rustc: pack size and alignment in vtables

xdoardo · xdoardo · commit 379047e6223a · 2025-12-05T14:54:37.000+01:00
diff --git a/cheri/tests/vtable.rs b/cheri/tests/vtable.rs
@@ -0,0 +1,55 @@
+#![no_std]
+
+extern crate alloc;
+extern crate cheriot;
+
+use alloc::string::String;
+
+trait Chair {
+    fn count_legs(&self) -> u32;
+}
+
+struct Monobloc {
+    dropme: String,
+}
+
+impl Chair for Monobloc {
+    fn count_legs(&self) -> u32 {
+        4
+    }
+}
+
+struct CuttyStool {}
+
+impl Chair for CuttyStool {
+    fn count_legs(&self) -> u32 {
+        3
+    }
+}
+
+struct RockingChair {}
+
+impl Chair for RockingChair {
+    fn count_legs(&self) -> u32 {
+        0
+    }
+}
+
+#[no_mangle]
+extern "C" fn test_vtable() -> i32 {
+    fn observe_chair(chair: &dyn Chair) -> u32 {
+        chair.count_legs()
+    }
+    core::hint::black_box({
+        let chair = Monobloc { dropme: String::new() };
+        assert_eq!(observe_chair(&chair), 4);
+
+        let chair = CuttyStool {};
+        assert_eq!(observe_chair(&chair), 3);
+
+        let chair = RockingChair {};
+        assert_eq!(observe_chair(&chair), 0);
+    });
+
+    0
+}
diff --git a/compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs b/compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs
@@ -1440,32 +1440,35 @@ fn build_vtable_type_di_node<'ll, 'tcx>(
 ) -> &'ll DIType {
     let tcx = cx.tcx;
 
-    let vtable_entries = if let Some(poly_trait_ref) = poly_trait_ref {
-        let trait_ref = poly_trait_ref.with_self_ty(tcx, ty);
-        let trait_ref = tcx.erase_and_anonymize_regions(trait_ref);
-
-        tcx.vtable_entries(trait_ref)
-    } else {
-        TyCtxt::COMMON_VTABLE_ENTRIES
-    };
+    // FIXME(xdoardo): Figure out to remove the overhead for the computations relative to the sizes
+    // of the entries happens only for targets that need it (i.e. those where sizeof(usize) !=
+    // sizeof(ptr)
 
     // All function pointers are described as opaque pointers. This could be improved in the future
     // by describing them as actual function pointers.
     let void_pointer_ty = Ty::new_imm_ptr(tcx, tcx.types.unit);
     let void_pointer_type_di_node = type_di_node(cx, void_pointer_ty);
     let usize_di_node = type_di_node(cx, tcx.types.usize);
+    let usize_layout = cx.layout_of(tcx.types.usize);
     let pointer_layout = cx.layout_of(void_pointer_ty);
-    let pointer_size = pointer_layout.size;
     let pointer_align = pointer_layout.align.abi;
-    // If `usize` is not pointer-sized and -aligned then the size and alignment computations
-    // for the vtable as a whole would be wrong. Let's make sure this holds even on weird
-    // platforms.
-    assert_eq!(cx.size_and_align_of(tcx.types.usize), (pointer_size, pointer_align));
+    let size_zero = Size::from_bits(0);
+
+    let vtable_entries = if let Some(poly_trait_ref) = poly_trait_ref {
+        let trait_ref = poly_trait_ref.with_self_ty(tcx, ty);
+        let trait_ref = tcx.erase_and_anonymize_regions(trait_ref);
+
+        tcx.vtable_entries(trait_ref)
+    } else {
+        TyCtxt::COMMON_VTABLE_ENTRIES
+    };
 
     let vtable_type_name =
         compute_debuginfo_vtable_name(cx.tcx, ty, poly_trait_ref, VTableNameKind::Type);
     let unique_type_id = UniqueTypeId::for_vtable_ty(tcx, ty, poly_trait_ref);
-    let size = pointer_size * vtable_entries.len() as u64;
+    let entries_size = vtable_entries.iter().map(|e| e.memory_size(&tcx)).collect::<Vec<_>>();
+    let size = entries_size.iter().fold(size_zero, |a, s| a + *s);
+    let mut field_offset = size_zero;
 
