tools/testgen: add README documenting design, usage, and limitations

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tools/testgen/README.md

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+# gen-insn-tests.pl
+
+Pseudorandom test-case generator for NASM instruction patterns. For every
+non-`PSEUDO` mnemonic in `x86/insns.xda`, it generates a
+`travis/<mnemonic>/` test directory (`.asm` source, `.json` descriptor,
+golden `.bin16/32/64[.t]`/`.stderr16/32/64[.t]` files) that plugs directly
+into the existing `tools/travis/nasm-t.py` harness. This gives every
+instruction-set pattern regression coverage without hand-writing
+thousands of test cases.
+
+The generated tree currently checked in lives at `travis/insns/` (2432
+mnemonic subdirectories as of the last full run).
+
+## Usage
+
+```sh
+# Regenerate everything into travis/insns (run from repo root, after
+# ./nasm has been built and x86/insns.xda is up to date):
+perl tools/testgen/gen-insn-tests.pl --outdir travis/insns
+
+# Regenerate just one mnemonic, useful while iterating on the generator:
+perl tools/testgen/gen-insn-tests.pl --outdir /tmp/tg-scratch --only vaddps --verbose
+
+# Validate the full existing+generated suite (there's no generated
+# Makefile in a fresh checkout without running configure, so invoke the
+# harness directly rather than via `make travis`):
+python3 tools/travis/nasm-t.py --nasm=./nasm --directory=./travis run
+```
+
+Options (`perl tools/testgen/gen-insn-tests.pl --help` equivalent, see
+the `GetOptions` block at the top of the script for the authoritative
+list):
+
+| Option | Default | Meaning |
+|---|---|---|
+| `--xda FILE` | `x86/insns.xda` | source instruction-template file |
+| `--nasm PATH` | `./nasm` | nasm binary used to probe/assemble and to run `nasm-t.py update` |
+| `--outdir DIR` | `travis` | where to write `<mnemonic>/` subdirectories |
+| `--seed N` | `1` | base seed for the deterministic per-mnemonic PRNG |
+| `--per-mnemonic N` | `4` | max distinct operand-arity templates sampled per mnemonic |
+| `--variants N` | `2` | concrete random instruction instances generated per sampled template |
+| `--only MNEMONIC` | (all) | restrict generation to a single mnemonic, for iterating on the generator |
+| `--no-undoc` | off | exclude `UNDOC`-flagged templates (not recommended — see below) |
+| `--verbose` | off | print per-mnemonic progress as it generates |
+
+Generation is deterministic (fixed default seed), so re-running with no
+source changes reproduces byte-identical `.asm`/`.json` files. It should
+only need to be re-run when `x86/insns.dat` (and hence the regenerated
+`x86/insns.xda`) changes — day-to-day `travis` test *runs* use the
+existing harness and scale via `make -jN travis` as normal; generation is
+a separate, infrequent step.
+
+## Design: why `x86/insns.xda` and not `insnsa.c`/`insns.pl` hooks
+
+Three data sources were considered:
+
+1. **Parse the generated `insnsa.c`/`insnsb.c` directly.** Rejected:
+   these encode operands as opaque bitmask constants and index into the
+   shared `nasm_bytecodes[]` array, which `x86/bytecode.txt` explicitly
+   documents as unstable ("byte codes can be moved around and recycled
+   at any time"). Parsing this from outside `insns.pl` would be fragile
+   across NASM releases and high-effort for no real benefit.
+2. **Add new hooks to `x86/insns.pl`** to emit a purpose-built
+   intermediate format. Rejected: it would duplicate information
+   `insns.xda` already provides, adding maintenance risk to a
+   build-critical code generator for no real gain.
+3. **Parse `x86/insns.xda`** (chosen). This file is already generated by
+   `insns.pl` from `insns.dat` as part of a normal build, with macros
+   fully expanded, one instruction template per line, in a small stable
+   text grammar:
+
+   ```
+   MNEMONIC   optype1,optype2,...   [enc: bytecode...]   FLAGS
+   ```
+
+   Crucially, **we never need the bytecode/encoding field** — NASM's own
+   assembler selects the encoding from mnemonic + operand syntax at
+   assemble time. So the generator only needs to produce syntactically
+   valid operand text per operand-type token; it doesn't need to
+   understand or replicate NASM's instruction encoding at all. This
+   means the generated tests are **regression tests** — validated
+   against a golden captured from a known-good `nasm` build — not
+   independent correctness oracles. That's the same philosophy already
+   used by the rest of `travis/`.
+
+## Handling of UNDOC / OBSOLETE / NEVER instructions
+
+These are **included**, not excluded (only `PSEUDO` templates — which
+aren't real ISA instructions, e.g. internal directives — are skipped).
