Rollup merge of #138627 - EnzymeAD:autodiff-cleanups, r=oli-obk

Autodiff cleanups

Splitting out some cleanups to reduce the size of my batching PR and simplify ``@haenoe`` 's [PR](https://github.com/rust-lang/rust/pull/138314).

r? ``@oli-obk``

Tracking:

- https://github.com/rust-lang/rust/issues/124509

1 files changed, 95 insertions, 58 deletions

diff --git a/compiler/rustc_builtin_macros/src/autodiff.rs b/compiler/rustc_builtin_macros/src/autodiff.rs
index dcd3c1ce8d9..6591ed151cf 100644
--- a/compiler/rustc_builtin_macros/src/autodiff.rs
+++ b/compiler/rustc_builtin_macros/src/autodiff.rs
@@ -26,6 +26,16 @@ mod llvm_enzyme {
 
     use crate::errors;
 
+    pub(crate) fn outer_normal_attr(
+        kind: &P<rustc_ast::NormalAttr>,
+        id: rustc_ast::AttrId,
+        span: Span,
+    ) -> rustc_ast::Attribute {
+        let style = rustc_ast::AttrStyle::Outer;
+        let kind = rustc_ast::AttrKind::Normal(kind.clone());
+        rustc_ast::Attribute { kind, id, style, span }
+    }
+
     // If we have a default `()` return type or explicitley `()` return type,
     // then we often can skip doing some work.
     fn has_ret(ty: &FnRetTy) -> bool {
@@ -224,20 +234,8 @@ mod llvm_enzyme {
             .filter(|a| **a == DiffActivity::Active || **a == DiffActivity::ActiveOnly)
             .count() as u32;
         let (d_sig, new_args, idents, errored) = gen_enzyme_decl(ecx, &sig, &x, span);
-        let new_decl_span = d_sig.span;
         let d_body = gen_enzyme_body(
-            ecx,
-            &x,
-            n_active,
-            &sig,
-            &d_sig,
-            primal,
-            &new_args,
-            span,
-            sig_span,
-            new_decl_span,
-            idents,
-            errored,
+            ecx, &x, n_active, &sig, &d_sig, primal, &new_args, span, sig_span, idents, errored,
         );
         let d_ident = first_ident(&meta_item_vec[0]);
 
@@ -270,36 +268,39 @@ mod llvm_enzyme {
         };
         let inline_never_attr = P(ast::NormalAttr { item: inline_item, tokens: None });
         let new_id = ecx.sess.psess.attr_id_generator.mk_attr_id();
-        let attr: ast::Attribute = ast::Attribute {
-            kind: ast::AttrKind::Normal(rustc_ad_attr.clone()),
-            id: new_id,
-            style: ast::AttrStyle::Outer,
-            span,
-        };
+        let attr = outer_normal_attr(&rustc_ad_attr, new_id, span);
         let new_id = ecx.sess.psess.attr_id_generator.mk_attr_id();
-        let inline_never: ast::Attribute = ast::Attribute {
-            kind: ast::AttrKind::Normal(inline_never_attr),
-            id: new_id,
-            style: ast::AttrStyle::Outer,
-            span,
-        };
+        let inline_never = outer_normal_attr(&inline_never_attr, new_id, span);
+
+        // We're avoid duplicating the attributes `#[rustc_autodiff]` and `#[inline(never)]`.
+        fn same_attribute(attr: &ast::AttrKind, item: &ast::AttrKind) -> bool {
+            match (attr, item) {
+                (ast::AttrKind::Normal(a), ast::AttrKind::Normal(b)) => {
+                    let a = &a.item.path;
+                    let b = &b.item.path;
+                    a.segments.len() == b.segments.len()
+                        && a.segments.iter().zip(b.segments.iter()).all(|(a, b)| a.ident == b.ident)
+                }
+                _ => false,
+            }
+        }
 
         // Don't add it multiple times:
         let orig_annotatable: Annotatable = match item {
             Annotatable::Item(ref mut iitem) => {
-                if !iitem.attrs.iter().any(|a| a.id == attr.id) {
+                if !iitem.attrs.iter().any(|a| same_attribute(&a.kind, &attr.kind)) {
                     iitem.attrs.push(attr);
                 }
-                if !iitem.attrs.iter().any(|a| a.id == inline_never.id) {
+                if !iitem.attrs.iter().any(|a| same_attribute(&a.kind, &inline_never.kind)) {
                     iitem.attrs.push(inline_never.clone());
                 }
                 Annotatable::Item(iitem.clone())
             }
             Annotatable::AssocItem(ref mut assoc_item, i @ Impl) => {
-                if !assoc_item.attrs.iter().any(|a| a.id == attr.id) {
+                if !assoc_item.attrs.iter().any(|a| same_attribute(&a.kind, &attr.kind)) {
                     assoc_item.attrs.push(attr);
                 }
-                if !assoc_item.attrs.iter().any(|a| a.id == inline_never.id) {
+                if !assoc_item.attrs.iter().any(|a| same_attribute(&a.kind, &inline_never.kind)) {
                     assoc_item.attrs.push(inline_never.clone());
                 }
                 Annotatable::AssocItem(assoc_item.clone(), i)
@@ -314,13 +315,7 @@ mod llvm_enzyme {
             delim: rustc_ast::token::Delimiter::Parenthesis,
             tokens: ts,
         });
-        let d_attr: ast::Attribute = ast::Attribute {
-            kind: ast::AttrKind::Normal(rustc_ad_attr.clone()),
-            id: new_id,
-            style: ast::AttrStyle::Outer,
-            span,
-        };
-
+        let d_attr = outer_normal_attr(&rustc_ad_attr, new_id, span);
         let d_annotatable = if is_impl {
             let assoc_item: AssocItemKind = ast::AssocItemKind::Fn(asdf);
             let d_fn = P(ast::AssocItem {
@@ -361,30 +356,27 @@ mod llvm_enzyme {
         ty
     }
 
