Split autodiff into autodiff_forward and autodiff_reverse

compiler/rustc_builtin_macros/messages.ftl

@@ -71,7 +71,6 @@ builtin_macros_assert_requires_expression = macro requires an expression as an a
 
 builtin_macros_autodiff = autodiff must be applied to function
 builtin_macros_autodiff_missing_config = autodiff requires at least a name and mode
-builtin_macros_autodiff_mode = unknown Mode: `{$mode}`. Use `Forward` or `Reverse`
 builtin_macros_autodiff_mode_activity = {$act} can not be used in {$mode} Mode
 builtin_macros_autodiff_not_build = this rustc version does not support autodiff
 builtin_macros_autodiff_number_activities = expected {$expected} activities, but found {$found}

compiler/rustc_builtin_macros/src/autodiff.rs

@@ -86,27 +86,23 @@ mod llvm_enzyme {
         ecx: &mut ExtCtxt<'_>,
         meta_item: &ThinVec<MetaItemInner>,
         has_ret: bool,
+        mode: DiffMode,
     ) -> AutoDiffAttrs {
         let dcx = ecx.sess.dcx();
-        let mode = name(&meta_item[1]);
-        let Ok(mode) = DiffMode::from_str(&mode) else {
-            dcx.emit_err(errors::AutoDiffInvalidMode { span: meta_item[1].span(), mode });
-            return AutoDiffAttrs::error();
-        };
 
         // Now we check, whether the user wants autodiff in batch/vector mode, or scalar mode.
         // If he doesn't specify an integer (=width), we default to scalar mode, thus width=1.
-        let mut first_activity = 2;
+        let mut first_activity = 1;
 
-        let width = if let [_, _, x, ..] = &meta_item[..]
+        let width = if let [_, x, ..] = &meta_item[..]
             && let Some(x) = width(x)
         {
-            first_activity = 3;
+            first_activity = 2;
             match x.try_into() {
                 Ok(x) => x,
                 Err(_) => {
                     dcx.emit_err(errors::AutoDiffInvalidWidth {
-                        span: meta_item[2].span(),
+                        span: meta_item[1].span(),
                         width: x,
                     });
                     return AutoDiffAttrs::error();
@@ -165,6 +161,24 @@ mod llvm_enzyme {
         ts.push(TokenTree::Token(comma.clone(), Spacing::Alone));
     }
 
+    pub(crate) fn expand_forward(
+        ecx: &mut ExtCtxt<'_>,
+        expand_span: Span,
+        meta_item: &ast::MetaItem,
+        item: Annotatable,
+    ) -> Vec<Annotatable> {
+        expand_with_mode(ecx, expand_span, meta_item, item, DiffMode::Forward)
+    }
+
+    pub(crate) fn expand_reverse(
+        ecx: &mut ExtCtxt<'_>,
+        expand_span: Span,
+        meta_item: &ast::MetaItem,
+        item: Annotatable,
+    ) -> Vec<Annotatable> {
+        expand_with_mode(ecx, expand_span, meta_item, item, DiffMode::Reverse)
+    }
+
     /// We expand the autodiff macro to generate a new placeholder function which passes
     /// type-checking and can be called by users. The function body of the placeholder function will
     /// later be replaced on LLVM-IR level, so the design of the body is less important and for now
@@ -198,11 +212,12 @@ mod llvm_enzyme {
     /// ```
     /// FIXME(ZuseZ4): Once autodiff is enabled by default, make this a doc comment which is checked
     /// in CI.
-    pub(crate) fn expand(
+    pub(crate) fn expand_with_mode(
         ecx: &mut ExtCtxt<'_>,
         expand_span: Span,
         meta_item: &ast::MetaItem,
         mut item: Annotatable,
+        mode: DiffMode,
     ) -> Vec<Annotatable> {
         if cfg!(not(llvm_enzyme)) {
             ecx.sess.dcx().emit_err(errors::AutoDiffSupportNotBuild { span: meta_item.span });
@@ -243,29 +258,41 @@ mod llvm_enzyme {
         // create TokenStream from vec elemtents:
         // meta_item doesn't have a .tokens field
         let mut ts: Vec<TokenTree> = vec![];
-        if meta_item_vec.len() < 2 {
-            // At the bare minimum, we need a fnc name and a mode, even for a dummy function with no
-            // input and output args.
+        if meta_item_vec.len() < 1 {
+            // At the bare minimum, we need a fnc name.
             dcx.emit_err(errors::AutoDiffMissingConfig { span: item.span() });
             return vec![item];
         }
 
