Auto merge of #142508 - Mark-Simulacrum:skip-noop-drop-glue, r=<try>

Skip noop drop glue

Since #122662 this no longer gets used in vtables, so we're safe to fully
drop generating these empty functions. Those are eventually cleaned up
by LLVM, but it's wasteful to produce them in the first place.

Opening this for a perf run before asking for review.

Author: rust-bors[bot]

compiler/rustc_monomorphize/src/collector.rs

@@ -948,10 +948,14 @@ fn visit_instance_use<'tcx>(
             bug!("{:?} being reified", instance);
         }
         ty::InstanceKind::DropGlue(_, None) => {
-            // Don't need to emit noop drop glue if we are calling directly.
-            if !is_direct_call {
-                output.push(create_fn_mono_item(tcx, instance, source));
-            }
+            // No-op drop glue never needs to be emitted.
+            // In direct calls, we skip it in codegen. In indirect calls (vtables) we place a null
+            // pointer rather than codegen'ing a pointer to the empty drop.
+            //
+            // Note that if user code casts drop_in_place to a fn(...) that's not a DropGlue, so
+            // it won't hit this branch.
+            //
+            // If we get this wrong we'll see linker errors.
         }
         ty::InstanceKind::DropGlue(_, Some(_))
         | ty::InstanceKind::FutureDropPollShim(..)

tests/codegen-units/item-collection/instantiation-through-vtable.rs

@@ -24,15 +24,13 @@ impl<T> Trait for Struct<T> {
 pub fn start(_: isize, _: *const *const u8) -> isize {
     let s1 = Struct { _a: 0u32 };
 
-    //~ MONO_ITEM fn std::ptr::drop_in_place::<Struct<u32>> - shim(None) @@ instantiation_through_vtable-cgu.0[External]
     //~ MONO_ITEM fn <Struct<u32> as Trait>::foo
     //~ MONO_ITEM fn <Struct<u32> as Trait>::bar
     let r1 = &s1 as &Trait;
     r1.foo();
     r1.bar();
 
     let s1 = Struct { _a: 0u64 };
-    //~ MONO_ITEM fn std::ptr::drop_in_place::<Struct<u64>> - shim(None) @@ instantiation_through_vtable-cgu.0[External]
     //~ MONO_ITEM fn <Struct<u64> as Trait>::foo
     //~ MONO_ITEM fn <Struct<u64> as Trait>::bar
     let _ = &s1 as &Trait;

tests/codegen-units/item-collection/non-generic-closures.rs

@@ -1,4 +1,4 @@
-//@ compile-flags:-Clink-dead-code -Zinline-mir=no
+//@ compile-flags:-Clink-dead-code -Zinline-mir=no -Zhuman-readable-cgu-names
 
 #![deny(dead_code)]
 #![crate_type = "lib"]
@@ -22,9 +22,8 @@ fn assigned_to_variable_but_not_executed() {
 //~ MONO_ITEM fn assigned_to_variable_executed_indirectly @@ non_generic_closures-cgu.0[External]
 fn assigned_to_variable_executed_indirectly() {
     //~ MONO_ITEM fn assigned_to_variable_executed_indirectly::{closure#0} @@ non_generic_closures-cgu.0[External]
-    //~ MONO_ITEM fn <{closure@TEST_PATH:28:13: 28:21} as std::ops::FnOnce<(i32,)>>::call_once - shim @@ non_generic_closures-cgu.0[External]
-    //~ MONO_ITEM fn <{closure@TEST_PATH:28:13: 28:21} as std::ops::FnOnce<(i32,)>>::call_once - shim(vtable) @@ non_generic_closures-cgu.0[External]
-    //~ MONO_ITEM fn std::ptr::drop_in_place::<{closure@TEST_PATH:28:13: 28:21}> - shim(None) @@ non_generic_closures-cgu.0[External]
+    //~ MONO_ITEM fn <{closure@TEST_PATH:27:13: 27:21} as std::ops::FnOnce<(i32,)>>::call_once - shim @@ non_generic_closures-cgu.0[External]
+    //~ MONO_ITEM fn <{closure@TEST_PATH:27:13: 27:21} as std::ops::FnOnce<(i32,)>>::call_once - shim(vtable) @@ non_generic_closures-cgu.0[External]
     let f = |a: i32| {
         let _ = a + 2;
     };

tests/codegen-units/item-collection/unsizing.rs

@@ -47,25 +47,21 @@ impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Wrapper<U>> for Wrapper<T>
 pub fn start(_: isize, _: *const *const u8) -> isize {
     // simple case
     let bool_sized = &true;
-    //~ MONO_ITEM fn std::ptr::drop_in_place::<bool> - shim(None) @@ unsizing-cgu.0[Internal]
     //~ MONO_ITEM fn <bool as Trait>::foo
     let _bool_unsized = bool_sized as &Trait;
 
     let char_sized = &'a';
 
