ghostty-rs/docs/rewrite/notes/perf-P1.md

perf-P1: benchmarks and hot-path optimisation

What was added

• New crate crates/ghostty-bench (registered in root [workspace.dependencies]):
  • binary ghostty-bench +<action> [--opt=value], same actions and options as Zig's ghostty-bench: terminal-stream, terminal-parser, codepoint-width, grapheme-break, is-symbol, osc-parser, screen-clone (options --data, --terminal-rows/-cols, --mode). Like Zig it runs the step exactly once and prints nothing; time the process (hyperfine). Rust-only extras: --time (prints the step time to stderr), terminal-stream --handler=full (full stream_readonly handler).
  • binary ghostty-synthetic +ascii|+utf8|+osc [--p-valid --style] (port of src/synthetic, Zig's ghostty-gen) plus Rust-only +sgr (SGR/CSI-heavy colourised text).
  • Benchmark harness (benchmark.rs), generators (synthetic.rs, with the Zig generator tests), arg parsing (args.rs).
  • Deviations: codepoint-width --mode=wcwidth is a warn+noop (libc FFI is forbidden); --mode=simd uses the table; is-symbol modes uucode/table both use the generated table; PRNG is a local xoshiro256++ (seeded from time) instead of Zig's std.Random.
• terminal-parser drops the returned actions (drop(p.next(c))) like Zig's _ = actions.

Zig baseline

Zig 0.16.0 is installed but build.zig.zon needs 0.15.2, so 0.15.2 was downloaded (ziglang.org tarball) and the reference tree copied to /tmp/claude-1000/ghostty-zig-build/src (reference tree untouched). Build: zig build -Demit-bench -Doptimize=ReleaseFast -Dapp-runtime=none (~90 s) gives ghostty-bench and ghostty-gen. Data was generated with the Zig ghostty-gen (ascii 200 MB, utf8 100 MB, osc --style=parser 50 MB); the SGR file (100 MB) comes from the Rust ghostty-synthetic +sgr. Timings: hyperfine -N --warmup 1 --runs 5, min wall time of the whole process, same files, same machine, Rust built with --profile production. Scripts: /tmp/claude-1000/ghostty-zig-build/run.sh.

Results (production profile; lower ms is better)

benchmark	data	Zig ms	Rust before ms	Rust after ms	after/Zig
terminal-stream ascii	200 MB	1008 (198 MB/s)	2334 (86 MB/s)	1092 (183 MB/s)	1.08
terminal-stream utf8	100 MB	825 (121 MB/s)	2061 (49 MB/s)	797 (125 MB/s)	0.97
terminal-stream sgr/csi	100 MB	665 (150 MB/s)	991 (101 MB/s)	693 (144 MB/s)	1.04
terminal-parser ascii	200 MB	500	1914	268	0.54
terminal-parser utf8	100 MB	287	954	349	1.22
terminal-parser sgr/csi	100 MB	352	1068	538	1.53
codepoint-width table	100 MB utf8	326	300	296	0.91
codepoint-width simd	100 MB utf8	615	298	295	0.48
grapheme-break	100 MB utf8	361	313	314	0.87
is-symbol table	100 MB utf8	321	274	291	0.91
osc-parser	50 MB	27.7	79.7	52.9	1.91
screen-clone clone (1000x)	80x120	27.1	107	70.3	2.60
screen-clone render (1000x)	80x120	4.5	15.9	11.2	2.49

Release profile (debug-assertions + overflow checks on) terminal-stream: ascii 2.6 s, utf8 1.34 s, sgr 1.19 s (about 2x slower than production, as expected). Rust-only terminal-stream --handler=full (all actions applied) on 100 MB: ascii(200MB) 1.2 s, utf8 0.85 s, sgr 1.6 s (no Zig counterpart; profile shows no pathology).

Optimisations (behaviour-preserving)

• Terminal::print: single-lookup fast path (print_fast) for width-1 characters mid-line on a plain narrow cell (same writes as cursor_mark_dirty+print_cell+cursor_right); new PageList::tracked_pin_x_mut. Avoids ~10 tracked-pin/node/row re-resolutions per char.
• Page: grapheme chunk allocator is now a u64 bitmap with first-fit word scanning (was an O(chunks) scan of a Vec<bool> from index 0 each time; 38% of utf8 stream time); randomised test against the naive first-fit. Grapheme maps use a cheap integer hasher instead of SipHash.
• Page::clear_cells: fast path when row has no styled/grapheme/hyperlink flags; no row rescans for full-row clears; clone_partial_row_from skips redundant per-cell clears and bulk-copies plain cells.
• ghostty-vt: ASCII run fast path and hand-written strict UTF-8 decode (was str::from_utf8 per codepoint; exhaustive equivalence test); Parser::next splits the same-state fast path from the state-change path and builds Print/Execute directly (avoids store-forwarding stalls on the 64-byte Action temporaries); CSI param copy without libc memcpy; osc::Parser::next inlines the payload accumulate path.
• RenderState::update: per-cell grapheme vectors only allocated for rows that contain graphemes.
• Fix: Page::assert_integrity was debug-only but called unconditionally from terminal.rs (production/--release without debug-assertions failed to build); added a no-op stub.

Remaining gaps (> 1.5x)

• screen-clone clone/render (2.5-2.6x): dominated by Page::init zero-filling and page-faulting freshly allocated Vec<Cell> (Zig gets lazily-zeroed mmap memory); fixing needs a calloc-style zeroed allocation, not possible with safe Vec<Cell>.
• osc-parser (1.9x): per-byte Vec::push into the payload buffer (Zig appends to a fixed buffer); Parser::data() also clones the buffer.
• terminal-parser sgr (1.5x): moving the 64-byte Option<Action> array out of the parser; real stream path (terminal-stream sgr) is at parity.