rustc_codegen_llvm/builder/autodiff.rs
1use std::ptr;
2
3use rustc_ast::expand::autodiff_attrs::{AutoDiffAttrs, AutoDiffItem, DiffActivity, DiffMode};
4use rustc_codegen_ssa::ModuleCodegen;
5use rustc_codegen_ssa::back::write::ModuleConfig;
6use rustc_codegen_ssa::common::TypeKind;
7use rustc_codegen_ssa::traits::BaseTypeCodegenMethods;
8use rustc_errors::FatalError;
9use rustc_middle::bug;
10use tracing::{debug, trace};
11
12use crate::back::write::llvm_err;
13use crate::builder::SBuilder;
14use crate::context::SimpleCx;
15use crate::declare::declare_simple_fn;
16use crate::errors::{AutoDiffWithoutEnable, LlvmError};
17use crate::llvm::AttributePlace::Function;
18use crate::llvm::{Metadata, True};
19use crate::value::Value;
20use crate::{CodegenContext, LlvmCodegenBackend, ModuleLlvm, attributes, llvm};
21
22fn get_params(fnc: &Value) -> Vec<&Value> {
23 let param_num = llvm::LLVMCountParams(fnc) as usize;
24 let mut fnc_args: Vec<&Value> = vec![];
25 fnc_args.reserve(param_num);
26 unsafe {
27 llvm::LLVMGetParams(fnc, fnc_args.as_mut_ptr());
28 fnc_args.set_len(param_num);
29 }
30 fnc_args
31}
32
33fn has_sret(fnc: &Value) -> bool {
34 let num_args = llvm::LLVMCountParams(fnc) as usize;
35 if num_args == 0 {
36 false
37 } else {
38 unsafe { llvm::LLVMRustHasAttributeAtIndex(fnc, 0, llvm::AttributeKind::StructRet) }
39 }
40}
41
42// When we call the `__enzyme_autodiff` or `__enzyme_fwddiff` function, we need to pass all the
43// original inputs, as well as metadata and the additional shadow arguments.
44// This function matches the arguments from the outer function to the inner enzyme call.
45//
46// This function also considers that Rust level arguments not always match the llvm-ir level
47// arguments. A slice, `&[f32]`, for example, is represented as a pointer and a length on
48// llvm-ir level. The number of activities matches the number of Rust level arguments, so we
49// need to match those.
50// FIXME(ZuseZ4): This logic is a bit more complicated than it should be, can we simplify it
51// using iterators and peek()?
52fn match_args_from_caller_to_enzyme<'ll>(
53 cx: &SimpleCx<'ll>,
54 width: u32,
55 args: &mut Vec<&'ll llvm::Value>,
56 inputs: &[DiffActivity],
57 outer_args: &[&'ll llvm::Value],
58 has_sret: bool,
59) {
60 debug!("matching autodiff arguments");
61 // We now handle the issue that Rust level arguments not always match the llvm-ir level
62 // arguments. A slice, `&[f32]`, for example, is represented as a pointer and a length on
63 // llvm-ir level. The number of activities matches the number of Rust level arguments, so we
64 // need to match those.
65 // FIXME(ZuseZ4): This logic is a bit more complicated than it should be, can we simplify it
66 // using iterators and peek()?
67 let mut outer_pos: usize = 0;
68 let mut activity_pos = 0;
69
70 if has_sret {
71 // Then the first outer arg is the sret pointer. Enzyme doesn't know about sret, so the
72 // inner function will still return something. We increase our outer_pos by one,
73 // and once we're done with all other args we will take the return of the inner call and
74 // update the sret pointer with it
75 outer_pos = 1;
76 }
77
78 let enzyme_const = cx.create_metadata("enzyme_const".to_string()).unwrap();
79 let enzyme_out = cx.create_metadata("enzyme_out".to_string()).unwrap();
80 let enzyme_dup = cx.create_metadata("enzyme_dup".to_string()).unwrap();
81 let enzyme_dupnoneed = cx.create_metadata("enzyme_dupnoneed".to_string()).unwrap();
82
83 while activity_pos < inputs.len() {
84 let diff_activity = inputs[activity_pos as usize];
85 // Duplicated arguments received a shadow argument, into which enzyme will write the
86 // gradient.
