[phire]

True.

But most C code out there assumes malloc will always return something that is at least aligned to sizeof(void *), it's very rare to see aligned_alloc. So how is your alloc allocation going to know when it can get away with a smaller alignment?

Even if you are on a cpu that doesn't fault on unaligned memory access, any malloc implementation that doesn't align by default will have serious disadvantages in any benchmarks. IMO, There is no good reason to use an unaligned backpointer.

[CodesInChaos]

Yes, large allocations need to be aligned to `alignof(max_align_t)`. But small allocations could have a smaller alignment. For example a single byte allocation can actually be a single byte with no alignment, since types can't have an alignment larger than their size.