slightly cleaner, if more verbose, vtable handling in codegen backends

Author: RalfJung

Diff for: compiler/rustc_codegen_cranelift/src/constant.rs

@@ -195,11 +195,6 @@ pub(crate) fn codegen_const_value<'tcx>(
             }
             Scalar::Ptr(ptr, _size) => {
                 let (alloc_id, offset) = ptr.into_parts(); // we know the `offset` is relative
-                // For vtables, get the underlying data allocation.
-                let alloc_id = match fx.tcx.global_alloc(alloc_id) {
-                    GlobalAlloc::VTable(ty, trait_ref) => fx.tcx.vtable_allocation((ty, trait_ref)),
-                    _ => alloc_id,
-                };
                 let base_addr = match fx.tcx.global_alloc(alloc_id) {
                     GlobalAlloc::Memory(alloc) => {
                         let data_id = data_id_for_alloc_id(
@@ -221,7 +216,20 @@ pub(crate) fn codegen_const_value<'tcx>(
                             fx.module.declare_func_in_func(func_id, &mut fx.bcx.func);
                         fx.bcx.ins().func_addr(fx.pointer_type, local_func_id)
                     }
-                    GlobalAlloc::VTable(..) => bug!("vtables are already handled"),
+                    GlobalAlloc::VTable(ty, trait_ref) => {
+                        let alloc_id = fx.tcx.vtable_allocation((ty, trait_ref));
+                        let alloc = fx.tcx.global_alloc(alloc_id).unwrap_memory();
+                        // FIXME: factor this common code with the `Memory` arm into a function?
+                        let data_id = data_id_for_alloc_id(
+                            &mut fx.constants_cx,
+                            fx.module,
+                            alloc_id,
+                            alloc.inner().mutability,
+                        );
+                        let local_data_id =
+                            fx.module.declare_data_in_func(data_id, &mut fx.bcx.func);
+                        fx.bcx.ins().global_value(fx.pointer_type, local_data_id)
+                    }
                     GlobalAlloc::Static(def_id) => {
                         assert!(fx.tcx.is_static(def_id));
                         let data_id = data_id_for_static(fx.tcx, fx.module, def_id, false);

Diff for: compiler/rustc_codegen_gcc/src/common.rs

@@ -183,13 +183,6 @@ impl<'gcc, 'tcx> ConstMethods<'tcx> for CodegenCx<'gcc, 'tcx> {
             }
             Scalar::Ptr(ptr, _size) => {
                 let (alloc_id, offset) = ptr.into_parts();
-                // For vtables, get the underlying data allocation.
-                let alloc_id = match self.tcx.global_alloc(alloc_id) {
-                    GlobalAlloc::VTable(ty, trait_ref) => {
-                        self.tcx.vtable_allocation((ty, trait_ref))
-                    }
-                    _ => alloc_id,
-                };
                 let base_addr =
                     match self.tcx.global_alloc(alloc_id) {
                         GlobalAlloc::Memory(alloc) => {
@@ -208,7 +201,11 @@ impl<'gcc, 'tcx> ConstMethods<'tcx> for CodegenCx<'gcc, 'tcx> {
                         GlobalAlloc::Function(fn_instance) => {
                             self.get_fn_addr(fn_instance)
                         },
-                        GlobalAlloc::VTable(..) => panic!("vtables are already handled"),
+                        GlobalAlloc::VTable(ty, trait_ref) => {
+                            let alloc = self.tcx.global_alloc(self.tcx.vtable_allocation((ty, trait_ref))).unwrap_memory();
+                            let init = const_alloc_to_gcc(self, alloc);
+                            self.static_addr_of(init, alloc.inner().align, None)
+                        }
                         GlobalAlloc::Static(def_id) => {
                             assert!(self.tcx.is_static(def_id));
                             self.get_static(def_id).get_address(None)

Diff for: compiler/rustc_codegen_llvm/src/common.rs

@@ -240,13 +240,6 @@ impl<'ll, 'tcx> ConstMethods<'tcx> for CodegenCx<'ll, 'tcx> {
             }
             Scalar::Ptr(ptr, _size) => {
                 let (alloc_id, offset) = ptr.into_parts();
-                // For vtables, get the underlying data allocation.
-                let alloc_id = match self.tcx.global_alloc(alloc_id) {
-                    GlobalAlloc::VTable(ty, trait_ref) => {
-                        self.tcx.vtable_allocation((ty, trait_ref))
-                    }
-                    _ => alloc_id,
-                };
                 let (base_addr, base_addr_space) = match self.tcx.global_alloc(alloc_id) {
                     GlobalAlloc::Memory(alloc) => {
                         let init = const_alloc_to_llvm(self, alloc);
@@ -264,7 +257,15 @@ impl<'ll, 'tcx> ConstMethods<'tcx> for CodegenCx<'ll, 'tcx> {
                         self.get_fn_addr(fn_instance.polymorphize(self.tcx)),
                         self.data_layout().instruction_address_space,
                     ),
-                    GlobalAlloc::VTable(..) => bug!("vtables are already handled"),
+                    GlobalAlloc::VTable(ty, trait_ref) => {
+                        let alloc = self
+                            .tcx
+                            .global_alloc(self.tcx.vtable_allocation((ty, trait_ref)))
+                            .unwrap_memory();
+                        let init = const_alloc_to_llvm(self, alloc);
+                        let value = self.static_addr_of(init, alloc.inner().align, None);
+                        (value, AddressSpace::DATA)
+                    }
                     GlobalAlloc::Static(def_id) => {
                         assert!(self.tcx.is_static(def_id));
                         assert!(!self.tcx.is_thread_local_static(def_id));