Rollup merge of #122320 - erikdesjardins:vtable, r=nikic

Use ptradd for vtable indexing

Extension of #121665.

After this, the only remaining usages of GEP are [this](https://github.com/rust-lang/rust/blob/cd81f5b27ee00b49d413db50b5e6af871cebcf23/compiler/rustc_codegen_llvm/src/intrinsic.rs#L909-L920) kinda janky Emscription EH code, which I'll change in a future PR, and array indexing / pointer offsets, where there isn't yet a canonical `ptradd` form. (Out of curiosity I tried converting the latter to `ptradd(ptr, mul(size, index))`, but that causes codegen regressions right now.)

r? `@nikic`

2 files changed, 87 insertions, 2 deletions

diff --git a/tests/codegen/dst-offset.rs b/tests/codegen/dst-offset.rs
index f0157e5a106..ce735baeb6a 100644
--- a/tests/codegen/dst-offset.rs
+++ b/tests/codegen/dst-offset.rs
@@ -25,9 +25,9 @@ struct Dst<T: ?Sized> {
 pub fn dst_dyn_trait_offset(s: &Dst<dyn Drop>) -> &dyn Drop {
 // The alignment of dyn trait is unknown, so we compute the offset based on align from the vtable.
 
-// CHECK: [[SIZE_PTR:%[0-9]+]] = getelementptr inbounds {{.+}} [[VTABLE_PTR]]
+// CHECK: [[SIZE_PTR:%[0-9]+]] = getelementptr inbounds i8, ptr [[VTABLE_PTR]]
 // CHECK: load [[USIZE]], ptr [[SIZE_PTR]]
-// CHECK: [[ALIGN_PTR:%[0-9]+]] = getelementptr inbounds {{.+}} [[VTABLE_PTR]]
+// CHECK: [[ALIGN_PTR:%[0-9]+]] = getelementptr inbounds i8, ptr [[VTABLE_PTR]]
 // CHECK: load [[USIZE]], ptr [[ALIGN_PTR]]
 
 // CHECK: getelementptr inbounds i8, ptr [[DATA_PTR]]
diff --git a/tests/codegen/vtable-upcast.rs b/tests/codegen/vtable-upcast.rs
new file mode 100644
index 00000000000..41a4be26cb4
--- /dev/null
+++ b/tests/codegen/vtable-upcast.rs
@@ -0,0 +1,85 @@
+//! This file tests that we correctly generate GEP instructions for vtable upcasting.
+//@ compile-flags: -C no-prepopulate-passes -Copt-level=0
+
+#![crate_type = "lib"]
+#![feature(trait_upcasting)]
+
+pub trait Base {
+    fn base(&self);
+}
+
+pub trait A : Base {
+    fn a(&self);
+}
+
+pub trait B : Base {
+    fn b(&self);
+}
+
+pub trait Diamond : A + B {
+    fn diamond(&self);
+}
+
+// CHECK-LABEL: upcast_a_to_base
+#[no_mangle]
+pub fn upcast_a_to_base(x: &dyn A) -> &dyn Base {
+    // Requires no adjustment, since its vtable is extended from `Base`.
+
+    // CHECK: start:
+    // CHECK-NEXT: insertvalue
+    // CHECK-NEXT: insertvalue
+    // CHECK-NEXT: ret
+    x as &dyn Base
+}
+
+// CHECK-LABEL: upcast_b_to_base
+#[no_mangle]
+pub fn upcast_b_to_base(x: &dyn B) -> &dyn Base {
+    // Requires no adjustment, since its vtable is extended from `Base`.
+
+    // CHECK: start:
+    // CHECK-NEXT: insertvalue
+    // CHECK-NEXT: insertvalue
+    // CHECK-NEXT: ret
+    x as &dyn Base
+}
+
+// CHECK-LABEL: upcast_diamond_to_a
+#[no_mangle]
+pub fn upcast_diamond_to_a(x: &dyn Diamond) -> &dyn A {
+    // Requires no adjustment, since its vtable is extended from `A` (as the first supertrait).
+
+    // CHECK: start:
+    // CHECK-NEXT: insertvalue
+    // CHECK-NEXT: insertvalue
+    // CHECK-NEXT: ret
+    x as &dyn A
+}
+
+// CHECK-LABEL: upcast_diamond_to_b
+// CHECK-SAME: (ptr align {{[0-9]+}} [[DATA_PTR:%.+]], ptr align {{[0-9]+}} [[VTABLE_PTR:%.+]])
+#[no_mangle]
+pub fn upcast_diamond_to_b(x: &dyn Diamond) -> &dyn B {
+    // Requires adjustment, since it's a non-first supertrait.
+
+    // CHECK: start:
+    // CHECK-NEXT: [[UPCAST_SLOT_PTR:%.+]] = getelementptr inbounds i8, ptr [[VTABLE_PTR]]
+    // CHECK-NEXT: [[UPCAST_VTABLE_PTR:%.+]] = load ptr, ptr [[UPCAST_SLOT_PTR]]
+    // CHECK-NEXT: [[FAT_PTR_1:%.+]] = insertvalue { ptr, ptr } poison, ptr [[DATA_PTR]], 0
+    // CHECK-NEXT: [[FAT_PTR_2:%.+]] = insertvalue { ptr, ptr } [[FAT_PTR_1]], ptr [[UPCAST_VTABLE_PTR]], 1
+    // CHECK-NEXT: ret { ptr, ptr } [[FAT_PTR_2]]
+    x as &dyn B
+}
+
+// CHECK-LABEL: upcast_diamond_to_b
+#[no_mangle]
+pub fn upcast_diamond_to_base(x: &dyn Diamond) -> &dyn Base {
+    // Requires no adjustment, since `Base` is the first supertrait of `A`,
+    // which is the first supertrait of `Diamond`.
+
+    // CHECK: start:
+    // CHECK-NEXT: insertvalue
+    // CHECK-NEXT: insertvalue
+    // CHECK-NEXT: ret
+    x as &dyn Base
+}