codegen: implement repr(scalable)

Introduces `BackendRepr::ScalableVector` corresponding to scalable
vector types annotated with `repr(scalable)` which lowers to a scalable
vector type in LLVM.

Co-authored-by: Jamie Cunliffe <Jamie.Cunliffe@arm.com>

Author: davidtwco

src/builder.rs

@@ -943,6 +943,10 @@ impl<'a, 'gcc, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'gcc, 'tcx> {
             .get_address(self.location)
     }
 
+    fn scalable_alloca(&mut self, _elt: u64, _align: Align, _element_ty: Ty<'_>) -> RValue<'gcc> {
+        todo!()
+    }
+
     fn load(&mut self, pointee_ty: Type<'gcc>, ptr: RValue<'gcc>, align: Align) -> RValue<'gcc> {
         let block = self.llbb();
         let function = block.get_function();

src/intrinsic/mod.rs

@@ -504,7 +504,7 @@ impl<'a, 'gcc, 'tcx> IntrinsicCallBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tc
                 let layout = self.layout_of(tp_ty).layout;
                 let _use_integer_compare = match layout.backend_repr() {
                     Scalar(_) | ScalarPair(_, _) => true,
-                    SimdVector { .. } => false,
+                    SimdVector { .. } | ScalableVector { .. } => false,
                     Memory { .. } => {
                         // For rusty ABIs, small aggregates are actually passed
                         // as `RegKind::Integer` (see `FnAbi::adjust_for_abi`),

src/type_of.rs

@@ -85,6 +85,7 @@ fn uncached_gcc_type<'gcc, 'tcx>(
             );
         }
         BackendRepr::Memory { .. } => {}
+        BackendRepr::ScalableVector { .. } => todo!(),
     }
 
     let name = match *layout.ty.kind() {
@@ -179,6 +180,8 @@ impl<'tcx> LayoutGccExt<'tcx> for TyAndLayout<'tcx> {
     fn is_gcc_immediate(&self) -> bool {
         match self.backend_repr {
             BackendRepr::Scalar(_) | BackendRepr::SimdVector { .. } => true,
+            // FIXME(rustc_scalable_vector): Not yet implemented in rustc_codegen_gcc.
+            BackendRepr::ScalableVector { .. } => todo!(),
             BackendRepr::ScalarPair(..) | BackendRepr::Memory { .. } => false,
         }
     }
@@ -188,6 +191,7 @@ impl<'tcx> LayoutGccExt<'tcx> for TyAndLayout<'tcx> {
             BackendRepr::ScalarPair(..) => true,
             BackendRepr::Scalar(_)
             | BackendRepr::SimdVector { .. }
+            | BackendRepr::ScalableVector { .. }
             | BackendRepr::Memory { .. } => false,
         }
     }