refactor(autodiff): Simplify rlib dep handling; use fn_ptr_ty in adjust_activity_to_abi, drop mono-collection & cross-crate-inline workarounds

Author: TKanX

compiler/rustc_codegen_llvm/src/builder/autodiff.rs

@@ -6,7 +6,7 @@ use rustc_codegen_ssa::common::TypeKind;
 use rustc_codegen_ssa::traits::{BaseTypeCodegenMethods, BuilderMethods};
 use rustc_data_structures::thin_vec::ThinVec;
 use rustc_hir::attrs::RustcAutodiff;
-use rustc_middle::ty::{Instance, PseudoCanonicalInput, TyCtxt, TypingEnv};
+use rustc_middle::ty::{PseudoCanonicalInput, Ty, TyCtxt, TypingEnv};
 use rustc_middle::{bug, ty};
 use rustc_target::callconv::PassMode;
 use tracing::debug;
@@ -18,25 +18,23 @@ use crate::llvm::{self, TRUE, Type, Value};
 
 pub(crate) fn adjust_activity_to_abi<'tcx>(
     tcx: TyCtxt<'tcx>,
-    instance: Instance<'tcx>,
+    fn_ptr_ty: Ty<'tcx>,
     typing_env: TypingEnv<'tcx>,
     da: &mut ThinVec<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);
+    if !matches!(fn_ptr_ty.kind(), ty::FnPtr(..)) {
+        bug!("expected fn ptr for autodiff, got {:?}", fn_ptr_ty);
     }
 
     // We don't actually pass the types back into the type system.
     // All we do is decide how to handle the arguments.
-    let sig = fn_ty.fn_sig(tcx).skip_binder();
+    let fn_sig = fn_ptr_ty.fn_sig(tcx);
+    let sig = fn_sig.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())))
+    let Ok(fn_abi) = tcx.fn_abi_of_fn_ptr(typing_env.as_query_input((fn_sig, ty::List::empty())))
     else {
-        bug!("failed to get fn_abi of instance with empty varargs");
+        bug!("failed to get fn_abi of fn_ptr with empty varargs");
     };
 
     let mut new_activities = vec![];

compiler/rustc_codegen_llvm/src/intrinsic.rs

@@ -1374,9 +1374,11 @@ fn codegen_autodiff<'ll, 'tcx>(
         bug!("could not find autodiff attrs")
     };
 
+    let fn_ptr_ty =
+        Ty::new_fn_ptr(tcx, fn_source.ty(tcx, TypingEnv::fully_monomorphized()).fn_sig(tcx));
     adjust_activity_to_abi(
         tcx,
-        fn_source,
+        fn_ptr_ty,
         TypingEnv::fully_monomorphized(),
         &mut diff_attrs.input_activity,
     );

compiler/rustc_mir_transform/src/cross_crate_inline.rs

@@ -35,11 +35,6 @@ fn cross_crate_inlinable(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
         return true;
     }
 
-    // FIXME(autodiff): replace this as per discussion in https://github.com/rust-lang/rust/pull/149033#discussion_r2535465880
-    if find_attr!(tcx, def_id, RustcAutodiff(..)) {
-        return true;
-    }
-
     if find_attr!(tcx, def_id, RustcIntrinsic) {
         // Intrinsic fallback bodies are always cross-crate inlineable.
         // To ensure that the MIR inliner doesn't cluelessly try to inline fallback

compiler/rustc_monomorphize/src/collector.rs

@@ -205,8 +205,6 @@
 //! this is not implemented however: a mono item will be produced
 //! regardless of whether it is actually needed or not.
 
-mod autodiff;
-
 use std::cell::OnceCell;
 use std::ops::ControlFlow;
 
@@ -240,7 +238,6 @@ use rustc_span::source_map::{Spanned, dummy_spanned, respan};
 use rustc_span::{DUMMY_SP, Span};
 use tracing::{debug, instrument, trace};
 
-use crate::collector::autodiff::collect_autodiff_fn;
 use crate::errors::{
     self, EncounteredErrorWhileInstantiating, EncounteredErrorWhileInstantiatingGlobalAsm,
     NoOptimizedMir, RecursionLimit,
@@ -990,8 +987,6 @@ fn visit_instance_use<'tcx>(
         return;
     }
     if let Some(intrinsic) = tcx.intrinsic(instance.def_id()) {
-        collect_autodiff_fn(tcx, instance, intrinsic, output);
-
         if let Some(_requirement) = ValidityRequirement::from_intrinsic(intrinsic.name) {
             // The intrinsics assert_inhabited, assert_zero_valid, and assert_mem_uninitialized_valid will
             // be lowered in codegen to nothing or a call to panic_nounwind. So if we encounter any