compiler/rustc_codegen_llvm/src/builder/autodiff.rs - third_party/rust - Git at Google

 use std::ptr;

 use rustc_ast::expand::autodiff_attrs::{AutoDiffAttrs, AutoDiffItem, DiffActivity, DiffMode};
 use rustc_codegen_ssa::ModuleCodegen;
 use rustc_codegen_ssa::common::TypeKind;
 use rustc_codegen_ssa::traits::{BaseTypeCodegenMethods, BuilderMethods};
 use rustc_errors::FatalError;
 use rustc_middle::bug;
 use tracing::{debug, trace};

 use crate::back::write::llvm_err;
 use crate::builder::{Builder, PlaceRef, UNNAMED};
 use crate::context::SimpleCx;
 use crate::declare::declare_simple_fn;
 use crate::errors::{AutoDiffWithoutEnable, LlvmError};
 use crate::llvm::AttributePlace::Function;
 use crate::llvm::{Metadata, True, Type};
 use crate::value::Value;
 use crate::{CodegenContext, LlvmCodegenBackend, ModuleLlvm, attributes, llvm};

 fn _get_params(fnc: &Value) -> Vec<&Value> {
     let param_num = llvm::LLVMCountParams(fnc) as usize;
     let mut fnc_args: Vec<&Value> = vec![];
     fnc_args.reserve(param_num);
     unsafe {
         llvm::LLVMGetParams(fnc, fnc_args.as_mut_ptr());
         fnc_args.set_len(param_num);
     }
     fnc_args
 }

 fn _has_sret(fnc: &Value) -> bool {
     let num_args = llvm::LLVMCountParams(fnc) as usize;
     if num_args == 0 {
         false
     } else {
         unsafe { llvm::LLVMRustHasAttributeAtIndex(fnc, 0, llvm::AttributeKind::StructRet) }
     }
 }

 // When we call the `__enzyme_autodiff` or `__enzyme_fwddiff` function, we need to pass all the
 // original inputs, as well as metadata and the additional shadow arguments.
 // This function matches the arguments from the outer function to the inner enzyme call.
 //
 // This function also considers that Rust level arguments not always match the llvm-ir level
 // arguments. A slice, `&[f32]`, for example, is represented as a pointer and a length on
 // llvm-ir level. The number of activities matches the number of Rust level arguments, so we
 // need to match those.
 // FIXME(ZuseZ4): This logic is a bit more complicated than it should be, can we simplify it
 // using iterators and peek()?
 fn match_args_from_caller_to_enzyme<'ll, 'tcx>(
     cx: &SimpleCx<'ll>,
     builder: &mut Builder<'_, 'll, 'tcx>,
     width: u32,
     args: &mut Vec<&'ll llvm::Value>,
     inputs: &[DiffActivity],
     outer_args: &[&'ll llvm::Value],
 ) {
     debug!("matching autodiff arguments");
     // We now handle the issue that Rust level arguments not always match the llvm-ir level
     // arguments. A slice, `&[f32]`, for example, is represented as a pointer and a length on
     // llvm-ir level. The number of activities matches the number of Rust level arguments, so we
     // need to match those.
     // FIXME(ZuseZ4): This logic is a bit more complicated than it should be, can we simplify it
     // using iterators and peek()?
     let mut outer_pos: usize = 0;
     let mut activity_pos = 0;

     let enzyme_const = cx.create_metadata("enzyme_const".to_string()).unwrap();
     let enzyme_out = cx.create_metadata("enzyme_out".to_string()).unwrap();
     let enzyme_dup = cx.create_metadata("enzyme_dup".to_string()).unwrap();
     let enzyme_dupv = cx.create_metadata("enzyme_dupv".to_string()).unwrap();
     let enzyme_dupnoneed = cx.create_metadata("enzyme_dupnoneed".to_string()).unwrap();
     let enzyme_dupnoneedv = cx.create_metadata("enzyme_dupnoneedv".to_string()).unwrap();

