How to tell the maximum data alignment requirement in C++

Question

Referencing this question and answer Memory alignment : how to use alignof / alignas? "Alignment is a restriction on which memory positions a value's first byte can be stored." Is there a portable way in C++ to find out the highest granularity of alignment that is required for instructions not to fault (for example in ARM)? Is alignof(intmax_t) sufficient since its the largest integer primitive type?

Further, how does something in malloc align data to a struct's boundaries? For example if I have a struct like so

alignas(16) struct SomethingElse { ... };

the SomethingElse struct is required to be aligned to a boundary that is a multiple of 16 bytes. Now if I request memory for a struct from malloc like so

SomethingElse* ptr = malloc(sizeof(SomethingElse));

Then what happens when malloc returns a pointer that points to an address like 40? Is that invalid since the pointer to SomethingElse objects must be a multiple of 16?

Answer 1

You might be looking for std::max_align_t

Answer 2

How to tell the maximum data alignment requirement in C++

There is std::max_align_t which

is a POD type whose alignment requirement is at least as strict (as large) as that of every scalar type.

Vector instructions may use array operands that require a higher alignment than any scalar. I don't know if there is a portable way to find out the the upper bound for that.

To find the alignment requirement for a particular type, you can use alignof.

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Further, how does something in malloc align data to a struct's boundaries?

malloc aligns to some boundary that is enough for any scalar. In practice exactly the same as std::max_align_t.

It won't align correctly for "overaligned" types. By overaligned, I mean a type that has higher alignment requirement than std::max_align_t.

Then what happens when malloc returns a pointer that points to an address like 40?

Dereferencing such SomethingElse* would be have undefined behaviour.

Is that invalid since the pointer to SomethingElse objects must be a multiple of 16?

Yes.

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how even would one go about fixing that? There is no way to tell malloc the alignment that you want right?

To allocate memory for overaligned types, you need std::aligned_alloc which is to be introduced in C++17.

Until then, you can use platform specific functions like posix_memalign or you can over allocate with std::malloc (allocate the alignment - 1 worth of extra bytes), then use std::align to find the correct boundary. But then you must keep track of both the start of the buffer, and the start of the memory block separately.