-    /// We only want this function to type-check, since we will replace the body
-    /// later on llvm level. Using `loop {}` does not cover all return types anymore,
-    /// so instead we build something that should pass. We also add a inline_asm
-    /// line, as one more barrier for rustc to prevent inlining of this function.
-    /// FIXME(ZuseZ4): We still have cases of incorrect inlining across modules, see
-    /// <https://github.com/EnzymeAD/rust/issues/173>, so this isn't sufficient.
-    /// It also triggers an Enzyme crash if we due to a bug ever try to differentiate
-    /// this function (which should never happen, since it is only a placeholder).
-    /// Finally, we also add back_box usages of all input arguments, to prevent rustc
-    /// from optimizing any arguments away.
-    fn gen_enzyme_body(
+    // Will generate a body of the type:
+    // ```
+    // {
+    //   unsafe {
+    //   asm!("NOP");
+    //   }
+    //   ::core::hint::black_box(primal(args));
+    //   ::core::hint::black_box((args, ret));
+    //   <This part remains to be done by following function>
+    // }
+    // ```
+    fn init_body_helper(
         ecx: &ExtCtxt<'_>,
-        x: &AutoDiffAttrs,
-        n_active: u32,
-        sig: &ast::FnSig,
-        d_sig: &ast::FnSig,
+        span: Span,
         primal: Ident,
         new_names: &[String],
-        span: Span,
         sig_span: Span,
         new_decl_span: Span,
-        idents: Vec<Ident>,
+        idents: &[Ident],
         errored: bool,
-    ) -> P<ast::Block> {
+    ) -> (P<ast::Block>, P<ast::Expr>, P<ast::Expr>, P<ast::Expr>) {
         let blackbox_path = ecx.std_path(&[sym::hint, sym::black_box]);
         let noop = ast::InlineAsm {
             asm_macro: ast::AsmMacro::Asm,
@@ -433,6 +425,51 @@ mod llvm_enzyme {
         }
         body.stmts.push(ecx.stmt_semi(black_box_remaining_args));
 
+        (body, primal_call, black_box_primal_call, blackbox_call_expr)
+    }
+
+    /// We only want this function to type-check, since we will replace the body
+    /// later on llvm level. Using `loop {}` does not cover all return types anymore,
+    /// so instead we manually build something that should pass the type checker.
+    /// We also add a inline_asm line, as one more barrier for rustc to prevent inlining
+    /// or const propagation. inline_asm will also triggers an Enzyme crash if due to another
+    /// bug would ever try to accidentially differentiate this placeholder function body.
+    /// Finally, we also add back_box usages of all input arguments, to prevent rustc
+    /// from optimizing any arguments away.
+    fn gen_enzyme_body(
+        ecx: &ExtCtxt<'_>,
+        x: &AutoDiffAttrs,
+        n_active: u32,
+        sig: &ast::FnSig,
+        d_sig: &ast::FnSig,
+        primal: Ident,
+        new_names: &[String],
+        span: Span,
+        sig_span: Span,
+        idents: Vec<Ident>,
+        errored: bool,
+    ) -> P<ast::Block> {
+        let new_decl_span = d_sig.span;
+
+        // Just adding some default inline-asm and black_box usages to prevent early inlining
+        // and optimizations which alter the function signature.
+        //
+        // The bb_primal_call is the black_box call of the primal function. We keep it around,
+        // since it has the convenient property of returning the type of the primal function,
+        // Remember, we only care to match types here.
+        // No matter which return we pick, we always wrap it into a std::hint::black_box call,
+        // to prevent rustc from propagating it into the caller.
+        let (mut body, primal_call, bb_primal_call, bb_call_expr) = init_body_helper(
+            ecx,
+            span,
+            primal,
+            new_names,
+            sig_span,
+            new_decl_span,
+            &idents,
+            errored,
+        );
+
         if !has_ret(&d_sig.decl.output) {
             // there is no return type that we have to match, () works fine.
             return body;
@@ -444,7 +481,7 @@ mod llvm_enzyme {
 
         if primal_ret && n_active == 0 && x.mode.is_rev() {
             // We only have the primal ret.
-            body.stmts.push(ecx.stmt_expr(black_box_primal_call));
+            body.stmts.push(ecx.stmt_expr(bb_primal_call));
             return body;
         }
 
@@ -536,11 +573,11 @@ mod llvm_enzyme {
                 return body;
             }
             [arg] => {
-                ret = ecx.expr_call(new_decl_span, blackbox_call_expr, thin_vec![arg.clone()]);
+                ret = ecx.expr_call(new_decl_span, bb_call_expr, thin_vec![arg.clone()]);
             }
             args => {
                 let ret_tuple: P<ast::Expr> = ecx.expr_tuple(span, args.into());
-                ret = ecx.expr_call(new_decl_span, blackbox_call_expr, thin_vec![ret_tuple]);
+                ret = ecx.expr_call(new_decl_span, bb_call_expr, thin_vec![ret_tuple]);
             }
         }
         assert!(has_ret(&d_sig.decl.output));
@@ -553,7 +590,7 @@ mod llvm_enzyme {
         ecx: &ExtCtxt<'_>,
         span: Span,
         primal: Ident,
-        idents: Vec<Ident>,
+        idents: &[Ident],
     ) -> P<ast::Expr> {
         let has_self = idents.len() > 0 && idents[0].name == kw::SelfLower;
         if has_self {