Turn field accesses into a transmute if followed by a transmute anyway

Author: oli-obk

compiler/rustc_mir_transform/src/gvn.rs

@@ -1527,6 +1527,7 @@ impl<'body, 'a, 'tcx> VnState<'body, 'a, 'tcx> {
         }
     }
 
+    #[instrument(level = "trace", skip(self), ret)]
     fn simplify_cast(
         &mut self,
         initial_kind: &mut CastKind,
@@ -1582,6 +1583,30 @@ impl<'body, 'a, 'tcx> VnState<'body, 'a, 'tcx> {
                 }
             }
 
+            // Field-Access-then-Transmute can just transmute the original value,
+            // so long as the bytes of a value from only from a single field.
+            if let Transmute = kind
+                && let Value::Projection(field_value, ProjectionElem::Field(field_idx, ())) =
+                    self.get(value)
+            {
+                if let Value::Projection(
+                    _downcast_value,
+                    ProjectionElem::Downcast(_, _variant_idx),
+                ) = self.get(field_value)
+                {
+                } else if let Some((f_idx, field_ty)) =
+                    self.value_is_all_in_one_field(self.ty(field_value), FIRST_VARIANT)
+                {
+                    assert_eq!(field_idx, f_idx, "{from} -> {field_ty}");
+                    from = self.ty(field_value);
+                    value = field_value;
+                    was_updated_this_iteration = true;
+                    if field_ty == to {
+                        return Some(value);
+                    }
+                }
+            }
+
             // Aggregate-then-Transmute can just transmute the original field value,
             // so long as the bytes of a value from only from a single field.
             if let Transmute = kind

tests/mir-opt/const_prop/transmute.unreachable_box.GVN.32bit.diff

@@ -14,7 +14,7 @@
 -         _1 = const 1_usize as std::boxed::Box<Never> (Transmute);
 -         _2 = copy ((_1.0: std::ptr::Unique<Never>).0: std::ptr::NonNull<Never>) as *const Never (Transmute);
 +         _1 = const Box::<Never>(std::ptr::Unique::<Never> {{ pointer: NonNull::<Never> {{ pointer: {0x1 as *const Never} is !null }}, _marker: PhantomData::<Never> }}, std::alloc::Global);
-+         _2 = const std::ptr::NonNull::<Never> {{ pointer: {0x1 as *const Never} is !null }} as *const Never (Transmute);
++         _2 = const std::boxed::Box::<Never>(std::ptr::Unique::<Never> {{ pointer: std::ptr::NonNull::<Never> {{ pointer: {0x1 as *const Never} is !null }}, _marker: std::marker::PhantomData::<Never> }}, std::alloc::Global) as *const Never (Transmute);
           unreachable;
       }
   }

tests/mir-opt/const_prop/transmute.unreachable_box.GVN.64bit.diff

@@ -14,7 +14,7 @@
 -         _1 = const 1_usize as std::boxed::Box<Never> (Transmute);
 -         _2 = copy ((_1.0: std::ptr::Unique<Never>).0: std::ptr::NonNull<Never>) as *const Never (Transmute);
 +         _1 = const Box::<Never>(std::ptr::Unique::<Never> {{ pointer: NonNull::<Never> {{ pointer: {0x1 as *const Never} is !null }}, _marker: PhantomData::<Never> }}, std::alloc::Global);
-+         _2 = const std::ptr::NonNull::<Never> {{ pointer: {0x1 as *const Never} is !null }} as *const Never (Transmute);
++         _2 = const std::boxed::Box::<Never>(std::ptr::Unique::<Never> {{ pointer: std::ptr::NonNull::<Never> {{ pointer: {0x1 as *const Never} is !null }}, _marker: std::marker::PhantomData::<Never> }}, std::alloc::Global) as *const Never (Transmute);
           unreachable;
       }
   }

tests/mir-opt/dont_reset_cast_kind_without_updating_operand.test.GVN.32bit.panic-abort.diff

@@ -73,7 +73,8 @@
           StorageDead(_2);
           StorageLive(_5);
           _10 = copy (*_1);
-          _11 = copy ((_10.0: std::ptr::Unique<()>).0: std::ptr::NonNull<()>) as *const () (Transmute);
+-         _11 = copy ((_10.0: std::ptr::Unique<()>).0: std::ptr::NonNull<()>) as *const () (Transmute);
++         _11 = copy _10 as *const () (Transmute);
           _5 = &raw const (*_11);
           StorageLive(_6);
           StorageLive(_7);

tests/mir-opt/dont_reset_cast_kind_without_updating_operand.test.GVN.32bit.panic-unwind.diff

@@ -53,7 +53,8 @@
           StorageDead(_2);
           StorageLive(_5);
           _10 = copy (*_1);
-          _11 = copy ((_10.0: std::ptr::Unique<()>).0: std::ptr::NonNull<()>) as *const () (Transmute);
+-         _11 = copy ((_10.0: std::ptr::Unique<()>).0: std::ptr::NonNull<()>) as *const () (Transmute);
++         _11 = copy _10 as *const () (Transmute);
           _5 = &raw const (*_11);
           StorageLive(_6);
           StorageLive(_7);

tests/mir-opt/dont_reset_cast_kind_without_updating_operand.test.GVN.64bit.panic-abort.diff

@@ -73,7 +73,8 @@
           StorageDead(_2);
           StorageLive(_5);
           _10 = copy (*_1);
-          _11 = copy ((_10.0: std::ptr::Unique<()>).0: std::ptr::NonNull<()>) as *const () (Transmute);
+-         _11 = copy ((_10.0: std::ptr::Unique<()>).0: std::ptr::NonNull<()>) as *const () (Transmute);
++         _11 = copy _10 as *const () (Transmute);
           _5 = &raw const (*_11);
           StorageLive(_6);
           StorageLive(_7);

tests/mir-opt/dont_reset_cast_kind_without_updating_operand.test.GVN.64bit.panic-unwind.diff

@@ -53,7 +53,8 @@
           StorageDead(_2);
           StorageLive(_5);
           _10 = copy (*_1);
-          _11 = copy ((_10.0: std::ptr::Unique<()>).0: std::ptr::NonNull<()>) as *const () (Transmute);
+-         _11 = copy ((_10.0: std::ptr::Unique<()>).0: std::ptr::NonNull<()>) as *const () (Transmute);
++         _11 = copy _10 as *const () (Transmute);
           _5 = &raw const (*_11);
           StorageLive(_6);
           StorageLive(_7);