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perf(web): disable icount by default (4.7x faster book open)

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icount throttled TCG and starved the guest watchdog. Measured book open dropped from 146s to 31s. Off by default; ?icount restores it, ?noicount removed in favor of the new default. Harness now reports open-file timing.
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Evan Reichard
2026-07-21 20:19:46 -04:00
parent 223500ab09
commit 080a264b5c
3 changed files with 15 additions and 5 deletions
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ISSUES.md
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@@ -12,9 +12,13 @@ Opening `/Test/example.txt` through QEMU `vvfat` could corrupt coroutine/guest s
Opening a book panics **only in WASM** (`Invalid read ... reason: rejected` → Load access fault) inside `uzlib_uncompress` during the reader's font `prewarmCache` **if** compiled guest stores are enabled. Native QEMU from the same submodule renders fine.
Root cause (confirmed by full diff against pristine upstream): `wasm_tci_only_tb = true` in `tcg_out_qemu_st` is **upstream qemu-wasm's own design**. Upstream forces every store-containing TB to the TCI interpreter because its compiled restartable-block store path is not rewind-safe (a yielded softmmu helper replays the block; safe for loads, corrupting for stores). This is the dominant cost of book open (font inflate is store-heavy, so the hot loop never JIT-promotes).
Root cause (confirmed by full diff against pristine upstream): `wasm_tci_only_tb = true` in `tcg_out_qemu_st` is **upstream qemu-wasm's own design**. Upstream forces every store-containing TB to the TCI interpreter because its compiled restartable-block store path is not rewind-safe (a yielded softmmu helper replays the block; safe for loads, corrupting for stores). Font inflate is store-heavy, so that hot loop never JIT-promotes — a real but secondary perf ceiling.
The real fix is to make compiled stores rewind-safe and drop the blanket TCI — a substantial TCG-backend task, ~15 min per browser iteration, no native repro. Full analysis: `docs/wasm-inflate-panic.md`.
Remaining fix is to make compiled stores rewind-safe and drop the blanket TCI — a substantial TCG-backend task, ~15 min per browser iteration, no native repro. Full analysis: `docs/wasm-inflate-panic.md`.
## Resolved: `-icount` throttled book open ~4.7x
The browser ran with `-icount shift=auto,align=off,sleep=off`, which serialized TCG and starved the guest watchdog (the earlier `wdt timeout` panics). Measured book open: 146s with icount, 31s without. icount is now off by default in `web/app.js`; `?icount` restores it. Guest timekeeping tracks host wall-clock, which is fine here because running flat-out feeds the watchdog faster, not slower.
## Resolved: reader grayscale rendered inverted and muddled