Trailing padding in internal structure

Question

Compilers typically insert trailing padding on structures to satisfy alignment restrictions when they are used in arrays:

// Size 4, alignment 2
struct A {
    uint16_t x;
    uint8_t y;
    // Padding 1 byte
};

// Size 6, alignment 2
struct B {
    struct A xy;
    uint8_t z;
    // Padding 1 byte
};

Now consider combination of these using internal structure:

struct AB {
    struct {
        uint16_t x;
        uint8_t y;
    } xy;
    uint8_t z;
};

Following could fit in 4 bytes, while not breaking alignment restrictions. Also, internal structure xy has no type which could be used elsewhere, so trailing padding for it isn't needed.

Downside is that member xy would be incompatible with struct A, but there is no reason why it should be as they are in different type definitions.

Is compiler allowed to do this size optimization?

Or, to put it other way, does standard require that 2 structures with equal members will always result in equal layout?

Answer 1

The answer is likely given by the notion of compatible types. If t1 and t2 are compatible types, a pointer to t1 can be used to access memory that was set using type t2.

In the C11 standard:

6.2.7 Compatible type and composite type

1. … Moreover, two structure, union, or enumerated types declared in separate translation units are compatible if their tags and members satisfy the following requirements: If one is declared with a tag, the other shall be declared with the same tag. If both are completed anywhere within their respective translation units, then the following additional requirements apply: there shall be a one-to-one correspondence between their members such that each pair of corresponding members are declared with compatible types; if one member of the pair is declared with an alignment specifier, the other is declared with an equivalent alignment specifier; and if one member of the pair is declared with a name, the other is declared with the same name. …

Two structs that do not have the same tag are not compatible types, and I do not see anything that force them to have the same layout.