Simplify the canonical clone method to copy

The optimized clone method ends up as the following MIR:

```
_2 = copy ((*_1).0: i32);
_3 = copy ((*_1).1: u64);
_4 = copy ((*_1).2: [i8; 3]);
_0 = Foo { a: move _2, b: move _3, c: move _4 };
```

We can transform this to:

```
_0 = copy (*_1);
```

compiler/rustc_mir_transform/src/gvn.rs

@@ -875,6 +875,95 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
         None
     }
 
+    fn try_as_place_elem(
+        &mut self,
+        proj: ProjectionElem<VnIndex, Ty<'tcx>>,
+        loc: Location,
+    ) -> Option<PlaceElem<'tcx>> {
+        Some(match proj {
+            ProjectionElem::Deref => ProjectionElem::Deref,
+            ProjectionElem::Field(idx, ty) => ProjectionElem::Field(idx, ty),
+            ProjectionElem::Index(idx) => {
+                let Some(local) = self.try_as_local(idx, loc) else {
+                    return None;
+                };
+                self.reused_locals.insert(local);
+                ProjectionElem::Index(local)
+            }
+            ProjectionElem::ConstantIndex { offset, min_length, from_end } => {
+                ProjectionElem::ConstantIndex { offset, min_length, from_end }
+            }
+            ProjectionElem::Subslice { from, to, from_end } => {
+                ProjectionElem::Subslice { from, to, from_end }
+            }
+            ProjectionElem::Downcast(symbol, idx) => ProjectionElem::Downcast(symbol, idx),
+            ProjectionElem::OpaqueCast(idx) => ProjectionElem::OpaqueCast(idx),
+            ProjectionElem::Subtype(idx) => ProjectionElem::Subtype(idx),
+        })
+    }
+
+    fn simplify_aggregate_to_copy(
+        &mut self,
+        rvalue: &mut Rvalue<'tcx>,
+        location: Location,
+        fields: &[VnIndex],
+        variant_index: VariantIdx,
+    ) -> Option<VnIndex> {
+        let Some(&first_field) = fields.first() else {
+            return None;
+        };
+        let Value::Projection(copy_from_value, _) = *self.get(first_field) else {
+            return None;
+        };
+        // All fields must correspond one-to-one and come from the same aggregate value.
+        if fields.iter().enumerate().any(|(index, &v)| {
+            if let Value::Projection(pointer, ProjectionElem::Field(from_index, _)) = *self.get(v)
+                && copy_from_value == pointer
+                && from_index.index() == index
+            {
+                return false;
+            }
+            true
+        }) {
+            return None;
+        }
+
+        let mut copy_from_local_value = copy_from_value;
+        if let Value::Projection(pointer, proj) = *self.get(copy_from_value)
+            && let ProjectionElem::Downcast(_, read_variant) = proj
+        {
+            if variant_index == read_variant {
+                // When copying a variant, there is no need to downcast.
+                copy_from_local_value = pointer;
+            } else {
+                // The copied variant must be identical.
+                return None;
+            }
+        }
+
+        let tcx = self.tcx;
+        let mut projection = SmallVec::<[PlaceElem<'tcx>; 1]>::new();
+        loop {
+            if let Some(local) = self.try_as_local(copy_from_local_value, location) {
+                projection.reverse();
+                let place = Place { local, projection: tcx.mk_place_elems(projection.as_slice()) };
+                if rvalue.ty(self.local_decls, tcx) == place.ty(self.local_decls, tcx).ty {
+                    self.reused_locals.insert(local);
+                    *rvalue = Rvalue::Use(Operand::Copy(place));
+                    return Some(copy_from_value);
+                }
+                return None;
+            } else if let Value::Projection(pointer, proj) = *self.get(copy_from_local_value)
+                && let Some(proj) = self.try_as_place_elem(proj, location)
+            {
+                projection.push(proj);
+                copy_from_local_value = pointer;
+            } else {
+                return None;
+            }
+        }
+    }
+
     fn simplify_aggregate(
         &mut self,
         rvalue: &mut Rvalue<'tcx>,
@@ -971,6 +1060,13 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
             }
         }
 
+        if let AggregateTy::Def(_, _) = ty
+            && let Some(value) =
+                self.simplify_aggregate_to_copy(rvalue, location, &fields, variant_index)
+        {
+            return Some(value);
+        }
+
         Some(self.insert(Value::Aggregate(ty, variant_index, fields)))
     }
 
