test(encoder): pin AND-immediate byte-range encoding + document fold bound (VCR-RA-001)#249
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…old bound (VCR-RA-001) Investigating immediate folding (the biggest win the #248 evidence pointed to) surfaced an encoder limitation: the `And { Operand2::Imm }` path packs the low 12 bits straight into the `i:imm3:imm8` field WITHOUT ThumbExpandImm (the modified-immediate expansion). For imm <= 0xFF (gale's int8 clamps #0x7e/#0x7f) that is correct — `and r2,r0,#0x7e` encodes to the canonical `00 f0 7e 02`. For imm >= 0x100 the field needs a true rotation/replication pattern that is not implemented, so it would silently encode a different value. This path is currently DEAD (the selector never emits And-imm), so no live bug — but it sets the precondition for the immediate-folding transform: **fold only imm <= 0xFF** until the encoder is hardened to ThumbExpandImm / Ok-or-Err (the "encoder must be Ok-or-Err, never silently wrong" principle, #180/#185). That bound covers the measured flat_flight waste. Pins the safe-range encoding as a regression guard; no codegen change. Part of #242. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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Subsumed into #250 — the encoder-bound test is included there alongside the immediate-folding transform that uses it. |
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…r bound (VCR-RA-001) (#250) * test(encoder): pin AND-immediate byte-range encoding + document the fold bound (VCR-RA-001) Investigating immediate folding (the biggest win the #248 evidence pointed to) surfaced an encoder limitation: the `And { Operand2::Imm }` path packs the low 12 bits straight into the `i:imm3:imm8` field WITHOUT ThumbExpandImm (the modified-immediate expansion). For imm <= 0xFF (gale's int8 clamps #0x7e/#0x7f) that is correct — `and r2,r0,#0x7e` encodes to the canonical `00 f0 7e 02`. For imm >= 0x100 the field needs a true rotation/replication pattern that is not implemented, so it would silently encode a different value. This path is currently DEAD (the selector never emits And-imm), so no live bug — but it sets the precondition for the immediate-folding transform: **fold only imm <= 0xFF** until the encoder is hardened to ThumbExpandImm / Ok-or-Err (the "encoder must be Ok-or-Err, never silently wrong" principle, #180/#185). That bound covers the measured flat_flight waste. Pins the safe-range encoding as a regression guard; no codegen change. Part of #242. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com> * feat(selector): fold small constants into i32.and immediates (VCR-RA-001) The FIRST delta-emitting codegen transform on the allocator track. Evidence (#248) showed the dominant flat_flight-shape waste is redundant const materialization: `i32.const C; i32.and` lowered to `movw rN,#C; and rD,rA,rN` even when C is a small constant the AND instruction can take as an immediate. Fix: when the operand pushed immediately before `i32.and` is `i32.const C` with C in 0..=0xFF AND its `movw` is cleanly at the instruction tail (not spilled), fold to `and rD, rA, #C` and drop the materialization (foldable_bitwise_imm + drop_prev_const_materialization, mirroring the const-divisor pattern). Bounded to 0..=0xFF: the encoder's AND-immediate path is not yet ThumbExpandImm- complete (#249) — larger modified immediates await an encoder hardening to Ok-or-Err. 0..=0xFF covers gale's int8 clamps. Measured delta: the (p & 0x7e) + (p & 0x7e) pattern drops 8 → 6 instructions (both `movw #126` eliminated; each AND uses the immediate). Better than const-CSE — no materialization at all. GATE (full, codegen change): 282 lib tests + 20 integration suites pass; the three frozen differential fixtures stay RESULT-identical (control_step 0x00210A55, flight_seam 0x07FDF307, div_const 338/338); tests i32_and_folds_small_const_into_immediate (folded shape) + i32_and_does_not_fold_out_of_range_const (0x140 stays a register operand — the encoder safety bound). CI fuzz adds totality. Part of #242. Supersedes the #248 evidence (the redundancy it pinned is now folded away). Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com> --------- Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
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Investigating immediate folding (the biggest win per #248's evidence) surfaced an encoder limitation.
Finding
The
And { Operand2::Imm }encoder (arm_encoder.rs:1886) packs the low 12 bits straight into thei:imm3:imm8field without applying ThumbExpandImm (modified-immediate expansion):imm <= 0xFF(gale's clamps#0x7e/#0x7f): correct —and r2,r0,#0x7e→00 f0 7e 02(pinned by this test).imm >= 0x100: needs a rotation/replication pattern that isn't implemented → would silently encode a different value.This path is currently dead (the selector never emits
And-imm), so there's no live bug — but it sets the precondition for the immediate-folding transform: fold onlyimm <= 0xFFuntil the encoder is hardened to ThumbExpandImm / Ok-or-Err (the "encoder must be Ok-or-Err, never silently wrong" principle, #180/#185). That bound covers the measuredflat_flightwaste.Pins the safe-range encoding as a regression guard. No codegen change.
Part of #242.
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