     while activity_pos < inputs.len() {
         let diff_activity = inputs[activity_pos as usize];
         // Duplicated arguments received a shadow argument, into which enzyme will write the
         // gradient.
         let (activity, duplicated): (&Metadata, bool) = match diff_activity {
             DiffActivity::None => panic!("not a valid input activity"),
             DiffActivity::Const => (enzyme_const, false),
             DiffActivity::Active => (enzyme_out, false),
             DiffActivity::ActiveOnly => (enzyme_out, false),
             DiffActivity::Dual => (enzyme_dup, true),
             DiffActivity::Dualv => (enzyme_dupv, true),
             DiffActivity::DualOnly => (enzyme_dupnoneed, true),
             DiffActivity::DualvOnly => (enzyme_dupnoneedv, true),
             DiffActivity::Duplicated => (enzyme_dup, true),
             DiffActivity::DuplicatedOnly => (enzyme_dupnoneed, true),
             DiffActivity::FakeActivitySize(_) => (enzyme_const, false),
         };
         let outer_arg = outer_args[outer_pos];
         args.push(cx.get_metadata_value(activity));
         if matches!(diff_activity, DiffActivity::Dualv) {
             let next_outer_arg = outer_args[outer_pos + 1];
             let elem_bytes_size: u64 = match inputs[activity_pos + 1] {
                 DiffActivity::FakeActivitySize(Some(s)) => s.into(),
                 _ => bug!("incorrect Dualv handling recognized."),
             };
             // stride: sizeof(T) * n_elems.
             // n_elems is the next integer.
             // Now we multiply `4 * next_outer_arg` to get the stride.
             let mul = unsafe {
                 llvm::LLVMBuildMul(
                     builder.llbuilder,
                     cx.get_const_int(cx.type_i64(), elem_bytes_size),
                     next_outer_arg,
                     UNNAMED,
                 )
             };
             args.push(mul);
         }
         args.push(outer_arg);
         if duplicated {
             // We know that duplicated args by construction have a following argument,
             // so this can not be out of bounds.
             let next_outer_arg = outer_args[outer_pos + 1];
             let next_outer_ty = cx.val_ty(next_outer_arg);
             // FIXME(ZuseZ4): We should add support for Vec here too, but it's less urgent since
             // vectors behind references (&Vec<T>) are already supported. Users can not pass a
             // Vec by value for reverse mode, so this would only help forward mode autodiff.
             let slice = {
                 if activity_pos + 1 >= inputs.len() {
                     // If there is no arg following our ptr, it also can't be a slice,
                     // since that would lead to a ptr, int pair.
                     false
                 } else {
                     let next_activity = inputs[activity_pos + 1];
                     // We analyze the MIR types and add this dummy activity if we visit a slice.
                     matches!(next_activity, DiffActivity::FakeActivitySize(_))
                 }
             };
             if slice {
                 // A duplicated slice will have the following two outer_fn arguments:
                 // (..., ptr1, int1, ptr2, int2, ...). We add the following llvm-ir to our __enzyme call:
                 // (..., metadata! enzyme_dup, ptr, ptr, int1, ...).
                 // FIXME(ZuseZ4): We will upstream a safety check later which asserts that
                 // int2 >= int1, which means the shadow vector is large enough to store the gradient.
                 assert_eq!(cx.type_kind(next_outer_ty), TypeKind::Integer);

                 let iterations =
                     if matches!(diff_activity, DiffActivity::Dualv) { 1 } else { width as usize };

