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.
irenaeus•2d ago
"to get individual free() working, the allocator needs to remember something about every allocation it handed out. and that’s the moment metadata stops being optional."
That's just a very nice distillation of an important concept.
nathaah3•2d ago
HexDecOctBin•24m ago
The conventional approach for allocating memory on GPUs for games and other applications is to use a real-time allocator such as TLSF. However, it is not usually discussed that TLSF is real-time because it stores metadata in-band. It is possible to create a variant of TLSF that preserves its real-time properties while storing metadata out-of-band, but this requires careful consideration.
geocar•14m ago
If you can convince the caller to keep track of that metadata themselves you obviously don’t need to. That can be important.
Something I noticed is that _very often_ the code that is calling malloc(n) is keeping track of n somehow for its own reasons (bounds checking, grow/gap pointers, etc) so merging the value halves stack churn and it’s an easy win.
lifthrasiir•11m ago