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TypeTree support in autodiff
# TypeTrees for Autodiff
## What are TypeTrees?
Memory layout descriptors for Enzyme. Tell Enzyme exactly how types are structured in memory so it can compute derivatives efficiently.
## Structure
```rust
TypeTree(Vec<Type>)
Type {
offset: isize, // byte offset (-1 = everywhere)
size: usize, // size in bytes
kind: Kind, // Float, Integer, Pointer, etc.
child: TypeTree // nested structure
}
```
## Example: `fn compute(x: &f32, data: &[f32]) -> f32`
**Input 0: `x: &f32`**
```rust
TypeTree(vec![Type {
offset: -1, size: 8, kind: Pointer,
child: TypeTree(vec![Type {
offset: -1, size: 4, kind: Float,
child: TypeTree::new()
}])
}])
```
**Input 1: `data: &[f32]`**
```rust
TypeTree(vec![Type {
offset: -1, size: 8, kind: Pointer,
child: TypeTree(vec![Type {
offset: -1, size: 4, kind: Float, // -1 = all elements
child: TypeTree::new()
}])
}])
```
**Output: `f32`**
```rust
TypeTree(vec![Type {
offset: -1, size: 4, kind: Float,
child: TypeTree::new()
}])
```
## Why Needed?
- Enzyme can't deduce complex type layouts from LLVM IR
- Prevents slow memory pattern analysis
- Enables correct derivative computation for nested structures
- Tells Enzyme which bytes are differentiable vs metadata
## What Enzyme Does With This Information:
Without TypeTrees (current state):
```llvm
; Enzyme sees generic LLVM IR:
define float ``@distance(ptr*`` %p1, ptr* %p2) {
; Has to guess what these pointers point to
; Slow analysis of all memory operations
; May miss optimization opportunities
}
```
With TypeTrees (our implementation):
```llvm
define "enzyme_type"="{[]:Float@float}" float ``@distance(``
ptr "enzyme_type"="{[]:Pointer}" %p1,
ptr "enzyme_type"="{[]:Pointer}" %p2
) {
; Enzyme knows exact type layout
; Can generate efficient derivative code directly
}
```
# TypeTrees - Offset and -1 Explained
## Type Structure
```rust
Type {
offset: isize, // WHERE this type starts
size: usize, // HOW BIG this type is
kind: Kind, // WHAT KIND of data (Float, Int, Pointer)
child: TypeTree // WHAT'S INSIDE (for pointers/containers)
}
```
## Offset Values
### Regular Offset (0, 4, 8, etc.)
**Specific byte position within a structure**
```rust
struct Point {
x: f32, // offset 0, size 4
y: f32, // offset 4, size 4
id: i32, // offset 8, size 4
}
```
TypeTree for `&Point` (internal representation):
```rust
TypeTree(vec![
Type { offset: 0, size: 4, kind: Float }, // x at byte 0
Type { offset: 4, size: 4, kind: Float }, // y at byte 4
Type { offset: 8, size: 4, kind: Integer } // id at byte 8
])
```
Generates LLVM:
```llvm
"enzyme_type"="{[]:Float@float}"
```
### Offset -1 (Special: "Everywhere")
**Means "this pattern repeats for ALL elements"**
#### Example 1: Array `[f32; 100]`
```rust
TypeTree(vec![Type {
offset: -1, // ALL positions
size: 4, // each f32 is 4 bytes
kind: Float, // every element is float
}])
```
Instead of listing 100 separate Types with offsets `0,4,8,12...396`
#### Example 2: Slice `&[i32]`
```rust
// Pointer to slice data
TypeTree(vec![Type {
offset: -1, size: 8, kind: Pointer,
child: TypeTree(vec![Type {
offset: -1, // ALL slice elements
size: 4, // each i32 is 4 bytes
kind: Integer
}])
}])
```
#### Example 3: Mixed Structure
```rust
struct Container {
header: i64, // offset 0
data: [f32; 1000], // offset 8, but elements use -1
}
```
```rust
TypeTree(vec![
Type { offset: 0, size: 8, kind: Integer }, // header
Type { offset: 8, size: 4000, kind: Pointer,
child: TypeTree(vec![Type {
offset: -1, size: 4, kind: Float // ALL array elements
}])
}
])
```
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cg_llvm: Replace some DIBuilder wrappers with LLVM-C API bindings (part 5)
- Part of rust-lang/rust#134001
- Follow-up to rust-lang/rust#146673
---
This is another batch of LLVMDIBuilder binding migrations, replacing some our own LLVMRust bindings with bindings to upstream LLVM-C APIs.
Some of these are a little more complex than most of the previous migrations, because they split one LLVMRust binding into multiple LLVM bindings, but nothing too fancy.
This appears to be the last of the low-hanging fruit. As noted in https://github.com/rust-lang/rust/issues/134001#issuecomment-2524979268, the remaining bindings are difficult or impossible to migrate at present.
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cg_llvm: Move target machine command-line quoting from C++ to Rust
When this code was introduced in rust-lang/rust#130446 and rust-lang/rust#131805, it was complicated by the need to maintain compatibility with earlier versions of LLVM.
Now that LLVM 20 is the baseline (rust-lang/rust#145071), we can do all of the quoting in pure Rust code, and pass two flat strings to LLVM to be used as-is.