                 for i in 0..iterations {
                     let next_outer_arg2 = outer_args[outer_pos + 2 * (i + 1)];
                     let next_outer_ty2 = cx.val_ty(next_outer_arg2);
                     assert_eq!(cx.type_kind(next_outer_ty2), TypeKind::Pointer);
                     let next_outer_arg3 = outer_args[outer_pos + 2 * (i + 1) + 1];
                     let next_outer_ty3 = cx.val_ty(next_outer_arg3);
                     assert_eq!(cx.type_kind(next_outer_ty3), TypeKind::Integer);
                     args.push(next_outer_arg2);
                 }
                 args.push(cx.get_metadata_value(enzyme_const));
                 args.push(next_outer_arg);
                 outer_pos += 2 + 2 * iterations;
                 activity_pos += 2;
             } else {
                 // A duplicated pointer will have the following two outer_fn arguments:
                 // (..., ptr, ptr, ...). We add the following llvm-ir to our __enzyme call:
                 // (..., metadata! enzyme_dup, ptr, ptr, ...).
                 if matches!(diff_activity, DiffActivity::Duplicated | DiffActivity::DuplicatedOnly)
                 {
                     assert_eq!(cx.type_kind(next_outer_ty), TypeKind::Pointer);
                 }
                 // In the case of Dual we don't have assumptions, e.g. f32 would be valid.
                 args.push(next_outer_arg);
                 outer_pos += 2;
                 activity_pos += 1;

                 // Now, if width > 1, we need to account for that
                 for _ in 1..width {
                     let next_outer_arg = outer_args[outer_pos];
                     args.push(next_outer_arg);
                     outer_pos += 1;
                 }
             }
         } else {
             // We do not differentiate with resprect to this argument.
             // We already added the metadata and argument above, so just increase the counters.
             outer_pos += 1;
             activity_pos += 1;
         }
     }
 }

 /// When differentiating `fn_to_diff`, take a `outer_fn` and generate another
 /// function with expected naming and calling conventions[^1] which will be
 /// discovered by the enzyme LLVM pass and its body populated with the differentiated
 /// `fn_to_diff`. `outer_fn` is then modified to have a call to the generated
 /// function and handle the differences between the Rust calling convention and
 /// Enzyme.
 /// [^1]: <https://enzyme.mit.edu/getting_started/CallingConvention/>
 // FIXME(ZuseZ4): `outer_fn` should include upstream safety checks to
 // cover some assumptions of enzyme/autodiff, which could lead to UB otherwise.
 pub(crate) fn generate_enzyme_call<'ll, 'tcx>(
     builder: &mut Builder<'_, 'll, 'tcx>,
     cx: &SimpleCx<'ll>,
     fn_to_diff: &'ll Value,
     outer_name: &str,
     ret_ty: &'ll Type,
     fn_args: &[&'ll Value],
     attrs: AutoDiffAttrs,
     dest: PlaceRef<'tcx, &'ll Value>,
 ) {
     // We have to pick the name depending on whether we want forward or reverse mode autodiff.
     let mut ad_name: String = match attrs.mode {
         DiffMode::Forward => "__enzyme_fwddiff",
         DiffMode::Reverse => "__enzyme_autodiff",
         _ => panic!("logic bug in autodiff, unrecognized mode"),
     }
     .to_string();

     // add outer_name to ad_name to make it unique, in case users apply autodiff to multiple
     // functions. Unwrap will only panic, if LLVM gave us an invalid string.
     ad_name.push_str(outer_name);

     // Let us assume the user wrote the following function square:
     //
     // ```llvm
     // define double @square(double %x) {
     // entry:
     //  %0 = fmul double %x, %x
     //  ret double %0
     // }
     // ```
     // define double @dsquare(double %x) {
     //  return 0.0;
     // }
     // ```
     //
     // so our `outer_fn` will be `dsquare`. The unsafe code section below now removes the placeholder
     // code and inserts an autodiff call. We also add a declaration for the __enzyme_autodiff call.
     // Again, the arguments to all functions are slightly simplified.
     // ```llvm
     // declare double @__enzyme_autodiff_square(...)
     //
     // define double @dsquare(double %x) {
     // entry:
     //   %0 = tail call double (...) @__enzyme_autodiff_square(double (double)* nonnull @square, double %x)
     //   ret double %0
     // }
     // ```
     let enzyme_ty = unsafe { llvm::LLVMFunctionType(ret_ty, ptr::null(), 0, True) };

