Single commit implementing the enzyme/autodiff frontend

Co-authored-by: Lorenz Schmidt <[email protected]>

Author: ZuseZ4

compiler/rustc_ast/src/expand/autodiff_attrs.rs

@@ -0,0 +1,283 @@
+//! This crate handles the user facing autodiff macro. For each `#[autodiff(...)]` attribute,
+//! we create an [`AutoDiffItem`] which contains the source and target function names. The source
+//! is the function to which the autodiff attribute is applied, and the target is the function
+//! getting generated by us (with a name given by the user as the first autodiff arg).
+
+use std::fmt::{self, Display, Formatter};
+use std::str::FromStr;
+
+use crate::expand::typetree::TypeTree;
+use crate::expand::{Decodable, Encodable, HashStable_Generic};
+use crate::ptr::P;
+use crate::{Ty, TyKind};
+
+/// Forward and Reverse Mode are well known names for automatic differentiation implementations.
+/// Enzyme does support both, but with different semantics, see DiffActivity. The First variants
+/// are a hack to support higher order derivatives. We need to compute first order derivatives
+/// before we compute second order derivatives, otherwise we would differentiate our placeholder
+/// functions. The proper solution is to recognize and resolve this DAG of autodiff invocations,
+/// as it's already done in the C++ and Julia frontend of Enzyme.
+///
+/// (FIXME) remove *First variants.
+/// Documentation for using [reverse](https://enzyme.mit.edu/rust/rev.html) and
+/// [forward](https://enzyme.mit.edu/rust/fwd.html) mode is available online.
+#[derive(Clone, Copy, Eq, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
+pub enum DiffMode {
+    /// No autodiff is applied (used during error handling).
+    Error,
+    /// The primal function which we will differentiate.
+    Source,
+    /// The target function, to be created using forward mode AD.
+    Forward,
+    /// The target function, to be created using reverse mode AD.
+    Reverse,
+    /// The target function, to be created using forward mode AD.
+    /// This target function will also be used as a source for higher order derivatives,
+    /// so compute it before all Forward/Reverse targets and optimize it through llvm.
+    ForwardFirst,
+    /// The target function, to be created using reverse mode AD.
+    /// This target function will also be used as a source for higher order derivatives,
+    /// so compute it before all Forward/Reverse targets and optimize it through llvm.
+    ReverseFirst,
+}
+
+/// Dual and Duplicated (and their Only variants) are getting lowered to the same Enzyme Activity.
+/// However, under forward mode we overwrite the previous shadow value, while for reverse mode
+/// we add to the previous shadow value. To not surprise users, we picked different names.
+/// Dual numbers is also a quite well known name for forward mode AD types.
+#[derive(Clone, Copy, Eq, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
+pub enum DiffActivity {
+    /// Implicit or Explicit () return type, so a special case of Const.
+    None,
+    /// Don't compute derivatives with respect to this input/output.
+    Const,
+    /// Reverse Mode, Compute derivatives for this scalar input/output.
+    Active,
+    /// Reverse Mode, Compute derivatives for this scalar output, but don't compute
+    /// the original return value.
+    ActiveOnly,
+    /// Forward Mode, Compute derivatives for this input/output and *overwrite* the shadow argument
+    /// with it.
+    Dual,
+    /// Forward Mode, Compute derivatives for this input/output and *overwrite* the shadow argument
+    /// with it. Drop the code which updates the original input/output for maximum performance.
+    DualOnly,
+    /// Reverse Mode, Compute derivatives for this &T or *T input and *add* it to the shadow argument.
+    Duplicated,
+    /// Reverse Mode, Compute derivatives for this &T or *T input and *add* it to the shadow argument.
+    /// Drop the code which updates the original input for maximum performance.
+    DuplicatedOnly,
