histogram: add pow2 allocation stats via MIMALLOC_SHOW_HISTOGRAM=1

[michaeljclark]

I do not intend this pull to be merged, but am surfacing the feature just in case anyone finds it useful. It is a bit easier to read if one is trying to assess allocation sizes with a power of two granularity. It is currently guarded behind #ifdef MI_STAT>1 but #if MI_DEBUG>0 might be more appropriate. It currently adds an atomic_increment to counters in a shared histogram array. It could be changed to use thread-local arrays that are merged at program termination.

[ghost]

All CLA requirements met.

[jserv]

Why not using mi_bsr? That is,

  if (mi_unlikely(size == 0)) return;
  size_t bucket = mi_bsr(size);

[michaeljclark]

Good idea. I have made these two changes. I think your suggestion for the zero check is necessary unless we were to add another element to the histogram array which also would eliminate a branch. I do a null check in the caller but didn't consider not null pointers with zero size but I believe it does happen when debug is disabled after looking at this conditional statement in mi_heap_malloc_small:

mimalloc/src/alloc.c

Lines 77 to 81 in 38a0322

	#if (MI_PADDING)
	if (size == 0) {
	size = sizeof(void*);
	}
	#endif

[jserv]

I appreciate the feedback from @michaeljclark. Last year, I was exploring mimalloc performance issues using the preliminary work. Thus, my proposed change was the part of the workbench.

[daanx]

Interesting pr; just based it on top of dev which is easier for me to experiment with. Thanks!