     // FIXME(ZuseZ4): the CC/Addr/Vis values are best effort guesses, we should look at tests and
     // think a bit more about what should go here.
     // FIXME(Sa4dUs): have to find a way to get the cc, using `FastCallConv` for now
     let cc = 8;
     let ad_fn = declare_simple_fn(
         cx,
         &ad_name,
         llvm::CallConv::try_from(cc).expect("invalid callconv"),
         llvm::UnnamedAddr::No,
         llvm::Visibility::Default,
         enzyme_ty,
     );

     // Otherwise LLVM might inline our temporary code before the enzyme pass has a chance to
     // do it's work.
     let attr = llvm::AttributeKind::NoInline.create_attr(cx.llcx);
     attributes::apply_to_llfn(ad_fn, Function, &[attr]);

     let num_args = llvm::LLVMCountParams(&fn_to_diff);
     let mut args = Vec::with_capacity(num_args as usize + 1);
     args.push(fn_to_diff);

     let enzyme_primal_ret = cx.create_metadata("enzyme_primal_return".to_string()).unwrap();
     if matches!(attrs.ret_activity, DiffActivity::Dual | DiffActivity::Active) {
         args.push(cx.get_metadata_value(enzyme_primal_ret));
     }
     if attrs.width > 1 {
         let enzyme_width = cx.create_metadata("enzyme_width".to_string()).unwrap();
         args.push(cx.get_metadata_value(enzyme_width));
         args.push(cx.get_const_int(cx.type_i64(), attrs.width as u64));
     }

     match_args_from_caller_to_enzyme(
         &cx,
         builder,
         attrs.width,
         &mut args,
         &attrs.input_activity,
         fn_args,
     );

     let call = builder.call(enzyme_ty, None, None, ad_fn, &args, None, None);

     builder.store_to_place(call, dest.val);
 }

 pub(crate) fn differentiate<'ll>(
     module: &'ll ModuleCodegen<ModuleLlvm>,
     cgcx: &CodegenContext<LlvmCodegenBackend>,
     diff_items: Vec<AutoDiffItem>,
 ) -> Result<(), FatalError> {
     // TODO(Sa4dUs): delete all this logic
     for item in &diff_items {
         trace!("{}", item);
     }

     let diag_handler = cgcx.create_dcx();

     let cx = SimpleCx::new(module.module_llvm.llmod(), module.module_llvm.llcx, cgcx.pointer_size);

     // First of all, did the user try to use autodiff without using the -Zautodiff=Enable flag?
     if !diff_items.is_empty()
         && !cgcx.opts.unstable_opts.autodiff.contains(&rustc_session::config::AutoDiff::Enable)
     {
         return Err(diag_handler.handle().emit_almost_fatal(AutoDiffWithoutEnable));
     }

     // Here we replace the placeholder code with the actual autodiff code, which calls Enzyme.
     for item in diff_items.iter() {
         let name = item.source.clone();
         let fn_def: Option<&llvm::Value> = cx.get_function(&name);
         let Some(_fn_def) = fn_def else {
             return Err(llvm_err(
                 diag_handler.handle(),
                 LlvmError::PrepareAutoDiff {
                     src: item.source.clone(),
                     target: item.target.clone(),
                     error: "could not find source function".to_owned(),
                 },
             ));
         };
         debug!(?item.target);
         let fn_target: Option<&llvm::Value> = cx.get_function(&item.target);
         let Some(_fn_target) = fn_target else {
             return Err(llvm_err(
                 diag_handler.handle(),
                 LlvmError::PrepareAutoDiff {
                     src: item.source.clone(),
                     target: item.target.clone(),
                     error: "could not find target function".to_owned(),
                 },
             ));
         };