+    /// All Integers must be Const, but these are used to mark the integer which represents the
+    /// length of a slice/vec. This is used for safety checks on slices.
+    FakeActivitySize,
+}
+/// We generate one of these structs for each `#[autodiff(...)]` attribute.
+#[derive(Clone, Eq, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
+pub struct AutoDiffItem {
+    /// The name of the function getting differentiated
+    pub source: String,
+    /// The name of the function being generated
+    pub target: String,
+    pub attrs: AutoDiffAttrs,
+    /// Describe the memory layout of input types
+    pub inputs: Vec<TypeTree>,
+    /// Describe the memory layout of the output type
+    pub output: TypeTree,
+}
+#[derive(Clone, Eq, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
+pub struct AutoDiffAttrs {
+    /// Conceptually either forward or reverse mode AD, as described in various autodiff papers and
+    /// e.g. in the [JAX
+    /// Documentation](https://jax.readthedocs.io/en/latest/_tutorials/advanced-autodiff.html#how-it-s-made-two-foundational-autodiff-functions).
+    pub mode: DiffMode,
+    pub ret_activity: DiffActivity,
+    pub input_activity: Vec<DiffActivity>,
+}
+
+impl DiffMode {
+    pub fn is_rev(&self) -> bool {
+        matches!(self, DiffMode::Reverse | DiffMode::ReverseFirst)
+    }
+    pub fn is_fwd(&self) -> bool {
+        matches!(self, DiffMode::Forward | DiffMode::ForwardFirst)
+    }
+}
+
+impl Display for DiffMode {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        match self {
+            DiffMode::Error => write!(f, "Error"),
+            DiffMode::Source => write!(f, "Source"),
+            DiffMode::Forward => write!(f, "Forward"),
+            DiffMode::Reverse => write!(f, "Reverse"),
+            DiffMode::ForwardFirst => write!(f, "ForwardFirst"),
+            DiffMode::ReverseFirst => write!(f, "ReverseFirst"),
+        }
+    }
+}
+
+/// Active(Only) is valid in reverse-mode AD for scalar float returns (f16/f32/...).
+/// Dual(Only) is valid in forward-mode AD for scalar float returns (f16/f32/...).
+/// Const is valid for all cases and means that we don't compute derivatives wrt. this output.
+/// That usually means we have a &mut or *mut T output and compute derivatives wrt. that arg,
+/// but this is too complex to verify here. Also it's just a logic error if users get this wrong.
+pub fn valid_ret_activity(mode: DiffMode, activity: DiffActivity) -> bool {
+    if activity == DiffActivity::None {
+        // Only valid if primal returns (), but we can't check that here.
+        return true;
+    }
+    match mode {
+        DiffMode::Error => false,
+        DiffMode::Source => false,
+        DiffMode::Forward | DiffMode::ForwardFirst => {
+            activity == DiffActivity::Dual
+                || activity == DiffActivity::DualOnly
+                || activity == DiffActivity::Const
+        }
+        DiffMode::Reverse | DiffMode::ReverseFirst => {
+            activity == DiffActivity::Const
+                || activity == DiffActivity::Active
+                || activity == DiffActivity::ActiveOnly
+        }
+    }
+}
+
+/// For indirections (ptr/ref) we can't use Active, since Active allocates a shadow value
+/// for the given argument, but we generally can't know the size of such a type.
+/// For scalar types (f16/f32/f64/f128) we can use Active and we can't use Duplicated,
+/// since Duplicated expects a mutable ref/ptr and we would thus end up with a shadow value
+/// who is an indirect type, which doesn't match the primal scalar type. We can't prevent
+/// users here from marking scalars as Duplicated, due to type aliases.
+pub fn valid_ty_for_activity(ty: &P<Ty>, activity: DiffActivity) -> bool {
+    use DiffActivity::*;
+    // It's always allowed to mark something as Const, since we won't compute derivatives wrt. it.
+    if matches!(activity, Const) {
+        return true;
+    }
+    if matches!(activity, Dual | DualOnly) {
+        return true;
+    }
+    // FIXME(ZuseZ4) We should make this more robust to also