87 let (activity, duplicated): (&Metadata, bool) = match diff_activity {
88 DiffActivity::None => panic!("not a valid input activity"),
89 DiffActivity::Const => (enzyme_const, false),
90 DiffActivity::Active => (enzyme_out, false),
91 DiffActivity::ActiveOnly => (enzyme_out, false),
92 DiffActivity::Dual => (enzyme_dup, true),
93 DiffActivity::DualOnly => (enzyme_dupnoneed, true),
94 DiffActivity::Duplicated => (enzyme_dup, true),
95 DiffActivity::DuplicatedOnly => (enzyme_dupnoneed, true),
96 DiffActivity::FakeActivitySize => (enzyme_const, false),
97 };
98 let outer_arg = outer_args[outer_pos];
99 args.push(cx.get_metadata_value(activity));
100 args.push(outer_arg);
101 if duplicated {
102 // We know that duplicated args by construction have a following argument,
103 // so this can not be out of bounds.
104 let next_outer_arg = outer_args[outer_pos + 1];
105 let next_outer_ty = cx.val_ty(next_outer_arg);
106 // FIXME(ZuseZ4): We should add support for Vec here too, but it's less urgent since
107 // vectors behind references (&Vec<T>) are already supported. Users can not pass a
108 // Vec by value for reverse mode, so this would only help forward mode autodiff.
109 let slice = {
110 if activity_pos + 1 >= inputs.len() {
111 // If there is no arg following our ptr, it also can't be a slice,
112 // since that would lead to a ptr, int pair.
113 false
114 } else {
115 let next_activity = inputs[activity_pos + 1];
116 // We analyze the MIR types and add this dummy activity if we visit a slice.
117 next_activity == DiffActivity::FakeActivitySize
118 }
119 };
120 if slice {
121 // A duplicated slice will have the following two outer_fn arguments:
122 // (..., ptr1, int1, ptr2, int2, ...). We add the following llvm-ir to our __enzyme call:
123 // (..., metadata! enzyme_dup, ptr, ptr, int1, ...).
124 // FIXME(ZuseZ4): We will upstream a safety check later which asserts that
125 // int2 >= int1, which means the shadow vector is large enough to store the gradient.
126 assert_eq!(cx.type_kind(next_outer_ty), TypeKind::Integer);
127
128 for i in 0..(width as usize) {
129 let next_outer_arg2 = outer_args[outer_pos + 2 * (i + 1)];
130 let next_outer_ty2 = cx.val_ty(next_outer_arg2);
131 assert_eq!(cx.type_kind(next_outer_ty2), TypeKind::Pointer);
132 let next_outer_arg3 = outer_args[outer_pos + 2 * (i + 1) + 1];
133 let next_outer_ty3 = cx.val_ty(next_outer_arg3);
134 assert_eq!(cx.type_kind(next_outer_ty3), TypeKind::Integer);
135 args.push(next_outer_arg2);
136 }
137 args.push(cx.get_metadata_value(enzyme_const));
138 args.push(next_outer_arg);
139 outer_pos += 2 + 2 * width as usize;
140 activity_pos += 2;
141 } else {
142 // A duplicated pointer will have the following two outer_fn arguments:
143 // (..., ptr, ptr, ...). We add the following llvm-ir to our __enzyme call:
144 // (..., metadata! enzyme_dup, ptr, ptr, ...).
145 if matches!(diff_activity, DiffActivity::Duplicated | DiffActivity::DuplicatedOnly)
146 {
147 assert_eq!(cx.type_kind(next_outer_ty), TypeKind::Pointer);
148 }
149 // In the case of Dual we don't have assumptions, e.g. f32 would be valid.
150 args.push(next_outer_arg);
151 outer_pos += 2;
152 activity_pos += 1;
153
154 // Now, if width > 1, we need to account for that
155 for _ in 1..width {
156 let next_outer_arg = outer_args[outer_pos];
157 args.push(next_outer_arg);
158 outer_pos += 1;
159 }
160 }
161 } else {
162 // We do not differentiate with resprect to this argument.
163 // We already added the metadata and argument above, so just increase the counters.
164 outer_pos += 1;
165 activity_pos += 1;
166 }
167 }
168}
169
170// On LLVM-IR, we can luckily declare __enzyme_ functions without specifying the input
171// arguments. We do however need to declare them with their correct return type.
172// We already figured the correct return type out in our frontend, when generating the outer_fn,
173// so we can now just go ahead and use that. This is not always trivial, e.g. because sret.
174// Beyond sret, this article describes our challenges nicely:
175// <https://yorickpeterse.com/articles/the-mess-that-is-handling-structure-arguments-and-returns-in-llvm/>
176// I.e. (i32, f32) will get merged into i64, but we don't handle that yet.