         // generate_enzyme_call(&cx, fn_def, fn_target, item.attrs.clone());
     }

     // FIXME(ZuseZ4): support SanitizeHWAddress and prevent illegal/unsupported opts

     trace!("done with differentiate()");

     Ok(())
 }
	use std::ptr;

	use rustc_ast::expand::autodiff_attrs::{AutoDiffAttrs, AutoDiffItem, DiffActivity, DiffMode};
	use rustc_codegen_ssa::ModuleCodegen;
	use rustc_codegen_ssa::common::TypeKind;
	use rustc_codegen_ssa::traits::{BaseTypeCodegenMethods, BuilderMethods};
	use rustc_errors::FatalError;
	use rustc_middle::bug;
	use tracing::{debug, trace};

	use crate::back::write::llvm_err;
	use crate::builder::{Builder, PlaceRef, UNNAMED};
	use crate::context::SimpleCx;
	use crate::declare::declare_simple_fn;
	use crate::errors::{AutoDiffWithoutEnable, LlvmError};
	use crate::llvm::AttributePlace::Function;
	use crate::llvm::{Metadata, True, Type};
	use crate::value::Value;
	use crate::{CodegenContext, LlvmCodegenBackend, ModuleLlvm, attributes, llvm};

	fn _get_params(fnc: &Value) -> Vec<&Value> {
	let param_num = llvm::LLVMCountParams(fnc) as usize;
	let mut fnc_args: Vec<&Value> = vec![];
	fnc_args.reserve(param_num);
	unsafe {
	llvm::LLVMGetParams(fnc, fnc_args.as_mut_ptr());
	fnc_args.set_len(param_num);
	}
	fnc_args
	}

	fn _has_sret(fnc: &Value) -> bool {
	let num_args = llvm::LLVMCountParams(fnc) as usize;
	if num_args == 0 {
	false
	} else {
	unsafe { llvm::LLVMRustHasAttributeAtIndex(fnc, 0, llvm::AttributeKind::StructRet) }
	}
	}

	// When we call the `__enzyme_autodiff` or `__enzyme_fwddiff` function, we need to pass all the
	// original inputs, as well as metadata and the additional shadow arguments.
	// This function matches the arguments from the outer function to the inner enzyme call.
	//
	// This function also considers that Rust level arguments not always match the llvm-ir level
	// arguments. A slice, `&[f32]`, for example, is represented as a pointer and a length on
	// llvm-ir level. The number of activities matches the number of Rust level arguments, so we
	// need to match those.
	// FIXME(ZuseZ4): This logic is a bit more complicated than it should be, can we simplify it
	// using iterators and peek()?
	fn match_args_from_caller_to_enzyme<'ll, 'tcx>(
	cx: &SimpleCx<'ll>,
	builder: &mut Builder<'_, 'll, 'tcx>,
	width: u32,
	args: &mut Vec<&'ll llvm::Value>,
	inputs: &[DiffActivity],
	outer_args: &[&'ll llvm::Value],
	) {
	debug!("matching autodiff arguments");
	// We now handle the issue that Rust level arguments not always match the llvm-ir level
	// arguments. A slice, `&[f32]`, for example, is represented as a pointer and a length on
	// llvm-ir level. The number of activities matches the number of Rust level arguments, so we
	// need to match those.
	// FIXME(ZuseZ4): This logic is a bit more complicated than it should be, can we simplify it
	// using iterators and peek()?
	let mut outer_pos: usize = 0;
	let mut activity_pos = 0;

	let enzyme_const = cx.create_metadata("enzyme_const".to_string()).unwrap();
	let enzyme_out = cx.create_metadata("enzyme_out".to_string()).unwrap();
	let enzyme_dup = cx.create_metadata("enzyme_dup".to_string()).unwrap();
	let enzyme_dupv = cx.create_metadata("enzyme_dupv".to_string()).unwrap();
	let enzyme_dupnoneed = cx.create_metadata("enzyme_dupnoneed".to_string()).unwrap();
	let enzyme_dupnoneedv = cx.create_metadata("enzyme_dupnoneedv".to_string()).unwrap();