+    // handle type aliases. Once that is done, we can be more restrictive here.
+    if matches!(activity, Active | ActiveOnly) {
+        return true;
+    }
+    matches!(ty.kind, TyKind::Ptr(_) | TyKind::Ref(..))
+        && matches!(activity, Duplicated | DuplicatedOnly)
+}
+pub fn valid_input_activity(mode: DiffMode, activity: DiffActivity) -> bool {
+    use DiffActivity::*;
+    return match mode {
+        DiffMode::Error => false,
+        DiffMode::Source => false,
+        DiffMode::Forward | DiffMode::ForwardFirst => {
+            matches!(activity, Dual | DualOnly | Const)
+        }
+        DiffMode::Reverse | DiffMode::ReverseFirst => {
+            matches!(activity, Active | ActiveOnly | Duplicated | DuplicatedOnly | Const)
+        }
+    };
+}
+
+impl Display for DiffActivity {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        match self {
+            DiffActivity::None => write!(f, "None"),
+            DiffActivity::Const => write!(f, "Const"),
+            DiffActivity::Active => write!(f, "Active"),
+            DiffActivity::ActiveOnly => write!(f, "ActiveOnly"),
+            DiffActivity::Dual => write!(f, "Dual"),
+            DiffActivity::DualOnly => write!(f, "DualOnly"),
+            DiffActivity::Duplicated => write!(f, "Duplicated"),
+            DiffActivity::DuplicatedOnly => write!(f, "DuplicatedOnly"),
+            DiffActivity::FakeActivitySize => write!(f, "FakeActivitySize"),
+        }
+    }
+}
+
+impl FromStr for DiffMode {
+    type Err = ();
+
+    fn from_str(s: &str) -> Result<DiffMode, ()> {
+        match s {
+            "Error" => Ok(DiffMode::Error),
+            "Source" => Ok(DiffMode::Source),
+            "Forward" => Ok(DiffMode::Forward),
+            "Reverse" => Ok(DiffMode::Reverse),
+            "ForwardFirst" => Ok(DiffMode::ForwardFirst),
+            "ReverseFirst" => Ok(DiffMode::ReverseFirst),
+            _ => Err(()),
+        }
+    }
+}
+impl FromStr for DiffActivity {
+    type Err = ();
+
+    fn from_str(s: &str) -> Result<DiffActivity, ()> {
+        match s {
+            "None" => Ok(DiffActivity::None),
+            "Active" => Ok(DiffActivity::Active),
+            "ActiveOnly" => Ok(DiffActivity::ActiveOnly),
+            "Const" => Ok(DiffActivity::Const),
+            "Dual" => Ok(DiffActivity::Dual),
+            "DualOnly" => Ok(DiffActivity::DualOnly),
+            "Duplicated" => Ok(DiffActivity::Duplicated),
+            "DuplicatedOnly" => Ok(DiffActivity::DuplicatedOnly),
+            _ => Err(()),
+        }
+    }
+}
+
+impl AutoDiffAttrs {
+    pub fn has_ret_activity(&self) -> bool {
+        self.ret_activity != DiffActivity::None
+    }
+    pub fn has_active_only_ret(&self) -> bool {
+        self.ret_activity == DiffActivity::ActiveOnly
+    }
+
+    pub fn error() -> Self {
+        AutoDiffAttrs {
+            mode: DiffMode::Error,
+            ret_activity: DiffActivity::None,
+            input_activity: Vec::new(),
+        }
+    }
+    pub fn source() -> Self {
+        AutoDiffAttrs {
+            mode: DiffMode::Source,
+            ret_activity: DiffActivity::None,
+            input_activity: Vec::new(),
+        }
+    }
+
+    pub fn is_active(&self) -> bool {
+        self.mode != DiffMode::Error
+    }
+
+    pub fn is_source(&self) -> bool {
+        self.mode == DiffMode::Source
+    }
+    pub fn apply_autodiff(&self) -> bool {
+        !matches!(self.mode, DiffMode::Error | DiffMode::Source)
+    }
+
+    pub fn into_item(
+        self,
+        source: String,
+        target: String,
+        inputs: Vec<TypeTree>,
+        output: TypeTree,
+    ) -> AutoDiffItem {
+        AutoDiffItem { source, target, inputs, output, attrs: self }
+    }
+}
+
+impl fmt::Display for AutoDiffItem {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "Differentiating {} -> {}", self.source, self.target)?;
+        write!(f, " with attributes: {:?}", self.attrs)?;
+        write!(f, " with inputs: {:?}", self.inputs)?;
+        write!(f, " with output: {:?}", self.output)
+    }
+}

