repr(C) AIX Struct Alignment

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1

On PowerPC AIX, there are special layout alignment rules for structs not represented by the repr(C) attribute in Rust. Specifically, the layout of the following struct is misaligned:

#[repr(C)]
#[derive(Copy, Clone)]
pub struct Floats {
    a: f64,
    b: u8, // currently has 7 bytes of padding
    c: f64,
}

While the same struct generated by Clang and GCC

struct Floats {
    double a;
    char b;
    double c;
};

has the following layout

Layout: <ASTRecordLayout
  Size:192
  DataSize:192
  Alignment:32
  PreferredAlignment:64
  FieldOffsets: [0, 64, 96]>
Layout: <CGRecordLayout
  LLVMType:%struct.Floats = type <{ double, i8, [3 x i8], double, [4 x i8] }>
  IsZeroInitializable:1
  BitFields:[
]>

There are already existing infrastructure in rustc_target to customize the target ABI calling convention for extern "C". I'm wondering what would be the best course of action to implement this special layout rule for repr(C)?

2

This might be a topic better suited for IRLO. (Put a link here if you cross-post.)

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3

Cross Posted on the Internal Forum. repr(C) AIX Struct Alignment - compiler - Rust Internals

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4

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