v3: make the FastC selfhost build 5x faster and fix its generation chain

[medvednikov]

Summary

v3 -selfhost -b fastc v3.v took 1.68 s and 5.2 GB RSS on one thread — slower and far heavier than the full parallel pipeline (~0.92 s / ~1 GB), despite FastC having no AST, checker, or transformer. Sampling showed nearly all time under two quadratic scans, both fixed by precomputing an index once per program:

• Parser.semantic_type_key cloned every declared-type key and re-derived each key's C spelling via string.replace on every query (~4 calls per return statement alone). Replaced with a C-spelling → type-key map built once; first key per spelling wins, matching the old first-match scan over the insertion-ordered key list.
• Parser.temporary_namespace rescanned every function key through fastc_c_function_name_for_key (two sanitize passes per key) per temporary. Only __v_fastc_-prefixed C names can ever collide with a temporary-namespace candidate, so the collision scan now walks a precomputed short list of those names.

Results (M-series macOS, -selfhost -b fastc -nocache)

	before	after	regular C backend
fastc parse+gen	1590 ms	241 ms	(parallel pipeline)
total	1682 ms	303 ms	~924 ms
peak RSS	5160 MB	363 MB	~1004 MB

Generated C is byte-identical to the previous compiler on the same input.

Generation-chain fixes

The FastC self-host chain (v4f building v5f) was broken in two pre-existing ways, masked one behind the other:

1. The array-membership template nested '!${found_name}' inside another interpolation's ${if ...} block. The FastC selfhost parser renders such nested blocks literally, so second-generation compilers emitted a raw !${found_name} into user C (error: '$' undeclared). The predicate is now hoisted into a variable, exactly like the neighboring string/map membership cases already do.
2. fastcdriver never mirrored the driver's TinyCC compatibility defines (add_v3_tcc_compat_defines), so descendant generations compiled builtin with tcc_backtrace calls on macOS arm64, where TCC's backtrace runtime cannot be linked (unresolved reference to '_tcc_backtrace'). The same no_backtrace define the first generation uses is now applied.

Verification

• v3 → v4f → v5f → v6f completes with V_MACOS_V3_NO_FALLBACK=1; generation-2 and generation-3 C outputs are byte-identical (fixed point).
• Pre-change and post-change compilers emit byte-identical C for the same input.
• Non-selfhost -b fastc smoke test (loop + interpolation) builds and runs correctly.
• v fmt -verify passes on both files.

🤖 Generated with Claude Code