	while activity_pos < inputs.len() {
	let diff_activity = inputs[activity_pos as usize];
	// Duplicated arguments received a shadow argument, into which enzyme will write the
	// gradient.
	let (activity, duplicated): (&Metadata, bool) = match diff_activity {
	DiffActivity::None => panic!("not a valid input activity"),
	DiffActivity::Const => (enzyme_const, false),
	DiffActivity::Active => (enzyme_out, false),
	DiffActivity::ActiveOnly => (enzyme_out, false),
	DiffActivity::Dual => (enzyme_dup, true),
	DiffActivity::Dualv => (enzyme_dupv, true),
	DiffActivity::DualOnly => (enzyme_dupnoneed, true),
	DiffActivity::DualvOnly => (enzyme_dupnoneedv, true),
	DiffActivity::Duplicated => (enzyme_dup, true),
	DiffActivity::DuplicatedOnly => (enzyme_dupnoneed, true),
	DiffActivity::FakeActivitySize(_) => (enzyme_const, false),
	};
	let outer_arg = outer_args[outer_pos];
	args.push(cx.get_metadata_value(activity));
	if matches!(diff_activity, DiffActivity::Dualv) {
	let next_outer_arg = outer_args[outer_pos + 1];
	let elem_bytes_size: u64 = match inputs[activity_pos + 1] {
	DiffActivity::FakeActivitySize(Some(s)) => s.into(),
	_ => bug!("incorrect Dualv handling recognized."),
	};
	// stride: sizeof(T) * n_elems.
	// n_elems is the next integer.
	// Now we multiply `4 * next_outer_arg` to get the stride.
	let mul = unsafe {
	llvm::LLVMBuildMul(
	builder.llbuilder,
	cx.get_const_int(cx.type_i64(), elem_bytes_size),
	next_outer_arg,
	UNNAMED,
	)
	};
	args.push(mul);
	}
	args.push(outer_arg);
	if duplicated {
	// We know that duplicated args by construction have a following argument,
	// so this can not be out of bounds.
	let next_outer_arg = outer_args[outer_pos + 1];
	let next_outer_ty = cx.val_ty(next_outer_arg);
	// FIXME(ZuseZ4): We should add support for Vec here too, but it's less urgent since
	// vectors behind references (&Vec<T>) are already supported. Users can not pass a
	// Vec by value for reverse mode, so this would only help forward mode autodiff.
	let slice = {
	if activity_pos + 1 >= inputs.len() {
	// If there is no arg following our ptr, it also can't be a slice,
	// since that would lead to a ptr, int pair.
	false
	} else {
	let next_activity = inputs[activity_pos + 1];
	// We analyze the MIR types and add this dummy activity if we visit a slice.
	matches!(next_activity, DiffActivity::FakeActivitySize(_))
	}
	};
	if slice {
	// A duplicated slice will have the following two outer_fn arguments:
	// (..., ptr1, int1, ptr2, int2, ...). We add the following llvm-ir to our __enzyme call:
	// (..., metadata! enzyme_dup, ptr, ptr, int1, ...).
	// FIXME(ZuseZ4): We will upstream a safety check later which asserts that
	// int2 >= int1, which means the shadow vector is large enough to store the gradient.
	assert_eq!(cx.type_kind(next_outer_ty), TypeKind::Integer);

	let iterations =
	if matches!(diff_activity, DiffActivity::Dualv) { 1 } else { width as usize };

