tests/mir-opt/slice_drop_shim.core.ptr-drop_glue.[String;42].AddMovesForPackedDrops.before.mir

@@ -2,62 +2,14 @@
 
 fn std::ptr::drop_glue(_1: &mut [String; 42]) -> () {
     let mut _0: ();
-    let mut _2: *mut [std::string::String; 42];
-    let mut _3: *mut [std::string::String];
-    let mut _4: usize;
-    let mut _5: usize;
-    let mut _6: *mut std::string::String;
-    let mut _7: bool;
-    let mut _8: *mut std::string::String;
-    let mut _9: bool;
+    let mut _2: &mut [std::string::String];
 
     bb0: {
-        goto -> bb9;
+        _2 = move _1 as &mut [std::string::String] (PointerCoercion(Unsize, Implicit));
+        _0 = std::ptr::drop_glue::<[String]>(move _2) -> [return: bb1, unwind continue];
     }
 
     bb1: {
         return;
     }
-
-    bb2 (cleanup): {
-        resume;
-    }
-
-    bb3 (cleanup): {
-        _6 = &raw mut (*_3)[_5];
-        _5 = Add(move _5, const 1_usize);
-        drop((*_6)) -> [return: bb4, unwind terminate(cleanup)];
-    }
-
-    bb4 (cleanup): {
-        _7 = Eq(copy _5, copy _4);
-        switchInt(move _7) -> [0: bb3, otherwise: bb2];
-    }
-
-    bb5: {
-        _8 = &raw mut (*_3)[_5];
-        _5 = Add(move _5, const 1_usize);
-        drop((*_8)) -> [return: bb6, unwind: bb4];
-    }
-
-    bb6: {
-        _9 = Eq(copy _5, copy _4);
-        switchInt(move _9) -> [0: bb5, otherwise: bb1];
-    }
-
-    bb7: {
-        _4 = PtrMetadata(copy _3);
-        _5 = const 0_usize;
-        goto -> bb6;
-    }
-
-    bb8: {
-        goto -> bb7;
-    }
-
-    bb9: {
-        _2 = &raw mut (*_1);
-        _3 = move _2 as *mut [std::string::String] (PointerCoercion(Unsize, Implicit));
-        goto -> bb8;
-    }
 }