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

oli-obk · oli-obk · commit 45a160304348 · 2026-03-09T11:38:47.000Z
diff --git a/compiler/rustc_mir_transform/src/gvn.rs b/compiler/rustc_mir_transform/src/gvn.rs
@@ -1494,6 +1494,7 @@ impl<'body, 'a, 'tcx> VnState<'body, 'a, 'tcx> {
         }
     }
 
+    #[instrument(level = "trace", skip(self), ret)]
     fn simplify_cast(
         &mut self,
         initial_kind: &mut CastKind,
@@ -1549,6 +1550,30 @@ 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
diff --git a/tests/mir-opt/const_prop/transmute.unreachable_box.GVN.32bit.diff b/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} }}, _marker: PhantomData::<Never> }}, std::alloc::Global);
-+         _2 = const std::ptr::NonNull::<Never> {{ pointer: {0x1 as *const Never} }} as *const Never (Transmute);
++         _2 = const std::boxed::Box::<Never>(std::ptr::Unique::<Never> {{ pointer: std::ptr::NonNull::<Never> {{ pointer: {0x1 as *const Never} }}, _marker: std::marker::PhantomData::<Never> }}, std::alloc::Global) as *const Never (Transmute);
           unreachable;
       }
   }
diff --git a/tests/mir-opt/const_prop/transmute.unreachable_box.GVN.64bit.diff b/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} }}, _marker: PhantomData::<Never> }}, std::alloc::Global);
-+         _2 = const std::ptr::NonNull::<Never> {{ pointer: {0x1 as *const Never} }} as *const Never (Transmute);
++         _2 = const std::boxed::Box::<Never>(std::ptr::Unique::<Never> {{ pointer: std::ptr::NonNull::<Never> {{ pointer: {0x1 as *const Never} }}, _marker: std::marker::PhantomData::<Never> }}, std::alloc::Global) as *const Never (Transmute);
           unreachable;
       }
   }
diff --git a/tests/mir-opt/dont_reset_cast_kind_without_updating_operand.test.GVN.32bit.panic-abort.diff b/tests/mir-opt/dont_reset_cast_kind_without_updating_operand.test.GVN.32bit.panic-abort.diff
@@ -74,7 +74,8 @@
 -         StorageLive(_5);
 +         nop;
           _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);
 +         nop;
diff --git a/tests/mir-opt/dont_reset_cast_kind_without_updating_operand.test.GVN.32bit.panic-unwind.diff b/tests/mir-opt/dont_reset_cast_kind_without_updating_operand.test.GVN.32bit.panic-unwind.diff
@@ -54,7 +54,8 @@
 -         StorageLive(_5);
 +         nop;
           _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);
 +         nop;
diff --git a/tests/mir-opt/dont_reset_cast_kind_without_updating_operand.test.GVN.64bit.panic-abort.diff b/tests/mir-opt/dont_reset_cast_kind_without_updating_operand.test.GVN.64bit.panic-abort.diff
@@ -74,7 +74,8 @@
 -         StorageLive(_5);
 +         nop;
           _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);
 +         nop;
diff --git a/tests/mir-opt/dont_reset_cast_kind_without_updating_operand.test.GVN.64bit.panic-unwind.diff b/tests/mir-opt/dont_reset_cast_kind_without_updating_operand.test.GVN.64bit.panic-unwind.diff
@@ -54,7 +54,8 @@
 -         StorageLive(_5);
 +         nop;
           _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);
 +         nop;
diff --git a/tests/mir-opt/pre-codegen/issue_117368_print_invalid_constant.main.GVN.32bit.panic-abort.diff b/tests/mir-opt/pre-codegen/issue_117368_print_invalid_constant.main.GVN.32bit.panic-abort.diff
@@ -69,7 +69,8 @@
           StorageLive(_3);
           StorageLive(_4);
           StorageLive(_5);
-          StorageLive(_6);
+-         StorageLive(_6);
++         nop;
           StorageLive(_7);
 -         _7 = copy _1;
 -         _6 = std::alloc::Global::alloc_impl_runtime(move _7, const false) -> [return: bb4, unwind unreachable];
@@ -93,11 +94,14 @@
       }
   
       bb6: {
-          _5 = move ((_6 as Ok).0: std::ptr::NonNull<[u8]>);
+-         _5 = move ((_6 as Ok).0: std::ptr::NonNull<[u8]>);
++         _5 = copy ((_6 as Ok).0: std::ptr::NonNull<[u8]>);
           StorageDead(_14);
           StorageDead(_10);
-          StorageDead(_6);
-          _4 = copy _5 as *mut [u8] (Transmute);
+-         StorageDead(_6);
+-         _4 = copy _5 as *mut [u8] (Transmute);
++         nop;
++         _4 = copy _6 as *mut [u8] (Transmute);
           StorageDead(_5);
           _3 = copy _4 as *mut u8 (PtrToPtr);
           StorageDead(_4);
diff --git a/tests/mir-opt/pre-codegen/issue_117368_print_invalid_constant.main.GVN.64bit.panic-abort.diff b/tests/mir-opt/pre-codegen/issue_117368_print_invalid_constant.main.GVN.64bit.panic-abort.diff
@@ -69,7 +69,8 @@
           StorageLive(_3);
           StorageLive(_4);
           StorageLive(_5);
-          StorageLive(_6);
+-         StorageLive(_6);
++         nop;
           StorageLive(_7);
 -         _7 = copy _1;
 -         _6 = std::alloc::Global::alloc_impl_runtime(move _7, const false) -> [return: bb4, unwind unreachable];
@@ -93,11 +94,14 @@
       }
   
       bb6: {
-          _5 = move ((_6 as Ok).0: std::ptr::NonNull<[u8]>);
+-         _5 = move ((_6 as Ok).0: std::ptr::NonNull<[u8]>);
++         _5 = copy ((_6 as Ok).0: std::ptr::NonNull<[u8]>);
           StorageDead(_14);
           StorageDead(_10);
-          StorageDead(_6);
-          _4 = copy _5 as *mut [u8] (Transmute);
+-         StorageDead(_6);
+-         _4 = copy _5 as *mut [u8] (Transmute);
++         nop;
++         _4 = copy _6 as *mut [u8] (Transmute);
           StorageDead(_5);
           _3 = copy _4 as *mut u8 (PtrToPtr);
           StorageDead(_4);

Original file line number	Diff line number	Diff line change
`@@ -14,7 +14,7 @@`
`14`	`14`	`- _1 = const 1_usize as std::boxed::Box<Never> (Transmute);`
`15`	`15`	`- _2 = copy ((_1.0: std::ptr::Unique<Never>).0: std::ptr::NonNull<Never>) as *const Never (Transmute);`
`16`	`16`	`+ _1 = const Box::<Never>(std::ptr::Unique::<Never> {{ pointer: NonNull::<Never> {{ pointer: {0x1 as *const Never} }}, _marker: PhantomData::<Never> }}, std::alloc::Global);`
`17`		`-+ _2 = const std::ptr::NonNull::<Never> {{ pointer: {0x1 as const Never} }} as const Never (Transmute);`
	`17`	`++ _2 = const std::boxed::Box::<Never>(std::ptr::Unique::<Never> {{ pointer: std::ptr::NonNull::<Never> {{ pointer: {0x1 as const Never} }}, _marker: std::marker::PhantomData::<Never> }}, std::alloc::Global) as const Never (Transmute);`
`18`	`18`	`unreachable;`
`19`	`19`	`}`
`20`	`20`	`}`