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[NFC] extract autodiff call lowering in cg_llvm into own function
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‎compiler/rustc_codegen_llvm/src/builder/autodiff.rs

Lines changed: 108 additions & 93 deletions
Original file line numberDiff line numberDiff line change
@@ -28,6 +28,113 @@ fn get_params(fnc: &Value) -> Vec<&Value> {
2828
}
2929
}
3030

31+
fn match_args_from_caller_to_enzyme<'ll>(
32+
cx: &SimpleCx<'ll>,
33+
args: &mut Vec<&'ll llvm::Value>,
34+
inputs: &[DiffActivity],
35+
outer_args: &[&'ll llvm::Value],
36+
) {
37+
debug!("matching autodiff arguments");
38+
// We now handle the issue that Rust level arguments not always match the llvm-ir level
39+
// arguments. A slice, `&[f32]`, for example, is represented as a pointer and a length on
40+
// llvm-ir level. The number of activities matches the number of Rust level arguments, so we
41+
// need to match those.
42+
// FIXME(ZuseZ4): This logic is a bit more complicated than it should be, can we simplify it
43+
// using iterators and peek()?
44+
let mut outer_pos: usize = 0;
45+
let mut activity_pos = 0;
46+
47+
let enzyme_const = cx.create_metadata("enzyme_const".to_string()).unwrap();
48+
let enzyme_out = cx.create_metadata("enzyme_out".to_string()).unwrap();
49+
let enzyme_dup = cx.create_metadata("enzyme_dup".to_string()).unwrap();
50+
let enzyme_dupnoneed = cx.create_metadata("enzyme_dupnoneed".to_string()).unwrap();
51+
52+
while activity_pos < inputs.len() {
53+
let diff_activity = inputs[activity_pos as usize];
54+
// Duplicated arguments received a shadow argument, into which enzyme will write the
55+
// gradient.
56+
let (activity, duplicated): (&Metadata, bool) = match diff_activity {
57+
DiffActivity::None => panic!("not a valid input activity"),
58+
DiffActivity::Const => (enzyme_const, false),
59+
DiffActivity::Active => (enzyme_out, false),
60+
DiffActivity::ActiveOnly => (enzyme_out, false),
61+
DiffActivity::Dual => (enzyme_dup, true),
62+
DiffActivity::DualOnly => (enzyme_dupnoneed, true),
63+
DiffActivity::Duplicated => (enzyme_dup, true),
64+
DiffActivity::DuplicatedOnly => (enzyme_dupnoneed, true),
65+
DiffActivity::FakeActivitySize => (enzyme_const, false),
66+
};
67+
let outer_arg = outer_args[outer_pos];
68+
args.push(cx.get_metadata_value(activity));
69+
args.push(outer_arg);
70+
if duplicated {
71+
// We know that duplicated args by construction have a following argument,
72+
// so this can not be out of bounds.
73+
let next_outer_arg = outer_args[outer_pos + 1];
74+
let next_outer_ty = cx.val_ty(next_outer_arg);
75+
// FIXME(ZuseZ4): We should add support for Vec here too, but it's less urgent since
76+
// vectors behind references (&Vec<T>) are already supported. Users can not pass a
77+