-        meta_item_inner_to_ts(&meta_item_vec[1], &mut ts);
+        let mode_symbol = match mode {
+            DiffMode::Forward => sym::Forward,
+            DiffMode::Reverse => sym::Reverse,
+            _ => unreachable!("Unsupported mode: {:?}", mode),
+        };
+
+        // Insert mode token
+        let mode_token = Token::new(TokenKind::Ident(mode_symbol, false.into()), Span::default());
+        ts.insert(0, TokenTree::Token(mode_token, Spacing::Joint));
+        ts.insert(
+            1,
+            TokenTree::Token(Token::new(TokenKind::Comma, Span::default()), Spacing::Alone),
+        );
 
         // Now, if the user gave a width (vector aka batch-mode ad), then we copy it.
         // If it is not given, we default to 1 (scalar mode).
         let start_position;
         let kind: LitKind = LitKind::Integer;
         let symbol;
-        if meta_item_vec.len() >= 3
-            && let Some(width) = width(&meta_item_vec[2])
+        if meta_item_vec.len() >= 2
+            && let Some(width) = width(&meta_item_vec[1])
         {
-            start_position = 3;
+            start_position = 2;
             symbol = Symbol::intern(&width.to_string());
         } else {
-            start_position = 2;
+            start_position = 1;
             symbol = sym::integer(1);
         }
+
         let l: Lit = Lit { kind, symbol, suffix: None };
         let t = Token::new(TokenKind::Literal(l), Span::default());
         let comma = Token::new(TokenKind::Comma, Span::default());
@@ -287,7 +314,7 @@ mod llvm_enzyme {
         ts.pop();
         let ts: TokenStream = TokenStream::from_iter(ts);
 
-        let x: AutoDiffAttrs = from_ast(ecx, &meta_item_vec, has_ret);
+        let x: AutoDiffAttrs = from_ast(ecx, &meta_item_vec, has_ret, mode);
         if !x.is_active() {
             // We encountered an error, so we return the original item.
             // This allows us to potentially parse other attributes.
@@ -966,4 +993,4 @@ mod llvm_enzyme {
     }
 }
 
-pub(crate) use llvm_enzyme::expand;
+pub(crate) use llvm_enzyme::{expand_forward, expand_reverse};

compiler/rustc_builtin_macros/src/errors.rs

@@ -187,14 +187,6 @@ mod autodiff {
         pub(crate) act: String,
     }
 
-    #[derive(Diagnostic)]
-    #[diag(builtin_macros_autodiff_mode)]
-    pub(crate) struct AutoDiffInvalidMode {
-        #[primary_span]
-        pub(crate) span: Span,
-        pub(crate) mode: String,
-    }
-
     #[derive(Diagnostic)]
     #[diag(builtin_macros_autodiff_width)]
     pub(crate) struct AutoDiffInvalidWidth {

compiler/rustc_builtin_macros/src/lib.rs

@@ -6,10 +6,10 @@
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::untranslatable_diagnostic)]
 #![cfg_attr(bootstrap, feature(let_chains))]
+#![cfg_attr(not(bootstrap), feature(autodiff))]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(assert_matches)]
-#![feature(autodiff)]
 #![feature(box_patterns)]
 #![feature(decl_macro)]
 #![feature(if_let_guard)]
@@ -113,7 +113,8 @@ pub fn register_builtin_macros(resolver: &mut dyn ResolverExpand) {
 
     register_attr! {
         alloc_error_handler: alloc_error_handler::expand,
-        autodiff: autodiff::expand,
+        autodiff_forward: autodiff::expand_forward,
+        autodiff_reverse: autodiff::expand_reverse,
         bench: test::expand_bench,
         cfg_accessible: cfg_accessible::Expander,
         cfg_eval: cfg_eval::expand,

compiler/rustc_passes/src/check_attr.rs

@@ -255,7 +255,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                             self.check_generic_attr(hir_id, attr, target, Target::Fn);
                             self.check_proc_macro(hir_id, target, ProcMacroKind::Derive)
                         }
-                        [sym::autodiff, ..] => {
+                        [sym::autodiff_forward, ..] | [sym::autodiff_reverse, ..] => {
                             self.check_autodiff(hir_id, attr, span, target)
                         }
                         [sym::coroutine, ..] => {

