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Tshepang Mbambo
2026-04-30 20:28:39 +02:00
parent 44034eca66
commit d12e52d852
1 changed files with 18 additions and 17 deletions
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src/doc/rustc-dev-guide/src/backend/backend-agnostic.md
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@@ -38,10 +38,11 @@ Here is the breakdown of the most important elements:
The goal of this refactoring is to separate inside this crate code that is
specific to LLVM from code that can be reused for other rustc backends.
For instance, the `mir` folder is almost entirely backend-specific but it relies
heavily on other parts of the crate.
The separation of the code must not affect
the logic of the code nor its performance.
For instance,
the `mir` folder is almost entirely backend-specific,
but it relies heavily on other parts of the crate.
The separation of the code must not affect the logic of the code,
nor its performance.
For these reasons, the separation process involves two transformations that
have to be done at the same time for the resulting code to compile:
@@ -102,10 +103,11 @@ struct LocalAnalyzer<'mir, 'a, 'tcx> {
}
```
However, the two most important structures `CodegenCx` and `Builder` are not
defined in the backend-agnostic code.
Indeed, their content is highly specific
of the backend and it makes more sense to leave their definition to the backend
However, the two most important structures,
`CodegenCx` and `Builder`,
are not defined in the backend-agnostic code.
Indeed, their content is highly specific to the backend,
and it makes more sense to leave their definition to the backend
implementor than to allow just a narrow spot via a generic field for the
backend's context.
@@ -129,9 +131,9 @@ pub fn codegen_instance<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
In this signature, we have the two lifetime parameters explained earlier and
the master type `Bx` which satisfies the trait `BuilderMethods` corresponding
to the interface satisfied by the `Builder` struct.
The `BuilderMethods`
defines an associated type `Bx::CodegenCx` that itself satisfies the
`CodegenMethods` traits implemented by the struct `CodegenCx`.
The `BuilderMethods` defines an associated type, `Bx::CodegenCx`,
that itself satisfies the `CodegenMethods` traits implemented by the struct,
`CodegenCx`.
On the trait side, here is an example with part of the definition of
`BuilderMethods` in `traits/builder.rs`:
@@ -183,10 +185,9 @@ in the LLVM implementation of the trait).
### State of the code after the refactoring
The traits offer an API which is very similar to the API of LLVM.
This is not
the best solution since LLVM has a very special way of doing things: when
adding another backend, the traits definition might be changed in order to
offer more flexibility.
This is not the best solution since LLVM has a very special way of doing things:
when adding another backend,
the traits definition might be changed in order to offer more flexibility.
However, the current separation between backend-agnostic and LLVM-specific code
has allowed the reuse of a significant part of the old `rustc_codegen_llvm`.
@@ -204,8 +205,8 @@ specific to LLVM.
Only its high-level features have been traitified.
The new `traits` folder has 1500 LOC only for trait definitions.
Overall, the
27,000 LOC-sized old `rustc_codegen_llvm` code has been split into the new
Overall,
the 27,000 LOC-sized old `rustc_codegen_llvm` code has been split into the new
18,500 LOC-sized new `rustc_codegen_llvm` and the 12,000 LOC-sized
`rustc_codegen_ssa`.
We can say that this refactoring allowed the reuse of