/*
 * Read-only information associated with each element of arena_t's bins array
 * is stored separately, partly to reduce memory usage (only one copy, rather
 * than one per arena), but mainly to avoid false cacheline sharing.
 *
 * Each run has the following layout:
 *
 *               /--------------------\
 *               | pad?               |
 *               |--------------------|
 *               | redzone            |
 *   reg0_offset | region 0           |
 *               | redzone            |
 *               |--------------------| \
 *               | redzone            | |
 *               | region 1           |  > reg_interval
 *               | redzone            | /
 *               |--------------------|
 *               | ...                |
 *               | ...                |
 *               | ...                |
 *               |--------------------|
 *               | redzone            |
 *               | region nregs-1     |
 *               | redzone            |
 *               |--------------------|
 *               | alignment pad?     |
 *               \--------------------/
 *
 * reg_interval has at least the same minimum alignment as reg_size; this
 * preserves the alignment constraint that sa2u() depends on.  Alignment pad is
 * either 0 or redzone_size; it is present only if needed to align reg0_offset.
 */