177fn compute_enzyme_fn_ty<'ll>(
178 cx: &SimpleCx<'ll>,
179 attrs: &AutoDiffAttrs,
180 fn_to_diff: &'ll Value,
181 outer_fn: &'ll Value,
182) -> &'ll llvm::Type {
183 let fn_ty = cx.get_type_of_global(outer_fn);
184 let mut ret_ty = cx.get_return_type(fn_ty);
185
186 let has_sret = has_sret(outer_fn);
187
188 if has_sret {
189 // Now we don't just forward the return type, so we have to figure it out based on the
190 // primal return type, in combination with the autodiff settings.
191 let fn_ty = cx.get_type_of_global(fn_to_diff);
192 let inner_ret_ty = cx.get_return_type(fn_ty);
193
194 let void_ty = unsafe { llvm::LLVMVoidTypeInContext(cx.llcx) };
195 if inner_ret_ty == void_ty {
196 // This indicates that even the inner function has an sret.
197 // Right now I only look for an sret in the outer function.
198 // This *probably* needs some extra handling, but I never ran
199 // into such a case. So I'll wait for user reports to have a test case.
200 bug!("sret in inner function");
201 }
202
203 if attrs.width == 1 {
204 // Enzyme returns a struct of style:
205 // `{ original_ret(if requested), float, float, ... }`
206 let mut struct_elements = vec![];
207 if attrs.has_primal_ret() {
208 struct_elements.push(inner_ret_ty);
209 }
210 // Next, we push the list of active floats, since they will be lowered to `enzyme_out`,
211 // and therefore part of the return struct.
212 let param_tys = cx.func_params_types(fn_ty);
213 for (act, param_ty) in attrs.input_activity.iter().zip(param_tys) {
214 if matches!(act, DiffActivity::Active) {
215 // Now find the float type at position i based on the fn_ty,
216 // to know what (f16/f32/f64/...) to add to the struct.
217 struct_elements.push(param_ty);
218 }
219 }
220 ret_ty = cx.type_struct(&struct_elements, false);
221 } else {
222 // First we check if we also have to deal with the primal return.
223 match attrs.mode {
224 DiffMode::Forward => match attrs.ret_activity {
225 DiffActivity::Dual => {
226 let arr_ty =
227 unsafe { llvm::LLVMArrayType2(inner_ret_ty, attrs.width as u64 + 1) };
228 ret_ty = arr_ty;
229 }
230 DiffActivity::DualOnly => {
231 let arr_ty =
232 unsafe { llvm::LLVMArrayType2(inner_ret_ty, attrs.width as u64) };
233 ret_ty = arr_ty;
234 }
235 DiffActivity::Const => {
236 todo!("Not sure, do we need to do something here?");
237 }
238 _ => {
239 bug!("unreachable");
240 }
241 },
242 DiffMode::Reverse => {
243 todo!("Handle sret for reverse mode");
244 }
245 _ => {
246 bug!("unreachable");
247 }
248 }
249 }
250 }
251
252 // LLVM can figure out the input types on it's own, so we take a shortcut here.
253 unsafe { llvm::LLVMFunctionType(ret_ty, ptr::null(), 0, True) }
254}
255
256/// When differentiating `fn_to_diff`, take a `outer_fn` and generate another
257/// function with expected naming and calling conventions[^1] which will be
258/// discovered by the enzyme LLVM pass and its body populated with the differentiated
259/// `fn_to_diff`. `outer_fn` is then modified to have a call to the generated
260/// function and handle the differences between the Rust calling convention and
261/// Enzyme.
262/// [^1]: <https://enzyme.mit.edu/getting_started/CallingConvention/>
263// FIXME(ZuseZ4): `outer_fn` should include upstream safety checks to
264// cover some assumptions of enzyme/autodiff, which could lead to UB otherwise.
265fn generate_enzyme_call<'ll>(
266 cx: &SimpleCx<'ll>,
267 fn_to_diff: &'ll Value,
268 outer_fn: &'ll Value,
269 attrs: AutoDiffAttrs,
270) {
271 // We have to pick the name depending on whether we want forward or reverse mode autodiff.