// Vec by value for reverse mode, so this would only help forward mode autodiff.
78+
let slice = {
79+
if activity_pos + 1 >= inputs.len() {
80+
// If there is no arg following our ptr, it also can't be a slice,
81+
// since that would lead to a ptr, int pair.
82+
false
83+
} else {
84+
let next_activity = inputs[activity_pos + 1];
85+
// We analyze the MIR types and add this dummy activity if we visit a slice.
86+
next_activity == DiffActivity::FakeActivitySize
87+
}
88+
};
89+
if slice {
90+
// A duplicated slice will have the following two outer_fn arguments:
91+
// (..., ptr1, int1, ptr2, int2, ...). We add the following llvm-ir to our __enzyme call:
92+
// (..., metadata! enzyme_dup, ptr, ptr, int1, ...).
93+
// FIXME(ZuseZ4): We will upstream a safety check later which asserts that
94+
// int2 >= int1, which means the shadow vector is large enough to store the gradient.
95+
assert!(unsafe {
96+
llvm::LLVMRustGetTypeKind(next_outer_ty) == llvm::TypeKind::Integer
97+
});
98+
let next_outer_arg2 = outer_args[outer_pos + 2];
99+
let next_outer_ty2 = cx.val_ty(next_outer_arg2);
100+
assert!(unsafe {
101+
llvm::LLVMRustGetTypeKind(next_outer_ty2) == llvm::TypeKind::Pointer
102+
});
103+
let next_outer_arg3 = outer_args[outer_pos + 3];
104+
let next_outer_ty3 = cx.val_ty(next_outer_arg3);
105+
assert!(unsafe {
106+
llvm::LLVMRustGetTypeKind(next_outer_ty3) == llvm::TypeKind::Integer
107+
});
108+
args.push(next_outer_arg2);
109+
args.push(cx.get_metadata_value(enzyme_const));
110+
args.push(next_outer_arg);
111+
outer_pos += 4;
112+
activity_pos += 2;
113+
} else {
114+
// A duplicated pointer will have the following two outer_fn arguments:
115+
// (..., ptr, ptr, ...). We add the following llvm-ir to our __enzyme call:
116+
// (..., metadata! enzyme_dup, ptr, ptr, ...).
117+
if matches!(diff_activity, DiffActivity::Duplicated | DiffActivity::DuplicatedOnly)
118+
{
119+
assert!(
120+
unsafe { llvm::LLVMRustGetTypeKind(next_outer_ty) }
121+
== llvm::TypeKind::Pointer
122+
);
123+
}
124+
// In the case of Dual we don't have assumptions, e.g. f32 would be valid.
125+
args.push(next_outer_arg);
126+
outer_pos += 2;
127+
activity_pos += 1;
128+
}
129+
} else {
130+
// We do not differentiate with resprect to this argument.
131+
// We already added the metadata and argument above, so just increase the counters.
132+
outer_pos += 1;
133+
activity_pos += 1;
134+
}
135+
}
136+
}
137+
31138
/// When differentiating `fn_to_diff`, take a `outer_fn` and generate another
32139
/// function with expected naming and calling conventions[^1] which will be
33140
/// discovered by the enzyme LLVM pass and its body populated with the differentiated
@@ -132,12 +239,7 @@ fn generate_enzyme_call<'ll>(
132239
let mut args = Vec::with_capacity(num_args as usize + 1);
133240
args.push(fn_to_diff);
134241