-    //~ MONO_ITEM fn std::ptr::drop_in_place::<char> - shim(None) @@ unsizing-cgu.0[Internal]
     //~ MONO_ITEM fn <char as Trait>::foo
     let _char_unsized = char_sized as &Trait;
 
     // struct field
     let struct_sized = &Struct { _a: 1, _b: 2, _c: 3.0f64 };
-    //~ MONO_ITEM fn std::ptr::drop_in_place::<f64> - shim(None) @@ unsizing-cgu.0[Internal]
     //~ MONO_ITEM fn <f64 as Trait>::foo
     let _struct_unsized = struct_sized as &Struct<Trait>;
 
     // custom coercion
     let wrapper_sized = Wrapper(&0u32);
-    //~ MONO_ITEM fn std::ptr::drop_in_place::<u32> - shim(None) @@ unsizing-cgu.0[Internal]
     //~ MONO_ITEM fn <u32 as Trait>::foo
     let _wrapper_sized = wrapper_sized as Wrapper<Trait>;
 

tests/codegen/sanitizer/cfi/emit-type-metadata-id-itanium-cxx-abi-drop-in-place.rs

@@ -1,5 +1,9 @@
 // Verifies that type metadata identifiers for drop functions are emitted correctly.
 //
+// Non needs_drop drop glue isn't codegen'd at all, so we don't try to check the IDs there. But we
+// do check it's not emitted which should help catch bugs if we do start generating it again in the
+// future.
+//
 //@ needs-sanitizer-cfi
 //@ compile-flags: -Clto -Cno-prepopulate-passes -Copt-level=0 -Zsanitizer=cfi -Ctarget-feature=-crt-static
 
@@ -10,18 +14,18 @@
 // CHECK:       call i1 @llvm.type.test(ptr {{%.+}}, metadata !"_ZTSFvPu3dynIu{{[0-9]+}}NtNtNtC{{[[:print:]]+}}_4core3ops4drop4Dropu6regionEE")
 
 struct EmptyDrop;
-// CHECK: define{{.*}}4core3ptr{{[0-9]+}}drop_in_place$LT${{.*}}EmptyDrop$GT${{.*}}!type ![[TYPE1]] !type !{{[0-9]+}} !type !{{[0-9]+}} !type !{{[0-9]+}}
+// CHECK-NOT: define{{.*}}4core3ptr{{[0-9]+}}drop_in_place$LT${{.*}}EmptyDrop$GT${{.*}}!type ![[TYPE1]] !type !{{[0-9]+}} !type !{{[0-9]+}} !type !{{[0-9]+}}
 
-struct NonEmptyDrop;
+struct PresentDrop;
 
-impl Drop for NonEmptyDrop {
+impl Drop for PresentDrop {
     fn drop(&mut self) {}
-    // CHECK: define{{.*}}4core3ptr{{[0-9]+}}drop_in_place$LT${{.*}}NonEmptyDrop$GT${{.*}}!type ![[TYPE1]] !type !{{[0-9]+}} !type !{{[0-9]+}} !type !{{[0-9]+}}
+    // CHECK: define{{.*}}4core3ptr{{[0-9]+}}drop_in_place$LT${{.*}}PresentDrop$GT${{.*}}!type ![[TYPE1]] !type !{{[0-9]+}} !type !{{[0-9]+}} !type !{{[0-9]+}}
 }
 
 pub fn foo() {
     let _ = Box::new(EmptyDrop) as Box<dyn Send>;
-    let _ = Box::new(NonEmptyDrop) as Box<dyn Send>;
+    let _ = Box::new(PresentDrop) as Box<dyn Send>;
 }
 
 // CHECK: ![[TYPE1]] = !{i64 0, !"_ZTSFvPu3dynIu{{[0-9]+}}NtNtNtC{{[[:print:]]+}}_4core3ops4drop4Dropu6regionEE"}