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078015bc34
Targeted branchless revert + the pre-matched form fast path, and a fix
for a pre-existing bug the latter surfaced.
(a) Revert the two speculative-write spots from the prior branchless pass
(legacy-prefix emission, widened displacement store, ENCODE_TAIL_SLACK)
back to predicted branches. In real streams a legacy prefix is almost
always absent and disp size is stable, so those branches are ~free and
the unconditional stores only added work. Every class got faster
(RET 19->17.5, MOV r,r 52->46.6, VADDPS 42.8->39.3 ns).
(b) Pre-matched form hint. Instruction.enc_hint (in the existing 11-byte
padding, idx+1 biased; 0 = matcher path) lets a typed builder that maps
to a single value-independent form bake the global form index, so
encode() skips the O(forms) match scan -- and, in a varied stream, its
unpredictable branches. Generated for non-immediate forms only (value-
dependent imm8/imm32 selection stays on the matcher). On a 100k mixed
typed-builder stream: 47.3 -> 30.2 ns/inst (-36%), byte-identical to the
matcher path -- ~2x the original baseline for codegen.
Repair the typed inst_/emit_ builders. They were non-functional: the
generator cast the hw-only typed enum straight to Register
(Register(GPR64.RAX) -> class 0), so every typed-builder operand was
rejected by the matcher (encode returned empty). Untested because the
suite builds via the generic constructors. Now they build through the
class-correct op_gpr64/op_xmm/... path (op_* already used by 3+ operand
builders), emit_ reuses inst_, and a new 30-case consistency suite
asserts typed == generic (llvm-verified) and hint == matcher.
gen/builders/check/test/idempotent all green; 2276 cases.