272 let mut ad_name: String = match attrs.mode {
273 DiffMode::Forward => "__enzyme_fwddiff",
274 DiffMode::Reverse => "__enzyme_autodiff",
275 _ => panic!("logic bug in autodiff, unrecognized mode"),
276 }
277 .to_string();
278
279 // add outer_fn name to ad_name to make it unique, in case users apply autodiff to multiple
280 // functions. Unwrap will only panic, if LLVM gave us an invalid string.
281 let name = llvm::get_value_name(outer_fn);
282 let outer_fn_name = std::str::from_utf8(name).unwrap();
283 ad_name.push_str(outer_fn_name);
284
285 // Let us assume the user wrote the following function square:
286 //
287 // ```llvm
288 // define double @square(double %x) {
289 // entry:
290 // %0 = fmul double %x, %x
291 // ret double %0
292 // }
293 // ```
294 //
295 // The user now applies autodiff to the function square, in which case fn_to_diff will be `square`.
296 // Our macro generates the following placeholder code (slightly simplified):
297 //
298 // ```llvm
299 // define double @dsquare(double %x) {
300 // ; placeholder code
301 // return 0.0;
302 // }
303 // ```
304 //
305 // so our `outer_fn` will be `dsquare`. The unsafe code section below now removes the placeholder
306 // code and inserts an autodiff call. We also add a declaration for the __enzyme_autodiff call.
307 // Again, the arguments to all functions are slightly simplified.
308 // ```llvm
309 // declare double @__enzyme_autodiff_square(...)
310 //
311 // define double @dsquare(double %x) {
312 // entry:
313 // %0 = tail call double (...) @__enzyme_autodiff_square(double (double)* nonnull @square, double %x)
314 // ret double %0
315 // }
316 // ```
317 unsafe {
318 let enzyme_ty = compute_enzyme_fn_ty(cx, &attrs, fn_to_diff, outer_fn);
319
320 // FIXME(ZuseZ4): the CC/Addr/Vis values are best effort guesses, we should look at tests and
321 // think a bit more about what should go here.
322 let cc = llvm::LLVMGetFunctionCallConv(outer_fn);
323 let ad_fn = declare_simple_fn(
324 cx,
325 &ad_name,
326 llvm::CallConv::try_from(cc).expect("invalid callconv"),
327 llvm::UnnamedAddr::No,
328 llvm::Visibility::Default,
329 enzyme_ty,
330 );
331
332 // Otherwise LLVM might inline our temporary code before the enzyme pass has a chance to
333 // do it's work.
334 let attr = llvm::AttributeKind::NoInline.create_attr(cx.llcx);
335 attributes::apply_to_llfn(ad_fn, Function, &[attr]);
336
337 // first, remove all calls from fnc
338 let entry = llvm::LLVMGetFirstBasicBlock(outer_fn);
339 let br = llvm::LLVMRustGetTerminator(entry);
340 llvm::LLVMRustEraseInstFromParent(br);
341
342 let last_inst = llvm::LLVMRustGetLastInstruction(entry).unwrap();
343 let mut builder = SBuilder::build(cx, entry);
344
345 let num_args = llvm::LLVMCountParams(&fn_to_diff);
346 let mut args = Vec::with_capacity(num_args as usize + 1);
347 args.push(fn_to_diff);
348
349 let enzyme_primal_ret = cx.create_metadata("enzyme_primal_return".to_string()).unwrap();
350 if matches!(attrs.ret_activity, DiffActivity::Dual | DiffActivity::Active) {
351 args.push(cx.get_metadata_value(enzyme_primal_ret));
352 }
353 if attrs.width > 1 {
354 let enzyme_width = cx.create_metadata("enzyme_width".to_string()).unwrap();
355 args.push(cx.get_metadata_value(enzyme_width));
356 args.push(cx.get_const_i64(attrs.width as u64));
357 }
358
359 let has_sret = has_sret(outer_fn);
360 let outer_args: Vec<&llvm::Value> = get_params(outer_fn);
361 match_args_from_caller_to_enzyme(
362 &cx,
363 attrs.width,
364 &mut args,
365 &attrs.input_activity,
366 &outer_args,
367 has_sret,
368 );
369
370 let call = builder.call(enzyme_ty, ad_fn, &args, None);
371
372 // This part is a bit iffy. LLVM requires that a call to an inlineable function has some
373 // metadata attached to it, but we just created this code oota. Given that the
374 // differentiated function already has partly confusing metadata, and given that this
375 // affects nothing but the auttodiff IR, we take a shortcut and just steal metadata from the
376 // dummy code which we inserted at a higher level.