     // This gets mapped to a DW_AT_containing_type attribute which allows GDB to correlate
     // the vtable to the type it is for.
@@ -1488,32 +1491,39 @@ fn build_vtable_type_di_node<'ll, 'tcx>(
                 .iter()
                 .enumerate()
                 .filter_map(|(index, vtable_entry)| {
-                    let (field_name, field_type_di_node) = match vtable_entry {
+                    let (field_name, layout, field_type_di_node) = match vtable_entry {
                         ty::VtblEntry::MetadataDropInPlace => {
-                            ("drop_in_place".to_string(), void_pointer_type_di_node)
+                            ("drop_in_place".to_string(), pointer_layout, void_pointer_type_di_node)
                         }
                         ty::VtblEntry::Method(_) => {
                             // Note: This code does not try to give a proper name to each method
                             //       because their might be multiple methods with the same name
                             //       (coming from different traits).
-                            (format!("__method{index}"), void_pointer_type_di_node)
+                            (format!("__method{index}"), pointer_layout, void_pointer_type_di_node)
+                        }
+                        ty::VtblEntry::TraitVPtr(_) => (
+                            format!("__super_trait_ptr{index}"),
+                            pointer_layout,
+                            void_pointer_type_di_node,
+                        ),
+                        ty::VtblEntry::MetadataAlign => {
+                            ("align".to_string(), usize_layout, usize_di_node)
                         }
-                        ty::VtblEntry::TraitVPtr(_) => {
-                            (format!("__super_trait_ptr{index}"), void_pointer_type_di_node)
+                        ty::VtblEntry::MetadataSize => {
+                            ("size".to_string(), usize_layout, usize_di_node)
                         }
-                        ty::VtblEntry::MetadataAlign => ("align".to_string(), usize_di_node),
-                        ty::VtblEntry::MetadataSize => ("size".to_string(), usize_di_node),
                         ty::VtblEntry::Vacant => return None,
                     };
 
-                    let field_offset = pointer_size * index as u64;
+                    let current_offset = field_offset;
+                    field_offset += entries_size[index];
 
                     Some(build_field_di_node(
                         cx,
                         vtable_type_di_node,
                         &field_name,
-                        pointer_layout,
-                        field_offset,
+                        layout,
+                        current_offset,
                         DIFlags::FlagZero,
                         field_type_di_node,
                         None,
diff --git a/compiler/rustc_codegen_ssa/src/meth.rs b/compiler/rustc_codegen_ssa/src/meth.rs
@@ -28,7 +28,19 @@ impl<'a, 'tcx> VirtualIndex {
 
         let llty = bx.fn_ptr_backend_type(fn_abi);
         let ptr_size = bx.data_layout().pointer_size();
-        let vtable_byte_offset = self.0 * ptr_size.bytes();
+        let usize_size = bx.data_layout().pointer_offset();
+
+        // FIXME(xdoardo) Find a way to remove the overhead for this comparison too?
+        let vtable_byte_offset = if ptr_size == usize_size {
+            ptr_size.bytes() * self.0
+        } else {
+            match self.0 {
+                0 => 0,
+                1 => usize_size.bytes(),
+                2 => usize_size.bytes() * 2,
+                n => usize_size.bytes() * 2 + ptr_size.bytes() * (n - 2),
+            }
+        };
 
         load_vtable(bx, llvtable, llty, vtable_byte_offset, ty, nonnull)
     }
diff --git a/compiler/rustc_middle/src/ty/vtable.rs b/compiler/rustc_middle/src/ty/vtable.rs
@@ -25,6 +25,25 @@ pub enum VtblEntry<'tcx> {
     TraitVPtr(TraitRef<'tcx>),
 }
 
