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::back::write::ModuleConfig;
 use rustc_codegen_ssa::traits::BaseTypeCodegenMethods as _;
 use rustc_errors::FatalError;
 use tracing::{debug, trace};

 use crate::back::write::llvm_err;
 use crate::builder::SBuilder;
 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};
 use crate::value::Value;
 use crate::{CodegenContext, LlvmCodegenBackend, ModuleLlvm, attributes, llvm};

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

 fn match_args_from_caller_to_enzyme<'ll>(
     cx: &SimpleCx<'ll>,
     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_dupnoneed = cx.create_metadata("enzyme_dupnoneed".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::DualOnly => (enzyme_dupnoneed, 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));
         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.
                     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!(unsafe {
                     llvm::LLVMRustGetTypeKind(next_outer_ty) == llvm::TypeKind::Integer
                 });
                 let next_outer_arg2 = outer_args[outer_pos + 2];
                 let next_outer_ty2 = cx.val_ty(next_outer_arg2);
                 assert!(unsafe {
                     llvm::LLVMRustGetTypeKind(next_outer_ty2) == llvm::TypeKind::Pointer
                 });
                 let next_outer_arg3 = outer_args[outer_pos + 3];
                 let next_outer_ty3 = cx.val_ty(next_outer_arg3);
                 assert!(unsafe {
                     llvm::LLVMRustGetTypeKind(next_outer_ty3) == llvm::TypeKind::Integer
                 });
                 args.push(next_outer_arg2);
                 args.push(cx.get_metadata_value(enzyme_const));
                 args.push(next_outer_arg);
                 outer_pos += 4;
                 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!(
                         unsafe { llvm::LLVMRustGetTypeKind(next_outer_ty) }
                             == llvm::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;
             }
         } 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.
 fn generate_enzyme_call<'ll>(
     cx: &SimpleCx<'ll>,
     fn_to_diff: &'ll Value,
     outer_fn: &'ll Value,
     attrs: AutoDiffAttrs,
 ) {
     // 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_fn 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.
     let name = llvm::get_value_name(outer_fn);
     let outer_fn_name = std::str::from_utf8(name).unwrap();
     ad_name.push_str(outer_fn_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
     // }
     // ```
     //
     // The user now applies autodiff to the function square, in which case fn_to_diff will be `square`.
     // Our macro generates the following placeholder code (slightly simplified):
     //
     // ```llvm
     // define double @dsquare(double %x) {
     //  ; placeholder code
     //  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
     // }
     // ```
     unsafe {
         // On LLVM-IR, we can luckily declare __enzyme_ functions without specifying the input
         // arguments. We do however need to declare them with their correct return type.
         // We already figured the correct return type out in our frontend, when generating the outer_fn,
         // so we can now just go ahead and use that. FIXME(ZuseZ4): This doesn't handle sret yet.
         let fn_ty = llvm::LLVMGlobalGetValueType(outer_fn);
         let ret_ty = llvm::LLVMGetReturnType(fn_ty);

         // LLVM can figure out the input types on it's own, so we take a shortcut here.
         let enzyme_ty = 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.
         let cc = llvm::LLVMGetFunctionCallConv(outer_fn);
         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]);

         // first, remove all calls from fnc
         let entry = llvm::LLVMGetFirstBasicBlock(outer_fn);
         let br = llvm::LLVMRustGetTerminator(entry);
         llvm::LLVMRustEraseInstFromParent(br);

         let last_inst = llvm::LLVMRustGetLastInstruction(entry).unwrap();
         let mut builder = SBuilder::build(cx, entry);

         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));
         }

         let outer_args: Vec<&llvm::Value> = get_params(outer_fn);
         match_args_from_caller_to_enzyme(&cx, &mut args, &attrs.input_activity, &outer_args);