compiler/rustc_ast/src/expand/mod.rs

@@ -7,6 +7,8 @@ use rustc_span::symbol::Ident;
 use crate::MetaItem;
 
 pub mod allocator;
+pub mod autodiff_attrs;
+pub mod typetree;
 
 #[derive(Debug, Clone, Encodable, Decodable, HashStable_Generic)]
 pub struct StrippedCfgItem<ModId = DefId> {

compiler/rustc_ast/src/expand/typetree.rs

@@ -0,0 +1,90 @@
+//! This module contains the definition of the `TypeTree` and `Type` structs.
+//! They are thin Rust wrappers around the TypeTrees used by Enzyme as the LLVM based autodiff
+//! backend. The Enzyme TypeTrees currently have various limitations and should be rewritten, so the
+//! Rust frontend obviously has the same limitations. The main motivation of TypeTrees is to
+//! represent how a type looks like "in memory". Enzyme can deduce this based on usage patterns in
+//! the user code, but this is extremely slow and not even always sufficient. As such we lower some
+//! information from rustc to help Enzyme. For a full explanation of their design it is necessary to
+//! analyze the implementation in Enzyme core itself. As a rough summary, `-1` in Enzyme speech means
+//! everywhere. That is `{0:-1: Float}` means at index 0 you have a ptr, if you dereference it it
+//! will be floats everywhere. Thus `* f32`. If you have `{-1:int}` it means int's everywhere,
+//! e.g. [i32; N]. `{0:-1:-1 float}` then means one pointer at offset 0, if you dereference it there
+//! will be only pointers, if you dereference these new pointers they will point to array of floats.
+//! Generally, it allows byte-specific descriptions.
+//! FIXME: This description might be partly inaccurate and should be extended, along with
+//! adding documentation to the corresponding Enzyme core code.
+//! FIXME: Rewrite the TypeTree logic in Enzyme core to reduce the need for the rustc frontend to
+//! provide typetree information.
+//! FIXME: We should also re-evaluate where we create TypeTrees from Rust types, since MIR
+//! representations of some types might not be accurate. For example a vector of floats might be
+//! represented as a vector of u8s in MIR in some cases.
+
+use std::fmt;
+
+use crate::expand::{Decodable, Encodable, HashStable_Generic};
+
+#[derive(Clone, Copy, Eq, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
+pub enum Kind {
+    Anything,
+    Integer,
+    Pointer,
+    Half,
+    Float,
+    Double,
+    Unknown,
+}
+
+#[derive(Clone, Eq, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
+pub struct TypeTree(pub Vec<Type>);
+
+impl TypeTree {
+    pub fn new() -> Self {
+        Self(Vec::new())
+    }
+    pub fn all_ints() -> Self {
+        Self(vec![Type { offset: -1, size: 1, kind: Kind::Integer, child: TypeTree::new() }])
+    }
+    pub fn int(size: usize) -> Self {
+        let mut ints = Vec::with_capacity(size);
+        for i in 0..size {
+            ints.push(Type {
+                offset: i as isize,
+                size: 1,
+                kind: Kind::Integer,
+                child: TypeTree::new(),
+            });
+        }
+        Self(ints)
+    }
+}
+
+#[derive(Clone, Eq, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
+pub struct FncTree {
+    pub args: Vec<TypeTree>,
+    pub ret: TypeTree,
+}
+
+#[derive(Clone, Eq, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
+pub struct Type {
+    pub offset: isize,
+    pub size: usize,
+    pub kind: Kind,
+    pub child: TypeTree,
+}
+
+impl Type {
+    pub fn add_offset(self, add: isize) -> Self {
+        let offset = match self.offset {
+            -1 => add,
+            x => add + x,
+        };
+
+        Self { size: self.size, kind: self.kind, child: self.child, offset }
+    }
+}
+
+impl fmt::Display for Type {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        <Self as fmt::Debug>::fmt(self, f)
+    }
+}

compiler/rustc_builtin_macros/Cargo.toml

@@ -3,6 +3,10 @@ name = "rustc_builtin_macros"
 version = "0.0.0"
 edition = "2021"
 
