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Update instruction_set to use the attribute template #1912

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Update instruction_set to use the attribute template #1912

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45 changes: 29 additions & 16 deletions src/attributes/codegen.md
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Expand Up @@ -614,34 +614,47 @@ trait object whose methods are attributed.
r[attributes.codegen.instruction_set]
## The `instruction_set` attribute

r[attributes.codegen.instruction_set.intro]
The *`instruction_set` [attribute]* specifies the instruction set that a function will use during code generation. This allows mixing more than one instruction set in a single program.

> [!EXAMPLE]
> <!-- ignore: arm-only -->
> ```rust,ignore
> #[instruction_set(arm::a32)]
> fn foo_arm_code() {}
>
> #[instruction_set(arm::t32)]
> fn bar_thumb_code() {}
> ```

r[attributes.codegen.instruction_set.syntax]
The `instruction_set` attribute uses the [MetaListPaths] syntax, and a path comprised of the architecture family name and instruction set name.

r[attributes.codegen.instruction_set.allowed-positions]
The *`instruction_set` [attribute]* may be applied to a function to control which instruction set the function will be generated for.
The `instruction_set` attribute may only be applied to:

r[attributes.codegen.instruction_set.behavior]
This allows mixing more than one instruction set in a single program on CPU architectures that support it.
- [Free functions][items.fn]
- [Inherent associated functions][items.associated.fn]
- [Trait impl functions][items.impl.trait]
- [Trait definition functions][items.traits] with a body
- [Closures][expr.closure]

r[attributes.codegen.instruction_set.syntax]
It uses the [MetaListPaths] syntax, and a path comprised of the architecture family name and instruction set name.
> [!NOTE]
> `rustc` currently ignores `instruction_set` in other positions. This may change in the future.

r[attributes.codegen.instruction_set.duplicates]
The `instruction_set` attribute may only be specified once on an item.

r[attributes.codegen.instruction_set.target-limits]
It is a compilation error to use the `instruction_set` attribute on a target that does not support it.
The `instruction_set` attribute may only be used with a target that supports the given value.

r[attributes.codegen.instruction_set.arm]
### On ARM
### `instruction_set` on ARM

For the `ARMv4T` and `ARMv5te` architectures, the following are supported:
* `arm::a32` --- Generate the function as A32 "ARM" code.
* `arm::t32` --- Generate the function as T32 "Thumb" code.

<!-- ignore: arm-only -->
```rust,ignore
#[instruction_set(arm::a32)]
fn foo_arm_code() {}

#[instruction_set(arm::t32)]
fn bar_thumb_code() {}
```

Using the `instruction_set` attribute has the following effects:

* If the address of the function is taken as a function pointer, the low bit of the address will be set to 0 (arm) or 1 (thumb) depending on the instruction set.
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