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

         // This part is a bit iffy. LLVM requires that a call to an inlineable function has some
         // metadata attachted to it, but we just created this code oota. Given that the
         // differentiated function already has partly confusing metadata, and given that this
         // affects nothing but the auttodiff IR, we take a shortcut and just steal metadata from the
         // dummy code which we inserted at a higher level.
         // FIXME(ZuseZ4): Work with Enzyme core devs to clarify what debug metadata issues we have,
         // and how to best improve it for enzyme core and rust-enzyme.
         let md_ty = cx.get_md_kind_id("dbg");
         if llvm::LLVMRustHasMetadata(last_inst, md_ty) {
             let md = llvm::LLVMRustDIGetInstMetadata(last_inst)
                 .expect("failed to get instruction metadata");
             let md_todiff = cx.get_metadata_value(md);
             llvm::LLVMSetMetadata(call, md_ty, md_todiff);
         } else {
             // We don't panic, since depending on whether we are in debug or release mode, we might
             // have no debug info to copy, which would then be ok.
             trace!("no dbg info");
         }

         // Now that we copied the metadata, get rid of dummy code.
         llvm::LLVMRustEraseInstUntilInclusive(entry, last_inst);

         if cx.val_ty(call) == cx.type_void() {
             builder.ret_void();
         } else {
             builder.ret(call);
         }

         // Let's crash in case that we messed something up above and generated invalid IR.
         llvm::LLVMRustVerifyFunction(
             outer_fn,
             llvm::LLVMRustVerifierFailureAction::LLVMAbortProcessAction,
         );
     }
 }

 pub(crate) fn differentiate<'ll>(
     module: &'ll ModuleCodegen<ModuleLlvm>,
     cgcx: &CodegenContext<LlvmCodegenBackend>,
     diff_items: Vec<AutoDiffItem>,
     _config: &ModuleConfig,
 ) -> Result<(), FatalError> {
     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)
     {
         let dcx = cgcx.create_dcx();
         return Err(dcx.handle().emit_almost_fatal(AutoDiffWithoutEnable));
     }

     // Before dumping the module, we want all the TypeTrees to become part of the module.
     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::back::write::ModuleConfig;
	use rustc_codegen_ssa::traits::BaseTypeCodegenMethods as _;
	use rustc_errors::FatalError;
	use tracing::{debug, trace};

	use crate::back::write::llvm_err;
	use crate::builder::SBuilder;
	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};
	use crate::value::Value;
	use crate::{CodegenContext, LlvmCodegenBackend, ModuleLlvm, attributes, llvm};

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

	fn match_args_from_caller_to_enzyme<'ll>(
	cx: &SimpleCx<'ll>,
	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_dupnoneed = cx.create_metadata("enzyme_dupnoneed".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::DualOnly => (enzyme_dupnoneed, 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));
	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.
	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!(unsafe {
	llvm::LLVMRustGetTypeKind(next_outer_ty) == llvm::TypeKind::Integer
	});
	let next_outer_arg2 = outer_args[outer_pos + 2];
	let next_outer_ty2 = cx.val_ty(next_outer_arg2);
	assert!(unsafe {
	llvm::LLVMRustGetTypeKind(next_outer_ty2) == llvm::TypeKind::Pointer
	});
	let next_outer_arg3 = outer_args[outer_pos + 3];
	let next_outer_ty3 = cx.val_ty(next_outer_arg3);
	assert!(unsafe {
	llvm::LLVMRustGetTypeKind(next_outer_ty3) == llvm::TypeKind::Integer
	});
	args.push(next_outer_arg2);
	args.push(cx.get_metadata_value(enzyme_const));
	args.push(next_outer_arg);
	outer_pos += 4;
	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!(
	unsafe { llvm::LLVMRustGetTypeKind(next_outer_ty) }
	== llvm::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;
	}
	} 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.
	fn generate_enzyme_call<'ll>(
	cx: &SimpleCx<'ll>,
	fn_to_diff: &'ll Value,
	outer_fn: &'ll Value,
	attrs: AutoDiffAttrs,
	) {
	// 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_fn 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.
	let name = llvm::get_value_name(outer_fn);
	let outer_fn_name = std::str::from_utf8(name).unwrap();
	ad_name.push_str(outer_fn_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
	// }
	// ```
	//
	// The user now applies autodiff to the function square, in which case fn_to_diff will be `square`.
	// Our macro generates the following placeholder code (slightly simplified):
	//
	// ```llvm
	// define double @dsquare(double %x) {
	// ; placeholder code
	// 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
	// }
	// ```
	unsafe {
	// On LLVM-IR, we can luckily declare __enzyme_ functions without specifying the input
	// arguments. We do however need to declare them with their correct return type.
	// We already figured the correct return type out in our frontend, when generating the outer_fn,
	// so we can now just go ahead and use that. FIXME(ZuseZ4): This doesn't handle sret yet.
	let fn_ty = llvm::LLVMGlobalGetValueType(outer_fn);
	let ret_ty = llvm::LLVMGetReturnType(fn_ty);

