[Bug libstdc++/96942] std::pmr::monotonic_buffer_resource causes CPU cache misses

[dmitriy.ovdienko at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96942

--- Comment #7 from Dmitriy Ovdienko <dmitriy.ovdienko at gmail dot com> ---
Following are CPU counters for single threaded code. Pre-allocation is enabled.
Memory pool is created inside the loop.

```cpp
  int poolSize(int depth)
  {
    return (1 << (depth + 1)) * sizeof(Node);
  }

  int count = 0;
  for(int i = 0; i < 20; ++i)
  {
    MemoryPool store (poolSize(stretch_depth));

    Node *c = make(stretch_depth, store);
    count += c->check();
    store.release();
  }
```

Depth = 21, Pool size = 134,217,728 Bytes

|                   |            PMR |        Malloc |
|-------------------|----------------|---------------|
| cache-references  |     60,180,483 |    60,205,187 |
| cache-misses      |     50,288,765 |    50,426,418 |
| cycles            |  7,587,314,879 | 6,076,106,356 |
| instructions      | 14,347,088,112 | 8,138,591,245 |
| branches          |  2,224,641,671 | 1,550,701,277 |
| branch-misses     |      8,074,211 |     7,307,996 |
| faults            |        655,503 |       655,485 |
| migrations        |              1 |             2 |
| time elapsed, sec |           2.16 |          1.75 |
| time (user, sec)  |           1.46 |          1.01 |
| time (sys, sec)   |           0.69 |          0.73 |

Depth = 18, Pool size = 16,777,216 Bytes

|                   |           PMR |      Malloc |
|-------------------|---------------|-------------|
| cache-references  |     8,186,788 |   3,450,642 |
| cache-misses      |     6,504,691 |   1,592,945 |
| cycles            |   992,526,559 | 472,979,689 |
| instructions      | 1,806,230,679 | 766,527,818 |
| branches          |   279,352,274 | 151,353,530 |
| branch-misses     |     1,072,404 |     474,648 |
| faults            |        82,063 |       8,314 |
| migrations        |             0 |           0 |
| time elapsed, sec |          0.28 |        0.14 |
| time (user, sec)  |          0.17 |        0.13 |
| time (sys, sec)   |          0.11 |        0.01 |

Depth = 17, Pool size: 8,388,608 Bytes

|                   |         PMR |      Malloc |
|-------------------|-------------|-------------|
| cache-references  |   1,624,992 |   1,707,061 |
| cache-misses      |     867,310 |     718,011 |
| cycles            | 312,687,116 | 255,951,365 |
| instructions      | 765,410,795 | 389,671,180 |
| branches          | 118,619,222 |  74,686,565 |
| branch-misses     |     272,286 |     263,916 |
| faults            |       4,221 |       4,219 |
| migrations        |           0 |           0 |
| time elapsed, sec |        0.10 |        0.08 |
| time (user, sec)  |        0.10 |        0.07 |
| time (sys, sec)   |        0.00 |        0.01 |