+
+[lints.rust]
+unexpected_cfgs = { level = "warn", check-cfg = ['cfg(llvm_enzyme)'] }
+
 [lib]
 doctest = false
 

compiler/rustc_builtin_macros/messages.ftl

@@ -69,6 +69,15 @@ builtin_macros_assert_requires_boolean = macro requires a boolean expression as
 builtin_macros_assert_requires_expression = macro requires an expression as an argument
     .suggestion = try removing semicolon
 
+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 {$found}
+builtin_macros_autodiff_ty_activity = {$act} can not be used for this type
+
+builtin_macros_autodiff_unknown_activity = did not recognize Activity: `{$act}`
 builtin_macros_bad_derive_target = `derive` may only be applied to `struct`s, `enum`s and `union`s
     .label = not applicable here
     .label2 = not a `struct`, `enum` or `union`

compiler/rustc_builtin_macros/src/autodiff.rs

@@ -145,6 +145,78 @@ pub(crate) struct AllocMustStatics {
     pub(crate) span: Span,
 }
 
+#[cfg(llvm_enzyme)]
+pub(crate) use autodiff::*;
+
+#[cfg(llvm_enzyme)]
+mod autodiff {
+    use super::*;
+    #[derive(Diagnostic)]
+    #[diag(builtin_macros_autodiff_missing_config)]
+    pub(crate) struct AutoDiffMissingConfig {
+        #[primary_span]
+        pub(crate) span: Span,
+    }
+    #[derive(Diagnostic)]
+    #[diag(builtin_macros_autodiff_unknown_activity)]
+    pub(crate) struct AutoDiffUnknownActivity {
+        #[primary_span]
+        pub(crate) span: Span,
+        pub(crate) act: String,
+    }
+    #[derive(Diagnostic)]
+    #[diag(builtin_macros_autodiff_ty_activity)]
+    pub(crate) struct AutoDiffInvalidTypeForActivity {
+        #[primary_span]
+        pub(crate) span: Span,
+        pub(crate) act: String,
+    }
+    #[derive(Diagnostic)]
+    #[diag(builtin_macros_autodiff_number_activities)]
+    pub(crate) struct AutoDiffInvalidNumberActivities {
+        #[primary_span]
+        pub(crate) span: Span,
+        pub(crate) expected: usize,
+        pub(crate) found: usize,
+    }
+    #[derive(Diagnostic)]
+    #[diag(builtin_macros_autodiff_mode_activity)]
+    pub(crate) struct AutoDiffInvalidApplicationModeAct {
+        #[primary_span]
+        pub(crate) span: Span,
+        pub(crate) mode: String,
+        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)]
+    pub(crate) struct AutoDiffInvalidApplication {
+        #[primary_span]
+        pub(crate) span: Span,
+    }
+}
+
+#[cfg(not(llvm_enzyme))]
+pub(crate) use ad_fallback::*;
+#[cfg(not(llvm_enzyme))]
+mod ad_fallback {
+    use super::*;
+    #[derive(Diagnostic)]
+    #[diag(builtin_macros_autodiff_not_build)]
+    pub(crate) struct AutoDiffSupportNotBuild {
+        #[primary_span]
+        pub(crate) span: Span,
+    }
+}
+
 #[derive(Diagnostic)]
 #[diag(builtin_macros_concat_bytes_invalid)]
 pub(crate) struct ConcatBytesInvalid {

compiler/rustc_builtin_macros/src/errors.rs

@@ -5,6 +5,7 @@
 #![allow(internal_features)]
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::untranslatable_diagnostic)]
+#![cfg_attr(not(bootstrap), feature(autodiff))]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(assert_matches)]
@@ -29,6 +30,7 @@ use crate::deriving::*;
 
 mod alloc_error_handler;
 mod assert;
+mod autodiff;
 mod cfg;
 mod cfg_accessible;
 mod cfg_eval;
@@ -106,6 +108,7 @@ pub fn register_builtin_macros(resolver: &mut dyn ResolverExpand) {
 
     register_attr! {
         alloc_error_handler: alloc_error_handler::expand,
+        autodiff: autodiff::expand,
         bench: test::expand_bench,
         cfg_accessible: cfg_accessible::Expander,
         cfg_eval: cfg_eval::expand,

compiler/rustc_builtin_macros/src/lib.rs

@@ -220,6 +220,10 @@ impl<'a> ExtCtxt<'a> {
         self.stmt_local(local, span)
     }
 