	for i in 0..iterations {
	let next_outer_arg2 = outer_args[outer_pos + 2 * (i + 1)];
	let next_outer_ty2 = cx.val_ty(next_outer_arg2);
	assert_eq!(cx.type_kind(next_outer_ty2), TypeKind::Pointer);
	let next_outer_arg3 = outer_args[outer_pos + 2 * (i + 1) + 1];
	let next_outer_ty3 = cx.val_ty(next_outer_arg3);
	assert_eq!(cx.type_kind(next_outer_ty3), TypeKind::Integer);
	args.push(next_outer_arg2);
	}
	args.push(cx.get_metadata_value(enzyme_const));
	args.push(next_outer_arg);
	outer_pos += 2 + 2 * iterations;
	activity_pos += 2;
	} else {
	// A duplicated pointer will have the following two outer_fn arguments:
	// (..., ptr, ptr, ...). We add the following llvm-ir to our __enzyme call:
	// (..., metadata! enzyme_dup, ptr, ptr, ...).
	if matches!(diff_activity, DiffActivity::Duplicated \| DiffActivity::DuplicatedOnly)
	{
	assert_eq!(cx.type_kind(next_outer_ty), TypeKind::Pointer);
	}
	// In the case of Dual we don't have assumptions, e.g. f32 would be valid.
	args.push(next_outer_arg);
	outer_pos += 2;
	activity_pos += 1;

	// Now, if width > 1, we need to account for that
	for _ in 1..width {
	let next_outer_arg = outer_args[outer_pos];
	args.push(next_outer_arg);
	outer_pos += 1;
	}
	}
	} else {
	// We do not differentiate with resprect to this argument.
	// We already added the metadata and argument above, so just increase the counters.
	outer_pos += 1;
	activity_pos += 1;
	}
	}
	}

	/// When differentiating `fn_to_diff`, take a `outer_fn` and generate another
	/// function with expected naming and calling conventions[^1] which will be
	/// discovered by the enzyme LLVM pass and its body populated with the differentiated
	/// `fn_to_diff`. `outer_fn` is then modified to have a call to the generated
	/// function and handle the differences between the Rust calling convention and
	/// Enzyme.
	/// [^1]: <https://enzyme.mit.edu/getting_started/CallingConvention/>
	// FIXME(ZuseZ4): `outer_fn` should include upstream safety checks to
	// cover some assumptions of enzyme/autodiff, which could lead to UB otherwise.
	pub(crate) fn generate_enzyme_call<'ll, 'tcx>(
	builder: &mut Builder<'_, 'll, 'tcx>,
	cx: &SimpleCx<'ll>,
	fn_to_diff: &'ll Value,
	outer_name: &str,
	ret_ty: &'ll Type,
	fn_args: &[&'ll Value],
	attrs: AutoDiffAttrs,
	dest: PlaceRef<'tcx, &'ll Value>,
	) {
	// We have to pick the name depending on whether we want forward or reverse mode autodiff.
	let mut ad_name: String = match attrs.mode {
	DiffMode::Forward => "__enzyme_fwddiff",
	DiffMode::Reverse => "__enzyme_autodiff",
	_ => panic!("logic bug in autodiff, unrecognized mode"),
	}
	.to_string();

	// add outer_name to ad_name to make it unique, in case users apply autodiff to multiple
	// functions. Unwrap will only panic, if LLVM gave us an invalid string.
	ad_name.push_str(outer_name);

	// Let us assume the user wrote the following function square:
	//
	// ```llvm
	// define double @square(double %x) {
	// entry:
	// %0 = fmul double %x, %x
	// ret double %0
	// }
	// ```
	// define double @dsquare(double %x) {
	// return 0.0;
	// }
	// ```
	//
	// so our `outer_fn` will be `dsquare`. The unsafe code section below now removes the placeholder
	// code and inserts an autodiff call. We also add a declaration for the __enzyme_autodiff call.
	// Again, the arguments to all functions are slightly simplified.
	// ```llvm
	// declare double @__enzyme_autodiff_square(...)
	//
	// define double @dsquare(double %x) {
	// entry:
	// %0 = tail call double (...) @__enzyme_autodiff_square(double (double)* nonnull @square, double %x)
	// ret double %0
	// }
	// ```
	let enzyme_ty = unsafe { llvm::LLVMFunctionType(ret_ty, ptr::null(), 0, True) };

