Adds 24 more mixed-arrangement mnemonics (72 forms) via specgen: three-different wide (SADDW/UADDW/SSUBW/USUBW), narrowing-halving (ADDHN/SUBHN/RADDHN/RSUBHN), and two-register narrowing (XTN/SQXTN/UQXTN/SQXTUN), plus their high-half '2' variants. All register-only (VD/VN/VM or VD/VN), no encoder change.
specgen refactored to a general arrangement-tuple mechanism (uniform + long/wide/narrow/XTN share one emit path). Verified: decode round-trips (SADDW/ADDHN/XTN/SQXTUN2), arm64 check + 461 tests pass.
Adds 26 widening long mnemonics (72 forms) via specgen: SADDL/UADDL/SSUBL/USUBL, SMULL/UMULL, SMLAL/UMLAL/SMLSL/UMLSL, SQDMULL/SQDMLAL/SQDMLSL and their high-half '2' variants. Destination arrangement is wider than the sources (Vd.8H, Vn.8B, Vm.8B; the '2' forms read the high half). Encoding stays VD/VN/VM, so no encoder change.
specgen gains a mixed-arrangement THREE_DIFF shape (low/high source-half pairs). Verified: decode round-trips (SMULL/SADDL2/SQDMULL/UMLAL), arm64 check + 461 tests pass.
Adds 10 Advanced-SIMD two-register-misc mnemonics (34 forms across valid arrangements) via specgen: NOT/RBIT/REV16/REV32/REV64/CLS/CLZ/CNT/URECPE/URSQRTE. llvm-mc filters illegal arrangements (CNT/NOT only 8B/16B, URECPE/URSQRTE only 2S/4S, ...).
specgen.lua generalized to a SHAPE table (THREE_SAME / TWO_SAME), so adding a family is one row. Verified: decode round-trips (NOT/REV64/CNT/URECPE), arm64 check + 461 tests pass.
Adds 25 Advanced-SIMD three-same integer mnemonics (153 forms across arrangements) to ENCODING_TABLE: SHADD/UHADD/SHSUB/UHSUB/SRHADD/URHADD, SQADD/UQADD/SQSUB/UQSUB, SMAX/UMAX/SMIN/UMIN, SABD/UABD/SABA/UABA, MLA/MLS, CMGE/CMHS/CMTST, SQDMULH/SQRDMULH.
Introduces tablegen/specgen.lua: compact specs (mnemonic + llvm name + arrangements) -> ENCODING_TABLE blocks, with bits taken from llvm-mc (the oracle) and mask derived empirically (vary registers 0..31). Invalid arrangements are auto-detected via llvm-mc and skipped. Output fills the SPECGEN:BEGIN..END region of encoding_table.odin in place; the hand-written core is untouched.
Verified: decode round-trips for SHADD/SQADD/CMGE/SQRDMULH; arm64 check + 461 tests pass; builders auto-generate (780 -> 805). Caveat: NEON builders currently collapse arrangements (one Register param per V operand) so inst_<mnem> exposes only the first arrangement -- an arrangement-aware builder-gen pass follows.
Author: Brendan Punsky (machine git config user.name is the login 'Flāvius').
