Patch: enumerate all live blocks

[Benoit Jacob]

Hello,

The attached patch instruments jemalloc 3.0, adding the ability to
enumerate all live blocks.

Currently, the only information stored about blocks is their (payload)
address and size, but the plan is to also store their allocation call
stack.

This is achieved by allocating larger blocks than requested, and using
the extra space to store doubly linked list elements.

 This is provided just in case it might be useful to anyone, not
considered ready for inclusion in jemalloc. It's been tested for 15
minutes in a Firefox build.

Details about the overhead, and how it could be reduced:
 * Memory overhead is, per block: 32 bytes on 32-bit systems, 48 bytes
on 64-bit systems (assuming size_t == uintptr_t). Could easily be
reduced to 16 bytes in both cases (by using a XOR linked list and
assuming that no block exceeds 4G).
 * Slowness overhead is essentially an additional mutex to lock on
every malloc/free call. Could be solved in various ways, either
copying what jemalloc does internally, or by using a lock-free list.

If you want to test it out, currently the only built-in way to output
the list of blocks is to call je_dump_list_of_blocks(void) e.g. from
your debugger. See its code to see the relevant calls. Sample output
from Firefox:

...snip...
Block 193965:  real block = 0x7fffba218580,  payload = 0x7fffba2185b0,
 payload size = 64
Block 193966:  real block = 0x7fffc02dd000,  payload = 0x7fffc02dd030,
 payload size = 1024
Block 193967:  real block = 0x7fffc2053ce0,  payload = 0x7fffc2053d10,
 payload size = 24
Block 193968:  real block = 0x7fffc5b4ed80,  payload = 0x7fffc5b4edb0,
 payload size = 80
Block 193969:  real block = 0x7fffd119e240,  payload = 0x7fffd119e270,
 payload size = 64
Block 193970:  real block = 0x7fffcfb4aa60,  payload = 0x7fffcfb4aa90,
 payload size = 24
Block 193971:  real block = 0x7fffbb85e1f0,  payload = 0x7fffbb85e220,
 payload size = 24
Block 193972:  real block = 0x7fffe20a62e0,  payload = 0x7fffe20a6310,
 payload size = 32

End enumeration of 193973 jemalloc blocks.

Cheers,
Benoit
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