---
In this PR, my priority has been to preserve the existing behaviour as much as possible, without worrying too much about what the behaviour *should* be. (Though I did avoid a leading space before the first argument.)
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Signed-off-by: Karan Janthe <karanjanthe@gmail.com>
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- Add F128 support to TypeTree Kind enum
- Implement TypeTree FFI bindings and conversion functions
- Add typetree.rs module for metadata attachment to LLVM functions
- Integrate TypeTree generation with autodiff intrinsic pipeline
- Support scalar types: f32, f64, integers, f16, f128
- Attach enzyme_type attributes as LLVM string metadata for Enzyme
Signed-off-by: Karan Janthe <karanjanthe@gmail.com>
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cg_llvm: Replace some DIBuilder wrappers with LLVM-C API bindings (part 4)
- Part of rust-lang/rust#134001
- Follow-up to rust-lang/rust#146631
---
This is another batch of LLVMDIBuilder binding migrations, replacing some our own LLVMRust bindings with bindings to upstream LLVM-C APIs.
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Make llvm_enzyme a regular cargo feature
This makes it clearer that it is set by the build system rather than by the rustc that compiles the current rustc. It also avoids bootstrap needing to pass `--check-cfg llvm_enzyme` to rustc.
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cg_llvm: Replace some DIBuilder wrappers with LLVM-C API bindings (part 3)
- Part of rust-lang/rust#134001
- Follow-up to rust-lang/rust#136375
- Follow-up to rust-lang/rust#136632
---
This is another batch of LLVMDIBuilder binding migrations, replacing some our own LLVMRust bindings with bindings to upstream LLVM-C APIs.
This PR migrates all of the bindings that were touched by rust-lang/rust#136632, plus `LLVMDIBuilderCreateStructType`.
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This makes it clearer that it is set by the build system rather than by
the rustc that compiles the current rustc. It also avoids bootstrap
needing to pass --check-cfg llvm_enzyme to rustc.
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It is always false nowadays. ThinLTO summary writing is instead done by
llvm_optimize.
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Use captures(address) instead of captures(none) for indirect args
While provenance cannot be captured through these arguments, the address / object identity can.
Fixes https://github.com/rust-lang/rust/issues/137668.
r? `@ghost`
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While provenance cannot be captured through these arguments, the
address / object identity can.
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The underlying implementation of `LLVMCreateConstantRangeAttribute` assumes
that each of `LowerWords` and `UpperWords` points to enough u64 values to
define an integer of the specified bit-length, and will encounter UB if that is
not the case.
Our safe wrapper function always passes pointers to `[u64; 2]` arrays,
regardless of the bit-length specified. That's fine in practice, because scalar
primitives never exceed 128 bits, but it is technically a soundness hole in a
safe function.
We can close the soundness hole by explicitly asserting `size_bits <= 128`.
This is effectively just a stricter version of the existing check that the
value must be small enough to fit in `c_uint`.
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This avoids the need for `#![allow(non_upper_case_globals)]`.
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`&Freeze` parameters are not only `readonly` within the function,
but any captures of the pointer can also only be used for reads.
This can now be encoded using the `captures(address, read_provenance)`
attribute.
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Remove `LlvmArchiveBuilder` and supporting code/bindings
Switching over to the newer Rust-based `ArArchiveBuilder` happened in rust-lang/rust#128936, a year ago.
Per the comment in `new_archive_builder`, that seems like enough time to justify removing the older, unused `LlvmArchiveBuilder` implementation and its associated bindings.
Fixes rust-lang/rust#128955.
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cg_llvm: Small cleanups to `owned_target_machine`
This PR contains a few tiny cleanups to the `owned_target_machine` code.
Each individual commit should be fairly straightforward.
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cg_llvm: Use LLVM-C bindings for `LLVMSetTailCallKind`, `LLVMGetTypeKind`
This PR replaces two existing `LLVMRust` bindings with equivalent calls to the LLVM-C API.
For `LLVMGetTypeKind`, we avoid the UB hazard by declaring the foreign function to return `RawEnum<TypeKind>` (which is a wrapper around `u32`), and then perform checked conversion from `u32` to `TypeKind`.
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As noted in the `ffi` module docs, passing pointer/length byte strings from
Rust to C++ is easier if we declare them as `*const c_uchar` on the Rust side,
but `const char *` (possibly signed) on the C++ side. This is allowed because
both pointer types are ABI-compatible, regardless of char signedness.
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Set the dead_on_return attribute (added in LLVM 21) for arguments
that are passed indirectly, but not byval.
This indicates that the value of the argument on return does not
matter, enabling additional dead store elimination.
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Implement support for `become` and explicit tail call codegen for the LLVM backend
This PR implements codegen of explicit tail calls via `become` in `rustc_codegen_ssa` and support within the LLVM backend. Completes a task on (https://github.com/rust-lang/rust/issues/112788). This PR implements all the necessary bits to make explicit tail calls usable, other backends have received stubs for now and will ICE if you use `become` on them. I suspect there is some bikeshedding to be done on how we should go about implementing this for other backends, but it should be relatively straightforward for GCC after this is merged.
During development I also put together a POC bytecode VM based on tail call dispatch to test these changes out and analyze the codegen to make sure it generates expected assembly. That is available [here](https://github.com/xacrimon/tcvm).
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