> so alloc() doesn’t just need to hand back a pointer. it needs to hand back a pointer that’s correctly aligned for whatever type the caller is about to store there.

Malloc doesn't know the required alignment (because has no idea what the type is, everything is cast through void). So all malloc implementations have a minimum alignment guarantee. Typically 16 bytes these days on x86, as that means even 128bit SSE values will end up aligned by default.

You couldn't go below the sizeof(void ) anyway, the backpointer needs to aligned too.

The padding only happens when you use memalign or aligned_malloc to specify a much larger alignment.

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CodesInChaos 4 hours ago | parent [-]

There is no reason an allocation needs to contain any inline metadata. And even if it does, the allocator could choose to make it unalined, and pay the cost of an unalined access on de-allocation.

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phire 4 hours ago | parent | next [-]

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 2 hours ago \| parent [-]
		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.

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adrian_b 2 hours ago | parent | prev [-]

This is explained in TFA, where it is mentioned that you can replace inline metadata with a pointer to metadata (or an index), which may be unaligned, if necessary.

However, the pointer to metadata is not really necessary.

The associated metadata could be stored in a table, and the index of the metadata could be computed from the offset of the pointer returned by malloc to the start of the heap (possibly using a hash function).

The ancient versions of the Microsoft C/C++ compilers were using a malloc with inline metadata. I have no idea if they replaced this more recently.