+    pub fn stmt_semi(&self, expr: P<ast::Expr>) -> ast::Stmt {
+        ast::Stmt { id: ast::DUMMY_NODE_ID, span: expr.span, kind: ast::StmtKind::Semi(expr) }
+    }
+
     pub fn stmt_local(&self, local: P<ast::Local>, span: Span) -> ast::Stmt {
         ast::Stmt { id: ast::DUMMY_NODE_ID, kind: ast::StmtKind::Let(local), span }
     }
@@ -287,6 +291,25 @@ impl<'a> ExtCtxt<'a> {
         self.expr(sp, ast::ExprKind::Paren(e))
     }
 
+    pub fn expr_method_call(
+        &self,
+        span: Span,
+        expr: P<ast::Expr>,
+        ident: Ident,
+        args: ThinVec<P<ast::Expr>>,
+    ) -> P<ast::Expr> {
+        let seg = ast::PathSegment::from_ident(ident);
+        self.expr(
+            span,
+            ast::ExprKind::MethodCall(Box::new(ast::MethodCall {
+                seg,
+                receiver: expr,
+                args,
+                span,
+            })),
+        )
+    }
+
     pub fn expr_call(
         &self,
         span: Span,
@@ -295,6 +318,12 @@ impl<'a> ExtCtxt<'a> {
     ) -> P<ast::Expr> {
         self.expr(span, ast::ExprKind::Call(expr, args))
     }
+    pub fn expr_loop(&self, sp: Span, block: P<ast::Block>) -> P<ast::Expr> {
+        self.expr(sp, ast::ExprKind::Loop(block, None, sp))
+    }
+    pub fn expr_asm(&self, sp: Span, expr: P<ast::InlineAsm>) -> P<ast::Expr> {
+        self.expr(sp, ast::ExprKind::InlineAsm(expr))
+    }
     pub fn expr_call_ident(
         &self,
         span: Span,

compiler/rustc_expand/src/build.rs

@@ -752,6 +752,11 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         template!(NameValueStr: "transparent|semitransparent|opaque"), ErrorFollowing,
         EncodeCrossCrate::Yes, "used internally for testing macro hygiene",
     ),
+    rustc_attr!(
+        rustc_autodiff, Normal,
+        template!(Word, List: r#""...""#), DuplicatesOk,
+        EncodeCrossCrate::No, INTERNAL_UNSTABLE
+    ),
 
     // ==========================================================================
     // Internal attributes, Diagnostics related:

compiler/rustc_feature/src/builtin_attrs.rs

@@ -49,6 +49,10 @@ passes_attr_crate_level =
 passes_attr_only_in_functions =
     `{$attr}` attribute can only be used on functions
 
+passes_autodiff_attr =
+    `#[autodiff]` should be applied to a function
+    .label = not a function
+
 passes_both_ffi_const_and_pure =
     `#[ffi_const]` function cannot be `#[ffi_pure]`
 

compiler/rustc_passes/messages.ftl

@@ -243,6 +243,9 @@ 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, ..] => {
+                    self.check_autodiff(hir_id, attr, span, target)
+                }
                 [sym::coroutine, ..] => {
                     self.check_coroutine(attr, target);
                 }
@@ -2345,6 +2348,18 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
             self.dcx().emit_err(errors::RustcPubTransparent { span, attr_span });
         }
     }
+
+    /// Checks if `#[autodiff]` is applied to an item other than a function item.
+    fn check_autodiff(&self, _hir_id: HirId, _attr: &Attribute, span: Span, target: Target) {
+        debug!("check_autodiff");
+        match target {
+            Target::Fn => {}
+            _ => {
+                self.dcx().emit_err(errors::AutoDiffAttr { attr_span: span });
+                self.abort.set(true);
+            }
+        }
+    }
 }
 
 impl<'tcx> Visitor<'tcx> for CheckAttrVisitor<'tcx> {

compiler/rustc_passes/src/check_attr.rs

@@ -20,6 +20,14 @@ use crate::lang_items::Duplicate;
 #[diag(passes_incorrect_do_not_recommend_location)]
 pub(crate) struct IncorrectDoNotRecommendLocation;
 