+impl<'tcx> VtblEntry<'tcx> {
+    /// Return the [`rustc_abi::Size`] it takes to represent in-memory the given entry kind.
+    pub fn memory_size(&self, ctxt: &impl rustc_abi::HasDataLayout) -> rustc_abi::Size {
+        let dl = ctxt.data_layout();
+        match self {
+            VtblEntry::MetadataSize | VtblEntry::MetadataAlign => dl.pointer_offset(),
+            VtblEntry::MetadataDropInPlace
+            | VtblEntry::Method(_)
+            | VtblEntry::TraitVPtr(_)
+            | VtblEntry::Vacant => dl.pointer_size(),
+        }
+    }
+
+    /// Return the [`rustc_abi::Size`] of the data the given entry kind can contain.
+    pub fn data_size(&self, ctxt: &impl rustc_abi::HasDataLayout) -> rustc_abi::Size {
+        ctxt.data_layout().pointer_offset()
+    }
+}
+
 impl<'tcx> fmt::Debug for VtblEntry<'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         // We want to call `Display` on `Instance` and `PolyTraitRef`,
@@ -85,6 +104,11 @@ pub(super) fn vtable_allocation_provider<'tcx>(
     tcx: TyCtxt<'tcx>,
     key: (Ty<'tcx>, Option<ty::ExistentialTraitRef<'tcx>>),
 ) -> AllocId {
+
+    // FIXME(xdoardo): Figure out to remove the overhead for the computations relative to the sizes
+    // of the entries happens only for targets that need it (i.e. those where sizeof(usize) !=
+    // sizeof(ptr)
+
     let (ty, poly_trait_ref) = key;
 
     let vtable_entries = if let Some(poly_trait_ref) = poly_trait_ref {
@@ -106,19 +130,25 @@ pub(super) fn vtable_allocation_provider<'tcx>(
     let size = layout.size.bytes();
     let align = layout.align.bytes();
 
-    let ptr_size = tcx.data_layout.pointer_size();
-    let ptr_capacity = tcx.data_layout.pointer_offset();
+    let usize_size = tcx.data_layout.pointer_offset();
     let ptr_align = tcx.data_layout.pointer_align().abi;
+    let size_zero = rustc_abi::Size::from_bits(0);
 
-    let vtable_size = ptr_size * u64::try_from(vtable_entries.len()).unwrap();
+    let entries_memory_size =
+        vtable_entries.iter().map(|e| e.memory_size(&tcx)).collect::<Vec<_>>();
+    let entries_data_size = vtable_entries.iter().map(|e| e.data_size(&tcx)).collect::<Vec<_>>();
+    let vtable_size = entries_memory_size.iter().fold(size_zero, |a, s| a + *s);
     let mut vtable = Allocation::new(vtable_size, ptr_align, AllocInit::Uninit, ());
+    let mut field_offset = size_zero;
 
-    // No need to do any alignment checks on the memory accesses below, because we know the
-    // allocation is correctly aligned as we created it above. Also we're only offsetting by
-    // multiples of `ptr_align`, which means that it will stay aligned to `ptr_align`.
+    // We must check that the offsetting in steps of `usizes` does not break the alignment
+    // requirements of the other entries.
+    assert!(
+        usize_size.bits() % ptr_align.bits() == 0 || ptr_align.bits() % usize_size.bits() == 0,
+        "usize_size: {usize_size:?}, ptr_alignment: {ptr_align:?}"
+    );
 