compiler/rustc_span/src/symbol.rs

@@ -253,6 +253,7 @@ symbols! {
         FnMut,
         FnOnce,
         Formatter,
+        Forward,
         From,
         FromIterator,
         FromResidual,
@@ -346,6 +347,7 @@ symbols! {
         Result,
         ResumeTy,
         Return,
+        Reverse,
         Right,
         Rust,
         RustaceansAreAwesome,
@@ -528,7 +530,8 @@ symbols! {
         audit_that,
         augmented_assignments,
         auto_traits,
-        autodiff,
+        autodiff_forward,
+        autodiff_reverse,
         automatically_derived,
         avx,
         avx512_target_feature,

library/core/src/lib.rs

@@ -226,10 +226,11 @@ pub mod assert_matches {
 
 // We don't export this through #[macro_export] for now, to avoid breakage.
 #[unstable(feature = "autodiff", issue = "124509")]
+#[cfg(not(bootstrap))]
 /// Unstable module containing the unstable `autodiff` macro.
 pub mod autodiff {
     #[unstable(feature = "autodiff", issue = "124509")]
-    pub use crate::macros::builtin::autodiff;
+    pub use crate::macros::builtin::{autodiff_forward, autodiff_reverse};
 }
 
 #[unstable(feature = "contracts", issue = "128044")]

library/core/src/macros/mod.rs

@@ -1519,20 +1519,40 @@ pub(crate) mod builtin {
         ($file:expr $(,)?) => {{ /* compiler built-in */ }};
     }
 
+    /// the derivative of a given function in the forward mode of differentiation.
+    /// It may only be applied to a function.
+    ///
+    /// The expected usage syntax is:
+    /// `#[autodiff_forward(NAME, INPUT_ACTIVITIES, OUTPUT_ACTIVITY)]`
+    ///
+    /// - `NAME`: A string that represents a valid function name.
+    /// - `INPUT_ACTIVITIES`: Specifies one valid activity for each input parameter.
+    /// - `OUTPUT_ACTIVITY`: Must not be set if the function implicitly returns nothing
+    ///   (or explicitly returns `-> ()`). Otherwise, it must be set to one of the allowed activities.
+    #[unstable(feature = "autodiff", issue = "124509")]
+    #[allow_internal_unstable(rustc_attrs)]
+    #[rustc_builtin_macro]
+    #[cfg(not(bootstrap))]
+    pub macro autodiff_forward($item:item) {
+        /* compiler built-in */
+    }
+
     /// Automatic Differentiation macro which allows generating a new function to compute
-    /// the derivative of a given function. It may only be applied to a function.
-    /// The expected usage syntax is
-    /// `#[autodiff(NAME, MODE, INPUT_ACTIVITIES, OUTPUT_ACTIVITY)]`
-    /// where:
-    /// NAME is a string that represents a valid function name.
-    /// MODE is any of Forward, Reverse, ForwardFirst, ReverseFirst.
-    /// INPUT_ACTIVITIES consists of one valid activity for each input parameter.
-    /// OUTPUT_ACTIVITY must not be set if we implicitly return nothing (or explicitly return
-    /// `-> ()`). Otherwise it must be set to one of the allowed activities.
+    /// the derivative of a given function in the reverse mode of differentiation.
+    /// It may only be applied to a function.
+    ///
+    /// The expected usage syntax is:
+    /// `#[autodiff_reverse(NAME, INPUT_ACTIVITIES, OUTPUT_ACTIVITY)]`
+    ///
+    /// - `NAME`: A string that represents a valid function name.
+    /// - `INPUT_ACTIVITIES`: Specifies one valid activity for each input parameter.
+    /// - `OUTPUT_ACTIVITY`: Must not be set if the function implicitly returns nothing
+    ///   (or explicitly returns `-> ()`). Otherwise, it must be set to one of the allowed activities.
     #[unstable(feature = "autodiff", issue = "124509")]
     #[allow_internal_unstable(rustc_attrs)]
     #[rustc_builtin_macro]
-    pub macro autodiff($item:item) {
+    #[cfg(not(bootstrap))]
+    pub macro autodiff_reverse($item:item) {
         /* compiler built-in */
     }
 