@@ -1485,7 +1581,7 @@ impl<'tcx> VnState<'_, 'tcx> {
     }
 
     /// If there is a local which is assigned `index`, and its assignment strictly dominates `loc`,
-    /// return it.
+    /// return it. If you used this local, add it to `reused_locals` to remove storage statements.
     fn try_as_local(&mut self, index: VnIndex, loc: Location) -> Option<Local> {
         let other = self.rev_locals.get(index)?;
         other

tests/codegen/clone_as_copy.rs

@@ -0,0 +1,40 @@
+//@ revisions: DEBUGINFO NODEBUGINFO
+//@ compile-flags: -O -Cno-prepopulate-passes
+//@ [DEBUGINFO] compile-flags: -Cdebuginfo=full
+
+// From https://github.com/rust-lang/rust/issues/128081.
+// Ensure that we only generate a memcpy instruction.
+
+#![crate_type = "lib"]
+
+#[derive(Clone)]
+struct SubCloneAndCopy {
+    v1: u32,
+    v2: u32,
+}
+
+#[derive(Clone)]
+struct CloneOnly {
+    v1: u8,
+    v2: u8,
+    v3: u8,
+    v4: u8,
+    v5: u8,
+    v6: u8,
+    v7: u8,
+    v8: u8,
+    v9: u8,
+    v_sub: SubCloneAndCopy,
+    v_large: [u8; 256],
+}
+
+// CHECK-LABEL: define {{.*}}@clone_only(
+#[no_mangle]
+pub fn clone_only(v: &CloneOnly) -> CloneOnly {
+    // CHECK-NOT: call {{.*}}clone
+    // CHECK-NOT: store i8
+    // CHECK-NOT: store i32
+    // CHECK: call void @llvm.memcpy
+    // CHECK-NEXT: ret void
+    v.clone()
+}

tests/codegen/enum/unreachable_enum_default_branch.rs

@@ -28,11 +28,13 @@ pub fn implicit_match(x: Int) -> bool {
 // The code is from https://github.com/rust-lang/rust/issues/110097.
 // We expect it to generate the same optimized code as a full match.
 // CHECK-LABEL: @if_let(
-// CHECK-NEXT:  start:
+// CHECK: start:
+// CHECK-NOT: zext
+// CHECK: select
 // CHECK-NEXT: insertvalue
 // CHECK-NEXT: insertvalue
 // CHECK-NEXT: ret
 #[no_mangle]
 pub fn if_let(val: Result<i32, ()>) -> Result<i32, ()> {
-    if let Ok(x) = val { Ok(x) } else { Err(()) }
+    if let Ok(x) = val { Ok(x * 2) } else { Err(()) }
 }

tests/mir-opt/gvn_clone.rs

@@ -0,0 +1,17 @@
+//@ test-mir-pass: GVN
+//@ compile-flags: -Zmir-enable-passes=+InstSimplify-before-inline
+
+// Check if we have transformed the default clone to copy in the specific pipeline.
+
+// EMIT_MIR gvn_clone.{impl#0}-clone.GVN.diff
+
+// CHECK-LABEL: ::clone(
+// CHECK-NOT: = AllCopy { {{.*}} };
+// CHECK: _0 = copy (*_1);
+// CHECK: return;
+#[derive(Clone)]
+struct AllCopy {
+    a: i32,
+    b: u64,
+    c: [i8; 3],
+}

tests/mir-opt/gvn_clone.{impl#0}-clone.GVN.diff

@@ -0,0 +1,71 @@
+- // MIR for `<impl at $DIR/gvn_clone.rs:12:10: 12:15>::clone` before GVN
++ // MIR for `<impl at $DIR/gvn_clone.rs:12:10: 12:15>::clone` after GVN
+
+  fn <impl at $DIR/gvn_clone.rs:12:10: 12:15>::clone(_1: &AllCopy) -> AllCopy {
+      debug self => _1;
+      let mut _0: AllCopy;
+      let mut _2: i32;
+      let mut _3: &i32;
+      let _4: &i32;
+      let mut _5: u64;
+      let mut _6: &u64;
+      let _7: &u64;
+      let mut _8: [i8; 3];
+      let mut _9: &[i8; 3];
+      let _10: &[i8; 3];
+
+      bb0: {
+          StorageLive(_2);
+          StorageLive(_3);
+-         StorageLive(_4);
++         nop;
+          _4 = &((*_1).0: i32);
+          _3 = copy _4;
+-         _2 = copy (*_3);
++         _2 = copy ((*_1).0: i32);
+          goto -> bb1;
+      }
+
+      bb1: {
+          StorageDead(_3);
+          StorageLive(_5);
+          StorageLive(_6);
+-         StorageLive(_7);
++         nop;
+          _7 = &((*_1).1: u64);
+          _6 = copy _7;
+-         _5 = copy (*_6);
++         _5 = copy ((*_1).1: u64);
+          goto -> bb2;
+      }
+
+      bb2: {
+          StorageDead(_6);
+          StorageLive(_8);
+          StorageLive(_9);
+-         StorageLive(_10);
++         nop;
+          _10 = &((*_1).2: [i8; 3]);
+          _9 = copy _10;
+-         _8 = copy (*_9);
++         _8 = copy ((*_1).2: [i8; 3]);
+          goto -> bb3;
+      }
+
+      bb3: {
+          StorageDead(_9);
+-         _0 = AllCopy { a: move _2, b: move _5, c: move _8 };
++         _0 = copy (*_1);
+          StorageDead(_8);
+          StorageDead(_5);
+          StorageDead(_2);
+-         StorageDead(_10);
+-         StorageDead(_7);
+-         StorageDead(_4);
++         nop;
++         nop;
++         nop;
+          return;
+      }
+  }
+

