9c1a484e embedded sec_t in pac_t (+~48B). Since pac sits before the hot
hpa/edata_cache fields in pa_shard_s, the larger pac_t shifts them and
regresses the free/extent paths ~2% on the fill-flush microbenchmark --
even though SEC is disabled by default and no SEC code runs; it is purely
a struct-layout effect.
Move pac to the last field so its growth shifts only the trailing
all_bins[] flex array. Pure reorder, no functional change.
Validated with the fill-flush microbenchmark (500 threads, 256 B,
MALLOC_CONF=tcache:true,prof:true), turbo pinned, interleaved medians:
on AMD EPYC 9D64 (x86-64) bad regresses ~2% and the reorder restores the
baseline; on ARM Neoverse-V2 (aarch64) there is no measurable regression
and the reorder is neutral.
db15a39d75