library/std/src/lib.rs

@@ -276,12 +276,12 @@
 // tidy-alphabetical-start
 
 // stabilization was reverted after it hit beta
+#![cfg_attr(not(bootstrap), feature(autodiff))]
 #![feature(alloc_error_handler)]
 #![feature(allocator_internals)]
 #![feature(allow_internal_unsafe)]
 #![feature(allow_internal_unstable)]
 #![feature(asm_experimental_arch)]
-#![feature(autodiff)]
 #![feature(cfg_sanitizer_cfi)]
 #![feature(cfg_target_thread_local)]
 #![feature(cfi_encoding)]
@@ -632,12 +632,15 @@ pub mod simd {
     #[doc(inline)]
     pub use crate::std_float::StdFloat;
 }
+
 #[unstable(feature = "autodiff", issue = "124509")]
+#[cfg(not(bootstrap))]
 /// This module provides support for automatic differentiation.
 pub mod autodiff {
     /// This macro handles automatic differentiation.
-    pub use core::autodiff::autodiff;
+    pub use core::autodiff::{autodiff_forward, autodiff_reverse};
 }
+
 #[stable(feature = "futures_api", since = "1.36.0")]
 pub mod task {
     //! Types and Traits for working with asynchronous tasks.

tests/codegen/autodiff/batched.rs

@@ -11,11 +11,11 @@
 
 #![feature(autodiff)]
 
-use std::autodiff::autodiff;
+use std::autodiff::autodiff_forward;
 
-#[autodiff(d_square3, Forward, Dual, DualOnly)]
-#[autodiff(d_square2, Forward, 4, Dual, DualOnly)]
-#[autodiff(d_square1, Forward, 4, Dual, Dual)]
+#[autodiff_forward(d_square3, Dual, DualOnly)]
+#[autodiff_forward(d_square2, 4, Dual, DualOnly)]
+#[autodiff_forward(d_square1, 4, Dual, Dual)]
 #[no_mangle]
 fn square(x: &f32) -> f32 {
     x * x

tests/codegen/autodiff/identical_fnc.rs

@@ -11,14 +11,14 @@
 // identical function calls in the LLVM-IR, while having two different calls in the Rust code.
 #![feature(autodiff)]
 
-use std::autodiff::autodiff;
+use std::autodiff::autodiff_reverse;
 
-#[autodiff(d_square, Reverse, Duplicated, Active)]
+#[autodiff_reverse(d_square, Duplicated, Active)]
 fn square(x: &f64) -> f64 {
     x * x
 }
 
-#[autodiff(d_square2, Reverse, Duplicated, Active)]
+#[autodiff_reverse(d_square2, Duplicated, Active)]
 fn square2(x: &f64) -> f64 {
     x * x
 }

tests/codegen/autodiff/inline.rs

@@ -4,9 +4,9 @@
 
 #![feature(autodiff)]
 
-use std::autodiff::autodiff;
+use std::autodiff::autodiff_reverse;
 
-#[autodiff(d_square, Reverse, Duplicated, Active)]
+#[autodiff_reverse(d_square, Duplicated, Active)]
 fn square(x: &f64) -> f64 {
     x * x
 }

tests/codegen/autodiff/scalar.rs

@@ -3,9 +3,9 @@
 //@ needs-enzyme
 #![feature(autodiff)]
 
-use std::autodiff::autodiff;
+use std::autodiff::autodiff_reverse;
 
-#[autodiff(d_square, Reverse, Duplicated, Active)]
+#[autodiff_reverse(d_square, Duplicated, Active)]
 #[no_mangle]
 fn square(x: &f64) -> f64 {
     x * x

tests/codegen/autodiff/sret.rs

@@ -9,10 +9,10 @@
 
 #![feature(autodiff)]
 
-use std::autodiff::autodiff;
+use std::autodiff::autodiff_reverse;
 
 #[no_mangle]
-#[autodiff(df, Reverse, Active, Active, Active)]
+#[autodiff_reverse(df, Active, Active, Active)]
 fn primal(x: f32, y: f32) -> f64 {
     (x * x * y) as f64
 }

tests/pretty/autodiff/autodiff_forward.pp

@@ -13,7 +13,7 @@
 
 // Test that forward mode ad macros are expanded correctly.
 
-use std::autodiff::autodiff;
+use std::autodiff::{autodiff_forward, autodiff_reverse};
 
 #[rustc_autodiff]
 #[inline(never)]