377 // FIXME(ZuseZ4): Work with Enzyme core devs to clarify what debug metadata issues we have,
378 // and how to best improve it for enzyme core and rust-enzyme.
379 let md_ty = cx.get_md_kind_id("dbg");
380 if llvm::LLVMRustHasMetadata(last_inst, md_ty) {
381 let md = llvm::LLVMRustDIGetInstMetadata(last_inst)
382 .expect("failed to get instruction metadata");
383 let md_todiff = cx.get_metadata_value(md);
384 llvm::LLVMSetMetadata(call, md_ty, md_todiff);
385 } else {
386 // We don't panic, since depending on whether we are in debug or release mode, we might
387 // have no debug info to copy, which would then be ok.
388 trace!("no dbg info");
389 }
390
391 // Now that we copied the metadata, get rid of dummy code.
392 llvm::LLVMRustEraseInstUntilInclusive(entry, last_inst);
393
394 if cx.val_ty(call) == cx.type_void() || has_sret {
395 if has_sret {
396 // This is what we already have in our outer_fn (shortened):
397 // define void @_foo(ptr <..> sret([32 x i8]) initializes((0, 32)) %0, <...>) {
398 // %7 = call [4 x double] (...) @__enzyme_fwddiff_foo(ptr @square, metadata !"enzyme_width", i64 4, <...>)
399 // <Here we are, we want to add the following two lines>
400 // store [4 x double] %7, ptr %0, align 8
401 // ret void
402 // }
403
404 // now store the result of the enzyme call into the sret pointer.
405 let sret_ptr = outer_args[0];
406 let call_ty = cx.val_ty(call);
407 if attrs.width == 1 {
408 assert_eq!(cx.type_kind(call_ty), TypeKind::Struct);
409 } else {
410 assert_eq!(cx.type_kind(call_ty), TypeKind::Array);
411 }
412 llvm::LLVMBuildStore(&builder.llbuilder, call, sret_ptr);
413 }
414 builder.ret_void();
415 } else {
416 builder.ret(call);
417 }
418
419 // Let's crash in case that we messed something up above and generated invalid IR.
420 llvm::LLVMRustVerifyFunction(
421 outer_fn,
422 llvm::LLVMRustVerifierFailureAction::LLVMAbortProcessAction,
423 );
424 }
425}
426
427pub(crate) fn differentiate<'ll>(
428 module: &'ll ModuleCodegen<ModuleLlvm>,
429 cgcx: &CodegenContext<LlvmCodegenBackend>,
430 diff_items: Vec<AutoDiffItem>,
431 _config: &ModuleConfig,
432) -> Result<(), FatalError> {
433 for item in &diff_items {
434 trace!("{}", item);
435 }
436
437 let diag_handler = cgcx.create_dcx();
438
439 let cx = SimpleCx::new(module.module_llvm.llmod(), module.module_llvm.llcx, cgcx.pointer_size);
440
441 // First of all, did the user try to use autodiff without using the -Zautodiff=Enable flag?
442 if !diff_items.is_empty()
443 && !cgcx.opts.unstable_opts.autodiff.contains(&rustc_session::config::AutoDiff::Enable)
444 {
445 return Err(diag_handler.handle().emit_almost_fatal(AutoDiffWithoutEnable));
446 }
447
448 // Before dumping the module, we want all the TypeTrees to become part of the module.
449 for item in diff_items.iter() {
450 let name = item.source.clone();
451 let fn_def: Option<&llvm::Value> = cx.get_function(&name);
452 let Some(fn_def) = fn_def else {
453 return Err(llvm_err(
454 diag_handler.handle(),
455 LlvmError::PrepareAutoDiff {
456 src: item.source.clone(),
457 target: item.target.clone(),
458 error: "could not find source function".to_owned(),
459 },
460 ));
461 };
462 debug!(?item.target);
463 let fn_target: Option<&llvm::Value> = cx.get_function(&item.target);
464 let Some(fn_target) = fn_target else {
465 return Err(llvm_err(
466 diag_handler.handle(),
467 LlvmError::PrepareAutoDiff {
468 src: item.source.clone(),
469 target: item.target.clone(),
470 error: "could not find target function".to_owned(),
471 },
472 ));
473 };
474
475 generate_enzyme_call(&cx, fn_def, fn_target, item.attrs.clone());
476 }
477
478 // FIXME(ZuseZ4): support SanitizeHWAddress and prevent illegal/unsupported opts
479
480 trace!("done with differentiate()");
481
482 Ok(())
483}