handle sret for scalar autodiff

Author: ZuseZ4

Diff for: compiler/rustc_ast/src/expand/autodiff_attrs.rs

@@ -92,6 +92,12 @@ pub struct AutoDiffAttrs {
     pub input_activity: Vec<DiffActivity>,
 }
 
+impl AutoDiffAttrs {
+    pub fn has_primal_ret(&self) -> bool {
+        matches!(self.ret_activity, DiffActivity::Active | DiffActivity::Dual)
+    }
+}
+
 impl DiffMode {
     pub fn is_rev(&self) -> bool {
         matches!(self, DiffMode::Reverse)

Diff for: compiler/rustc_codegen_llvm/src/builder/autodiff.rs

@@ -201,7 +201,23 @@ fn compute_enzyme_fn_ty<'ll>(
         }
 
         if attrs.width == 1 {
-            todo!("Handle sret for scalar ad");
+            // Enzyme returns a struct of style:
+            // `{ original_ret(if requested), float, float, ... }`
+            let mut struct_elements = vec![];
+            if attrs.has_primal_ret() {
+                struct_elements.push(inner_ret_ty);
+            }
+            // Next, we push the list of active floats, since they will be lowered to `enzyme_out`,
+            // and therefore part of the return struct.
+            let param_tys = cx.func_params_types(fn_ty);
+            for (act, param_ty) in attrs.input_activity.iter().zip(param_tys) {
+                if matches!(act, DiffActivity::Active) {
+                    // Now find the float type at position i based on the fn_ty,
+                    // to know what (f16/f32/f64/...) to add to the struct.
+                    struct_elements.push(param_ty);
+                }
+            }
+            ret_ty = cx.type_struct(&struct_elements, false);
         } else {
             // First we check if we also have to deal with the primal return.
             match attrs.mode {
@@ -388,7 +404,11 @@ fn generate_enzyme_call<'ll>(
                 // now store the result of the enzyme call into the sret pointer.
                 let sret_ptr = outer_args[0];
                 let call_ty = cx.val_ty(call);
-                assert_eq!(cx.type_kind(call_ty), TypeKind::Array);
+                if attrs.width == 1 {
+                    assert_eq!(cx.type_kind(call_ty), TypeKind::Struct);
+                } else {
+                    assert_eq!(cx.type_kind(call_ty), TypeKind::Array);
+                }
                 llvm::LLVMBuildStore(&builder.llbuilder, call, sret_ptr);
             }
             builder.ret_void();