     for (idx, entry) in vtable_entries.iter().enumerate() {
-        let idx: u64 = u64::try_from(idx).unwrap();
         let scalar = match *entry {
             VtblEntry::MetadataDropInPlace => {
                 if ty.needs_drop(tcx, ty::TypingEnv::fully_monomorphized()) {
@@ -130,9 +160,12 @@ pub(super) fn vtable_allocation_provider<'tcx>(
                     Scalar::from_maybe_pointer(Pointer::null(), &tcx)
                 }
             }
-            VtblEntry::MetadataSize => Scalar::from_uint(size, ptr_capacity),
-            VtblEntry::MetadataAlign => Scalar::from_uint(align, ptr_capacity),
-            VtblEntry::Vacant => continue,
+            VtblEntry::MetadataSize => Scalar::from_uint(size, usize_size),
+            VtblEntry::MetadataAlign => Scalar::from_uint(align, usize_size),
+            VtblEntry::Vacant => {
+                field_offset += entries_memory_size[idx];
+                continue;
+            }
             VtblEntry::Method(instance) => {
                 // Prepare the fn ptr we write into the vtable.
                 let fn_alloc_id = tcx.reserve_and_set_fn_alloc(instance, CTFE_ALLOC_SALT);
@@ -146,8 +179,13 @@ pub(super) fn vtable_allocation_provider<'tcx>(
                 Scalar::from_pointer(vptr, &tcx)
             }
         };
+
+        let current_offset = field_offset;
+        let field_data_size = entries_data_size[idx];
+        field_offset += entries_memory_size[idx];
+
         vtable
-            .write_scalar(&tcx, alloc_range(ptr_size * idx, ptr_capacity), scalar)
+            .write_scalar(&tcx, alloc_range(current_offset, field_data_size), scalar)
             .expect("failed to build vtable representation");
     }
 
diff --git a/tests/codegen-llvm/cheri/debug-vtable.rs b/tests/codegen-llvm/cheri/debug-vtable.rs
@@ -0,0 +1,62 @@
+//@ revisions: cheriot
+//@ [cheriot] compile-flags: --target riscv32cheriot-unknown-cheriotrtos -Cdebuginfo=2 -Copt-level=0 -Csymbol-mangling-version=v0
+//@ [cheriot] needs-llvm-components: riscv
+
+#![crate_type = "lib"]
+#![no_std]
+
+extern crate alloc;
+use alloc::boxed::Box;
+
+// cheriot: @ptable
+// Force emission for debuginfo for usize and *const() early..
+pub static mut XYZ: Option<(usize, *const ())> = None;
+
+pub struct Foo {
+    dropme: alloc::string::String,
+}
+
+pub trait SomeTrait {
+    fn method1(&self) -> u32;
+    fn method2(&self) -> u32;
+}
+
+impl SomeTrait for Foo {
+    fn method1(&self) -> u32 {
+        1
+    }
+    fn method2(&self) -> u32 {
+        2
+    }
+}
+
+pub trait SomeTraitWithGenerics<T, U> {
+    fn method1(&self) -> (T, U);
+}
+
+impl SomeTraitWithGenerics<u64, i8> for Foo {
+    fn method1(&self) -> (u64, i8) {
+        (1, 2)
+    }
+}
+
+pub fn foo(x: &Foo) -> (u32, (u64, i8), &dyn Send) {
+    let y: &dyn SomeTrait = x;
+    let z: &dyn SomeTraitWithGenerics<u64, i8> = x;
+    (y.method1(), z.method1(), x as &dyn Send)
+}
+
+// Constructing the debuginfo name for the FnOnce vtable below initially caused an ICE on MSVC
+// because the trait type contains a late bound region that needed to be erased before the type
+// layout for the niche enum `Option<&dyn Fn()>` could be computed.
+pub fn bar() -> Box<dyn FnOnce(Option<&dyn Fn()>)> {
+    Box::new(|_x: Option<&dyn Fn()>| {})
+}
+
+fn generic_closure<T: 'static>(x: T) -> Box<dyn FnOnce() -> T> {
+    Box::new(move || x)
+}
+
+pub fn instantiate_generic_closures() -> (Box<dyn FnOnce() -> u32>, Box<dyn FnOnce() -> bool>) {
+    (generic_closure(1u32), generic_closure(false))
+}
