A bug story: data alignment on x86

Re:A bug story: data alignment on x86 by Thiez on Monday November 07, 2016 @05:37PM (#12895879)

The zero length array still must be aligned correctly. You can create a pointer to it (e.g. by taking &something._alignment[..] to create a slice) and pointers must have correct alignment, so it follows that a zero length array has the same alignment requirements as a longer array. So padding must be inserted in your struct so that the address of the zero-length array is correctly aligned.

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Re:A bug story: data alignment on x86 by alkonaut on Tuesday November 08, 2016 @06:29AM (#12899358)

Hmm, I don't understand (sorry). The pointer to the internal array must be aligned. But how does the element size play into the padding. If we take these two examples?

   [repr(C)]
   struct StructA
   {
      pub foo: f32,
      _alignment: [SixteenBytes, 0]
   }

   [repr(C)]
   struct StructB
   {
      pub foo: f32,
      _alignment: [ThirtytwoBytes, 0]
   }

Are both just padded with 16-4 and 32-4 bytes respectively, so they are equivalent to making a padding like this in the first case?

   [repr(C)]
   struct StructAPadded
   {
      pub foo: f32,
      _padding : TwelveBytes;
   }

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