+#[derive(Diagnostic)]
+#[diag(passes_autodiff_attr)]
+pub(crate) struct AutoDiffAttr {
+    #[primary_span]
+    #[label]
+    pub attr_span: Span,
+}
+
 #[derive(LintDiagnostic)]
 #[diag(passes_outer_crate_level_attr)]
 pub(crate) struct OuterCrateLevelAttr;

compiler/rustc_passes/src/errors.rs

@@ -481,6 +481,8 @@ symbols! {
         audit_that,
         augmented_assignments,
         auto_traits,
+        autodiff,
+        autodiff_fallback,
         automatically_derived,
         avx,
         avx512_target_feature,
@@ -544,6 +546,7 @@ symbols! {
         cfg_accessible,
         cfg_attr,
         cfg_attr_multi,
+        cfg_autodiff_fallback,
         cfg_boolean_literals,
         cfg_doctest,
         cfg_eval,
@@ -998,6 +1001,7 @@ symbols! {
         hashset_iter_ty,
         hexagon_target_feature,
         hidden,
+        hint,
         homogeneous_aggregate,
         host,
         html_favicon_url,
@@ -1650,6 +1654,7 @@ symbols! {
         rustc_allow_incoherent_impl,
         rustc_allowed_through_unstable_modules,
         rustc_attrs,
+        rustc_autodiff,
         rustc_box,
         rustc_builtin_macro,
         rustc_capture_analysis,

compiler/rustc_span/src/symbol.rs

@@ -278,6 +278,15 @@ pub mod assert_matches {
     pub use crate::macros::{assert_matches, debug_assert_matches};
 }
 
+// We don't export this through #[macro_export] for now, to avoid breakage.
+#[cfg(not(bootstrap))]
+#[unstable(feature = "autodiff", issue = "124509")]
+/// Unstable module containing the unstable `autodiff` macro.
+pub mod autodiff {
+    #[unstable(feature = "autodiff", issue = "124509")]
+    pub use crate::macros::builtin::autodiff;
+}
+
 #[unstable(feature = "cfg_match", issue = "115585")]
 pub use crate::macros::cfg_match;
 

library/core/src/lib.rs

@@ -1539,6 +1539,24 @@ pub(crate) mod builtin {
         ($file:expr $(,)?) => {{ /* 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 implicitely return nothing (or explicitely return
+    /// `-> ()`. 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($item:item) {
+        /* compiler built-in */
+    }
+
     /// Asserts that a boolean expression is `true` at runtime.
     ///
     /// This will invoke the [`panic!`] macro if the provided expression cannot be

library/core/src/macros/mod.rs

@@ -267,6 +267,7 @@
 #![allow(unused_features)]
 //
 // Features:
+#![cfg_attr(not(bootstrap), feature(autodiff))]
 #![cfg_attr(test, feature(internal_output_capture, print_internals, update_panic_count, rt))]
 #![cfg_attr(
     all(target_vendor = "fortanix", target_env = "sgx"),
@@ -627,7 +628,13 @@ pub mod simd {
     #[doc(inline)]
     pub use crate::std_float::StdFloat;
 }
-
+#[cfg(not(bootstrap))]
+#[unstable(feature = "autodiff", issue = "124509")]
+/// This module provides support for automatic differentiation.
+pub mod autodiff {
+    /// This macro handles automatic differentiation.
+    pub use core::autodiff::autodiff;
+}
 #[stable(feature = "futures_api", since = "1.36.0")]
 pub mod task {
     //! Types and Traits for working with asynchronous tasks.