4 ms·

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

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.

phire 3mo ago
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 3mo ago
  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.
adrian_b 3mo ago
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.
- CodesInChaos 3mo ago
  I consider an inline-pointer a form of inline-metadata. When I mentioned that inline metadata isn't necessary I was referring to the kind of external lookup you describe.

4 ms·

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

by CodesInChaos 3mo ago

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.

phire 3mo ago
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 3mo ago
  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.
adrian_b 3mo ago
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.
- CodesInChaos 3mo ago
  I consider an inline-pointer a form of inline-metadata. When I mentioned that inline metadata isn't necessary I was referring to the kind of external lookup you describe.