Check ZST via PassMode

compiler/rustc_codegen_llvm/src/builder/autodiff.rs

@@ -3,8 +3,9 @@ use std::ptr;
 use rustc_ast::expand::autodiff_attrs::{AutoDiffAttrs, DiffActivity, DiffMode};
 use rustc_codegen_ssa::common::TypeKind;
 use rustc_codegen_ssa::traits::{BaseTypeCodegenMethods, BuilderMethods};
-use rustc_middle::ty::{PseudoCanonicalInput, Ty, TyCtxt, TypingEnv};
+use rustc_middle::ty::{Instance, PseudoCanonicalInput, TyCtxt, TypingEnv};
 use rustc_middle::{bug, ty};
+use rustc_target::callconv::PassMode;
 use tracing::debug;
 
 use crate::builder::{Builder, PlaceRef, UNNAMED};
@@ -16,9 +17,12 @@ use crate::value::Value;
 
 pub(crate) fn adjust_activity_to_abi<'tcx>(
     tcx: TyCtxt<'tcx>,
-    fn_ty: Ty<'tcx>,
+    instance: Instance<'tcx>,
+    typing_env: TypingEnv<'tcx>,
     da: &mut Vec<DiffActivity>,
 ) {
+    let fn_ty = instance.ty(tcx, typing_env);
+
     if !matches!(fn_ty.kind(), ty::FnDef(..)) {
         bug!("expected fn def for autodiff, got {:?}", fn_ty);
     }
@@ -27,6 +31,13 @@ pub(crate) fn adjust_activity_to_abi<'tcx>(
     // All we do is decide how to handle the arguments.
     let sig = fn_ty.fn_sig(tcx).skip_binder();
 
+    // FIXME(Sa4dUs): pass proper varargs once we have support for differentiating variadic functions
+    let Ok(fn_abi) =
+        tcx.fn_abi_of_instance(typing_env.as_query_input((instance, ty::List::empty())))
+    else {
+        bug!("failed to get fn_abi of instance with empty varargs");
+    };
+
     let mut new_activities = vec![];
     let mut new_positions = vec![];
     let mut del_activities = 0;
@@ -91,10 +102,13 @@ pub(crate) fn adjust_activity_to_abi<'tcx>(
             }
         };
 
+        let pass_mode = &fn_abi.args[i].mode;
+
         // For ZST, just ignore and don't add its activity, as this arg won't be present
         // in the LLVM passed to Enzyme.
+        // Some targets pass ZST indirectly in the C ABI, in that case, handle it as a normal arg
         // FIXME(Sa4dUs): Enforce ZST corresponding diff activity be `Const`
-        if layout.is_zst() {
+        if *pass_mode == PassMode::Ignore {
             del_activities += 1;
             da.remove(i);
         }

compiler/rustc_codegen_llvm/src/intrinsic.rs

@@ -1198,7 +1198,8 @@ fn codegen_autodiff<'ll, 'tcx>(
 
     adjust_activity_to_abi(
         tcx,
-        fn_source.ty(tcx, TypingEnv::fully_monomorphized()),
+        fn_source,
+        TypingEnv::fully_monomorphized(),
         &mut diff_attrs.input_activity,
     );