	// LLVM can figure out the input types on it's own, so we take a shortcut here.
	let enzyme_ty = 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.
	let cc = llvm::LLVMGetFunctionCallConv(outer_fn);
	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]);

	// first, remove all calls from fnc
	let entry = llvm::LLVMGetFirstBasicBlock(outer_fn);
	let br = llvm::LLVMRustGetTerminator(entry);
	llvm::LLVMRustEraseInstFromParent(br);

	let last_inst = llvm::LLVMRustGetLastInstruction(entry).unwrap();
	let mut builder = SBuilder::build(cx, entry);

	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));
	}

	let outer_args: Vec<&llvm::Value> = get_params(outer_fn);
	match_args_from_caller_to_enzyme(&cx, &mut args, &attrs.input_activity, &outer_args);

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

	// This part is a bit iffy. LLVM requires that a call to an inlineable function has some
	// metadata attachted to it, but we just created this code oota. Given that the
	// differentiated function already has partly confusing metadata, and given that this
	// affects nothing but the auttodiff IR, we take a shortcut and just steal metadata from the
	// dummy code which we inserted at a higher level.
	// FIXME(ZuseZ4): Work with Enzyme core devs to clarify what debug metadata issues we have,
	// and how to best improve it for enzyme core and rust-enzyme.
	let md_ty = cx.get_md_kind_id("dbg");
	if llvm::LLVMRustHasMetadata(last_inst, md_ty) {
	let md = llvm::LLVMRustDIGetInstMetadata(last_inst)
	.expect("failed to get instruction metadata");
	let md_todiff = cx.get_metadata_value(md);
	llvm::LLVMSetMetadata(call, md_ty, md_todiff);
	} else {
	// We don't panic, since depending on whether we are in debug or release mode, we might
	// have no debug info to copy, which would then be ok.
	trace!("no dbg info");
	}

	// Now that we copied the metadata, get rid of dummy code.
	llvm::LLVMRustEraseInstUntilInclusive(entry, last_inst);

	if cx.val_ty(call) == cx.type_void() {
	builder.ret_void();
	} else {
	builder.ret(call);
	}

	// Let's crash in case that we messed something up above and generated invalid IR.
	llvm::LLVMRustVerifyFunction(
	outer_fn,
	llvm::LLVMRustVerifierFailureAction::LLVMAbortProcessAction,
	);
	}
	}

	pub(crate) fn differentiate<'ll>(
	module: &'ll ModuleCodegen<ModuleLlvm>,
	cgcx: &CodegenContext<LlvmCodegenBackend>,
	diff_items: Vec<AutoDiffItem>,
	_config: &ModuleConfig,
	) -> Result<(), FatalError> {
	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)
	{
	let dcx = cgcx.create_dcx();
	return Err(dcx.handle().emit_almost_fatal(AutoDiffWithoutEnable));
	}

	// Before dumping the module, we want all the TypeTrees to become part of the module.
	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(())
	}