USES_POWER_ALIGNMENT in rustc_lint::types::improper_ctypes - Rust
Description: The `uses_power_alignment` lint detects specific `repr(C)` aggregates on AIX. In its platform C ABI, AIX uses the “power” (as in PowerPC) alignment rule (detailed in https://www.ibm.com/docs/en/xl-c-and-cpp-aix/16.1?topic=data-using-alignment-modes#alignment), which can also be set for XLC by `#pragma align(power)` or `-qalign=power`. Aggregates with a floating-point type as the recursively first field (as in “at offset 0”) modify the layout of subsequent fields of the associated structs to use an alignment value where the floating-point type is aligned on a 4-byte boundary.

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## USES_POWER_ALIGNMENT

# Static USES_POWER_ALIGNMENT

Source

    static USES_POWER_ALIGNMENT: &Lint

Expand description

The `uses_power_alignment` lint detects specific `repr(C)` aggregates on AIX. In its platform C ABI, AIX uses the “power” (as in PowerPC) alignment rule (detailed in https://www.ibm.com/docs/en/xl-c-and-cpp-aix/16.1?topic=data-using-alignment-modes#alignment), which can also be set for XLC by `#pragma align(power)` or `-qalign=power`. Aggregates with a floating-point type as the recursively first field (as in “at offset 0”) modify the layout of subsequent fields of the associated structs to use an alignment value where the floating-point type is aligned on a 4-byte boundary.

Effectively, subsequent floating-point fields act as-if they are `repr(packed(4))`. This would be unsound to do in a `repr(C)` type without all the restrictions that come with `repr(packed)`. Rust instead chooses a layout that maintains soundness of Rust code, at the expense of incompatibility with C code.

#### §Example

ⓘ

    #[repr(C)]
    pub struct Floats {
        a: f64,
        b: u8,
        c: f64,
    }

This will produce:

    warning: repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
     --> <source>:5:3
      |
    5 |   c: f64,
      |   ^^^^^^
      |
      = note: `#[warn(uses_power_alignment)]` on by default

#### §Explanation

The power alignment rule specifies that the above struct has the following alignment:

- offset_of!(Floats, a) == 0
- offset_of!(Floats, b) == 8
- offset_of!(Floats, c) == 12

However, Rust currently aligns `c` at `offset_of!(Floats, c) == 16`. Using offset 12 would be unsound since `f64` generally must be 8-aligned on this target. Thus, a warning is produced for the above struct.