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document #[align] on functions #1866

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23 changes: 23 additions & 0 deletions src/attributes/codegen.md
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Expand Up @@ -668,3 +668,26 @@ 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.
* Any inline assembly in the function must use the specified instruction set instead of the target default.

r[attributes.codegen.align]
## The `align` attribute

r[attributes.codegen.align.allowed-positions]
The *`align` [attribute]* may be applied to function items.

r[attributes.codegen.align.behavior]
This attribute raises the aligmnent of a function's generated code.

```rust
#[align(32)]
fn aligned_to_32_bytes() {}
```

> [!NOTE]
> Some targets use pointer tagging on function pointers. Consequently, on those targets, a function's address may not be a multiple of its alignment.

r[attributes.codegen.align.minimum]
The alignment value that is specified is a minimum. The actual alignment may be higher.

r[attributes.codegen.align.syntax]
The alignment is specified as an integer parameter in the form of `#[align(x)]`. The alignment value must be a power of two from 1 up to 2<sup>29</sup>.
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