	// FIXME(ZuseZ4): the CC/Addr/Vis values are best effort guesses, we should look at tests and
	// think a bit more about what should go here.
	// FIXME(Sa4dUs): have to find a way to get the cc, using `FastCallConv` for now
	let cc = 8;
	let ad_fn = declare_simple_fn(
	cx,
	&ad_name,
	llvm::CallConv::try_from(cc).expect("invalid callconv"),
	llvm::UnnamedAddr::No,
	llvm::Visibility::Default,
	enzyme_ty,
	);

	// Otherwise LLVM might inline our temporary code before the enzyme pass has a chance to
	// do it's work.
	let attr = llvm::AttributeKind::NoInline.create_attr(cx.llcx);
	attributes::apply_to_llfn(ad_fn, Function, &[attr]);

	let num_args = llvm::LLVMCountParams(&fn_to_diff);
	let mut args = Vec::with_capacity(num_args as usize + 1);
	args.push(fn_to_diff);

	let enzyme_primal_ret = cx.create_metadata("enzyme_primal_return".to_string()).unwrap();
	if matches!(attrs.ret_activity, DiffActivity::Dual \| DiffActivity::Active) {
	args.push(cx.get_metadata_value(enzyme_primal_ret));
	}
	if attrs.width > 1 {
	let enzyme_width = cx.create_metadata("enzyme_width".to_string()).unwrap();
	args.push(cx.get_metadata_value(enzyme_width));
	args.push(cx.get_const_int(cx.type_i64(), attrs.width as u64));
	}

	match_args_from_caller_to_enzyme(
	&cx,
	builder,
	attrs.width,
	&mut args,
	&attrs.input_activity,
	fn_args,
	);

	let call = builder.call(enzyme_ty, None, None, ad_fn, &args, None, None);

	builder.store_to_place(call, dest.val);
	}

	pub(crate) fn differentiate<'ll>(
	module: &'ll ModuleCodegen<ModuleLlvm>,
	cgcx: &CodegenContext<LlvmCodegenBackend>,
	diff_items: Vec<AutoDiffItem>,
	) -> Result<(), FatalError> {
	// TODO(Sa4dUs): delete all this logic
	for item in &diff_items {
	trace!("{}", item);
	}

	let diag_handler = cgcx.create_dcx();

	let cx = SimpleCx::new(module.module_llvm.llmod(), module.module_llvm.llcx, cgcx.pointer_size);

	// First of all, did the user try to use autodiff without using the -Zautodiff=Enable flag?
	if !diff_items.is_empty()
	&& !cgcx.opts.unstable_opts.autodiff.contains(&rustc_session::config::AutoDiff::Enable)
	{
	return Err(diag_handler.handle().emit_almost_fatal(AutoDiffWithoutEnable));
	}

	// Here we replace the placeholder code with the actual autodiff code, which calls Enzyme.
	for item in diff_items.iter() {
	let name = item.source.clone();
	let fn_def: Option<&llvm::Value> = cx.get_function(&name);
	let Some(_fn_def) = fn_def else {
	return Err(llvm_err(
	diag_handler.handle(),
	LlvmError::PrepareAutoDiff {
	src: item.source.clone(),
	target: item.target.clone(),
	error: "could not find source function".to_owned(),
	},
	));
	};
	debug!(?item.target);
	let fn_target: Option<&llvm::Value> = cx.get_function(&item.target);
	let Some(_fn_target) = fn_target else {
	return Err(llvm_err(
	diag_handler.handle(),
	LlvmError::PrepareAutoDiff {
	src: item.source.clone(),
	target: item.target.clone(),
	error: "could not find target function".to_owned(),
	},
	));
	};

	// generate_enzyme_call(&cx, fn_def, fn_target, item.attrs.clone());
	}

	// FIXME(ZuseZ4): support SanitizeHWAddress and prevent illegal/unsupported opts

	trace!("done with differentiate()");

	Ok(())
	}