megboyzzandClaude
80768652ea
notdirty_write called uc->memory_mapping() on every guest write, before
testing whether the region is executable, and used the result for that one
bit. A probe counted over a million such calls per 3 seconds with
exec_region=0 - the full find_memory_mapping -> address_space_translate ->
flatview_translate chain running and the answer then discarded.
The map is static once CreateConfiguredEngine has mapped its five spans, so
a 512-entry direct-mapped page -> MemoryRegion cache removes all but cold
misses.
interleaved A/B, engine start to first OnCarLoaded
cache on: 31.27 / 31.34 / 31.22 s mean 31.28
cache off: 32.27 / 32.12 / 32.44 s mean 32.28
3.1%, ranges fully separate, spreads under 0.35s. The profile confirms the
mechanism rather than just the outcome: flatview_translate 4.46% -> 3.17%,
find_memory_mapping 3.02% -> 2.36%.
THE FIRST VERSION OF THIS CACHE WAS SLOWER, and the measurement is why it
was caught. It used `static __thread`, which on Android resolves through
emulated TLS: __emutls_get_address appeared at 8.11% and pthread_getspecific
at 3.07% - together more than the 7.5% being removed. Storage now lives in
uc_struct, which is already per-thread here (one uc_engine per host thread),
so no TLS is involved. Do not reintroduce __thread on this path.
INVARIANT: the cache assumes mappings and permissions are fixed after engine
setup. Anyone changing them at runtime must clear mr_cache_ready.
Co-Authored-By: Claude <noreply@anthropic.com>