135-
let enzyme_const = cx.create_metadata("enzyme_const".to_string()).unwrap();
136-
let enzyme_out = cx.create_metadata("enzyme_out".to_string()).unwrap();
137-
let enzyme_dup = cx.create_metadata("enzyme_dup".to_string()).unwrap();
138-
let enzyme_dupnoneed = cx.create_metadata("enzyme_dupnoneed".to_string()).unwrap();
139242
let enzyme_primal_ret = cx.create_metadata("enzyme_primal_return".to_string()).unwrap();
140-
141243
match output {
142244
DiffActivity::Dual => {
143245
args.push(cx.get_metadata_value(enzyme_primal_ret));
@@ -148,95 +250,8 @@ fn generate_enzyme_call<'ll>(
148250
_ => {}
149251
}
150252

151-
debug!("matching autodiff arguments");
152-
// We now handle the issue that Rust level arguments not always match the llvm-ir level
153-
// arguments. A slice, `&[f32]`, for example, is represented as a pointer and a length on
154-
// llvm-ir level. The number of activities matches the number of Rust level arguments, so we
155-
// need to match those.
156-
// FIXME(ZuseZ4): This logic is a bit more complicated than it should be, can we simplify it
157-
// using iterators and peek()?
158-
let mut outer_pos: usize = 0;
159-
let mut activity_pos = 0;
160253
let outer_args: Vec<&llvm::Value> = get_params(outer_fn);
161-
while activity_pos < inputs.len() {
162-
let diff_activity = inputs[activity_pos as usize];
163-
// Duplicated arguments received a shadow argument, into which enzyme will write the
164-
// gradient.
165-
let (activity, duplicated): (&Metadata, bool) = match diff_activity {
166-
DiffActivity::None => panic!("not a valid input activity"),
167-
DiffActivity::Const => (enzyme_const, false),
168-
DiffActivity::Active => (enzyme_out, false),
169-
DiffActivity::ActiveOnly => (enzyme_out, false),
170-
DiffActivity::Dual => (enzyme_dup, true),
171-
DiffActivity::DualOnly => (enzyme_dupnoneed, true),
172-
DiffActivity::Duplicated => (enzyme_dup, true),
173-
DiffActivity::DuplicatedOnly => (enzyme_dupnoneed, true),
174-
DiffActivity::FakeActivitySize => (enzyme_const, false),
175-
};
176-
let outer_arg = outer_args[outer_pos];
177-
args.push(cx.get_metadata_value(activity));
178-
args.push(outer_arg);
179-
if duplicated {
180-
// We know that duplicated args by construction have a following argument,
181-
// so this can not be out of bounds.
182-
let next_outer_arg = outer_args[outer_pos + 1];
183-
let next_outer_ty = cx.val_ty(next_outer_arg);
184-
// FIXME(ZuseZ4): We should add support for Vec here too, but it's less urgent since
185-
// vectors behind references (&Vec<T>) are already supported. Users can not pass a
186-
// Vec by value for reverse mode, so this would only help forward mode autodiff.
187-
let slice = {
188-
if activity_pos + 1 >= inputs.len() {
189-
// If there is no arg following our ptr, it also can't be a slice,
190-
// since that would lead to a ptr, int pair.
191-
false
192-
} else {
193-
let next_activity = inputs[activity_pos + 1];
194-
// We analyze the MIR types and add this dummy activity if we visit a slice.
195-
next_activity == DiffActivity::FakeActivitySize
196-
}
197-
};
198-
if slice {
199-
// A duplicated slice will have the following two outer_fn arguments:
200-
// (..., ptr1, int1, ptr2, int2, ...). We add the following llvm-ir to our __enzyme call:
201-
// (..., metadata! enzyme_dup, ptr, ptr, int1, ...).
202-
// FIXME(ZuseZ4): We will upstream a safety check later which asserts that
203-
// int2 >= int1, which means the shadow vector is large enough to store the gradient.
204-
assert!(llvm::LLVMRustGetTypeKind(next_outer_ty) == llvm::TypeKind::Integer);
205-
let next_outer_arg2 = outer_args[outer_pos + 2];
206-
let next_outer_ty2 = cx.val_ty(next_outer_arg2);
207-
assert!(llvm::LLVMRustGetTypeKind(next_outer_ty2) == llvm::TypeKind::Pointer);
208-
let next_outer_arg3 = outer_args[outer_pos + 3];
209-
let next_outer_ty3 = cx.val_ty(next_outer_arg3);
210-
assert!(llvm::LLVMRustGetTypeKind(next_outer_ty3) == llvm::TypeKind::Integer);
211-
args.push(next_outer_arg2);
212-
args.push(cx.get_metadata_value(enzyme_const));
213-
args.push(next_outer_arg);
214-
outer_pos += 4;
215-
activity_pos += 2;
216-
} else {
217-
// A duplicated pointer will have the following two outer_fn arguments:
218-
// (..., ptr, ptr, ...). We add the following llvm-ir to our __enzyme call:
219-
// (..., metadata! enzyme_dup, ptr, ptr, ...).
220-
if matches!(
221-
diff_activity,
222-
DiffActivity::Duplicated | DiffActivity::DuplicatedOnly
223-
) {
224-
assert!(
225-
llvm::LLVMRustGetTypeKind(next_outer_ty) == llvm::TypeKind::Pointer
226-
);
227-
}
228-
// In the case of Dual we don't have assumptions, e.g. f32 would be valid.
229-
args.push(next_outer_arg);
230-
outer_pos += 2;
231-
activity_pos += 1;
232-
}
233-
} else {
234-
// We do not differentiate with resprect to this argument.
235-
// We already added the metadata and argument above, so just increase the counters.
236-
outer_pos += 1;
237-
activity_pos += 1;
238-
}
239-
}
254+
match_args_from_caller_to_enzyme(&cx, &mut args, &inputs, &outer_args);
240255

241256
let call = builder.call(enzyme_ty, ad_fn, &args, None);
242257

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