[llvm-dev] A struct {i8, i64} has size == 12, clang says size 16

[Martin J. O'Riordan via llvm-dev]

This is because of alignment requirements.  The alignment for the 'i64' is 8, so assuming the compiler is required to insert 7 bytes of padding between the first member and the second to ensure that the second 'i64' member is validly aligned.

You can use '__attribute__((packed))' to cause the compiler to remove the padding, and the size will become '9'.

	MartinO

-----Original Message-----
From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of edA-qa mort-ora-y via llvm-dev
Sent: 18 April 2018 20:05
To: LLVM Developers Mailing List <llvm-dev at lists.llvm.org>
Subject: [llvm-dev] A struct {i8,i64} has size == 12, clang says size 16

I'm creating a struct of `{i8,i64}` and `DataLayout::getTypeAllocSize` is returning `12`. `getStructLayout` also gives an `4` offset for the second element.

The native ABI, and clang, for the same type are producing a size of 16, with an alignment of 8, for the second element.

This is for the system triple "x86_64-linux-gnu"

What could be causing this difference in alignment and how can I fix it?


I'm verifying the native ABI with:

    // clang -emit-llvm ll_struct_arg.c -S -o /dev/tty
    #include <stddef.h>

    typedef struct vpt_data {
        _Bool c;
        long int b;
    } vpt_data;

    int main() {
        vpt_data v;
        vpt_test(v);
        auto a = sizeof(v);
        auto off = offsetof(vpt_data,b);
    }


16 is stored to `a` and 8 to `off`.

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