tests/mir-opt/gvn_copy_aggregate.all_copy.GVN.diff

@@ -0,0 +1,53 @@
+- // MIR for `all_copy` before GVN
++ // MIR for `all_copy` after GVN
+
+  fn all_copy(_1: &AllCopy) -> AllCopy {
+      debug v => _1;
+      let mut _0: AllCopy;
+      let _2: i32;
+      let mut _5: i32;
+      let mut _6: u64;
+      let mut _7: [i8; 3];
+      scope 1 {
+          debug a => _2;
+          let _3: u64;
+          scope 2 {
+              debug b => _3;
+              let _4: [i8; 3];
+              scope 3 {
+                  debug c => _4;
+              }
+          }
+      }
+
+      bb0: {
+-         StorageLive(_2);
++         nop;
+          _2 = copy ((*_1).0: i32);
+-         StorageLive(_3);
++         nop;
+          _3 = copy ((*_1).1: u64);
+-         StorageLive(_4);
++         nop;
+          _4 = copy ((*_1).2: [i8; 3]);
+          StorageLive(_5);
+          _5 = copy _2;
+          StorageLive(_6);
+          _6 = copy _3;
+          StorageLive(_7);
+          _7 = copy _4;
+-         _0 = AllCopy { a: move _5, b: move _6, c: move _7 };
++         _0 = copy (*_1);
+          StorageDead(_7);
+          StorageDead(_6);
+          StorageDead(_5);
+-         StorageDead(_4);
+-         StorageDead(_3);
+-         StorageDead(_2);
++         nop;
++         nop;
++         nop;
+          return;
+      }
+  }
+

tests/mir-opt/gvn_copy_aggregate.all_copy_2.GVN.diff

@@ -0,0 +1,64 @@
+- // MIR for `all_copy_2` before GVN
++ // MIR for `all_copy_2` after GVN
+
+  fn all_copy_2(_1: &&AllCopy) -> AllCopy {
+      debug v => _1;
+      let mut _0: AllCopy;
+      let _2: i32;
+      let mut _5: i32;
+      let mut _6: u64;
+      let mut _7: [i8; 3];
+      let mut _8: &AllCopy;
+      let mut _9: &AllCopy;
+      let mut _10: &AllCopy;
+      scope 1 {
+          debug a => _2;
+          let _3: u64;
+          scope 2 {
+              debug b => _3;
+              let _4: [i8; 3];
+              scope 3 {
+                  debug c => _4;
+              }
+          }
+      }
+
+      bb0: {
+-         StorageLive(_2);
+-         _8 = deref_copy (*_1);
++         nop;
++         _8 = copy (*_1);
+          _2 = copy ((*_8).0: i32);
+-         StorageLive(_3);
+-         _9 = deref_copy (*_1);
+-         _3 = copy ((*_9).1: u64);
+-         StorageLive(_4);
+-         _10 = deref_copy (*_1);
+-         _4 = copy ((*_10).2: [i8; 3]);
++         nop;
++         _9 = copy _8;
++         _3 = copy ((*_8).1: u64);
++         nop;
++         _10 = copy _8;
++         _4 = copy ((*_8).2: [i8; 3]);
+          StorageLive(_5);
+          _5 = copy _2;
+          StorageLive(_6);
+          _6 = copy _3;
+          StorageLive(_7);
+          _7 = copy _4;
+-         _0 = AllCopy { a: move _5, b: move _6, c: move _7 };
++         _0 = copy (*_8);
+          StorageDead(_7);
+          StorageDead(_6);
+          StorageDead(_5);
+-         StorageDead(_4);
+-         StorageDead(_3);
+-         StorageDead(_2);
++         nop;
++         nop;
++         nop;
+          return;
+      }
+  }
+