+Verifying that `OBSOLETE`/`NEVER`-flagged instructions produce the
+expected warning (typically `-w+obsolete-removed` at the assembler's
+default/highest CPU level) is itself part of the intended test coverage,
+not something to suppress. When the generator's probe step detects
+nonempty stderr for a template, it adds a `"stderr"` target to the
+`.json` descriptor so the warning text becomes part of the golden.
+`UNDOC` alone does not currently trigger a warning by default (verified
+against `SALC`), so most `UNDOC` mnemonics just get ordinary output-only
+targets.
+
+## Condition-code mnemonic families
+
+`insns.xda` leaves condition-code instruction families as literal,
+unexpanded placeholders: `Jcc`, `SETcc`, `CMOVcc`, `CFCMOVcc`. Unlike
+`$bwdq`-style width macros (which `insns.pl` *does* pre-expand into
+concrete lines in `.xda`), NASM's parser expands condition codes against
+its own cc-table at parse time — so the literal placeholder mnemonic
+isn't directly assemblable. The generator expands these four families
+itself into concrete mnemonics (`JE`, `SETNZ`, `CMOVGE`, ...) using the
+16 canonical condition-code suffixes
+(`o no b ae e ne be a s ns p np l ge le g`).
+
+**Known gap:** the newer APX condition-suffix families (`CCMPscc`,
+`CTESTscc`, `CMPccXADD`, `SETccZU`) are *not* expanded and are currently
+dropped as unsupported. Extending the `@cc_suffixes` expansion loop to
+cover these would close this gap.
+
+## Bit-width (16/32/64) handling
+
+Bit-mode support isn't derived from CPU/mode flags in `insns.xda` — the
+same generated `.asm` text is simply probed under `--bits 16/32/64`, and
+only the modes that actually assemble successfully become targets in the
+final `.json` (mirroring the pre-existing `travis/jmpxx/`-style pattern).
+Two wrinkles this created and how they're handled:
+
+- **64-bit-only tokens poisoning 16/32 probes.** A single sampled
+  template using a 64-bit-only operand (e.g. `reg64`, `imm64`) would
+  otherwise make the *whole file* fail to assemble at 16/32-bit, even
+  though other templates in the same mnemonic don't need 64-bit. The
+  generator classifies operand tokens via a `%needs64_token` table and
+  renders **two** asm bodies per mnemonic — a "narrow" one (16/32-safe)
+  and a "full" one (includes 64-bit-only forms) — reusing one file when
+  they'd be identical.
+- **Memory/vsib addressing across bit widths.** A fixed base-register
+  name (`rax` vs `eax` vs `ax`) isn't valid syntax across all three bit
+  widths. The generator sidesteps this by using **base-register-free**
+  addressing: bare-displacement memory operands (`[0x1234]`) and
+  base-free scaled vsib index forms (`[xmm0*1]`). This sacrifices
+  coverage of true `[base+index*scale+disp]` forms — a known,
+  documented limitation, not a bug.
+
+## Integration with `nasm-t.py`
+
+Golden capture is delegated entirely to the existing harness:
+
+```sh
+python3 tools/travis/nasm-t.py --nasm=./nasm update -t <outdir>/<mnemonic>
+```
+
+rather than hand-rolling nasm invocation, renaming output binaries, or
+hand-writing stderr golden files. This matters because
+`nasm-t.py`'s stderr/stdout comparison is an **exact byte-for-byte
+string match with no path normalization** — NASM embeds the literal
+source-path argument it was invoked with into warning/error text, so
+hand-rolled golden capture is easy to get subtly wrong. `update` uses
+its own internal, consistent path construction, so the generator's own
+cheap throwaway pre-probe (used only to decide per-bit-width viability
+and whether a `"stderr"` target is needed) doesn't need to match those
+paths exactly. If `nasm-t.py update` fails for a mnemonic (should only
+happen if the pre-probe's viability check was wrong), the generator
+drops that mnemonic's directory entirely rather than committing a
+broken test.
+
+## Output naming convention
+
+Multiple bit-width variants generated from one source share the
+established repo convention: `<mnemonic>.bin16` / `.bin32` / `.bin64`
+(binary; see e.g. `travis/jmpxx/jmpxx-ox.bin16`), with `.stderr16` /
+`.stderr32` / `.stderr64` analogously for expected-warning goldens — not
+`<mnemonic>16.bin`.
+
+## Known limitations / coverage gaps
+
+The generator prints a coverage summary at the end of each run
+(mnemonics generated, mnemonics dropped, and a frequency table of
+unsupported operand-type tokens), so gaps are self-documenting. As of
+the last full run:
+
+- **2432 / 2452 mnemonics generate at least one assemblable template.**
+- **20 mnemonics produce zero output and are dropped**, mostly:
+  - `HINT_NOP0` .. `HINT_NOP63` placeholder mnemonics,
+  - `LOADALL` / `LOADALL286`,
+  - a handful of exotic AMX-transpose / APX instructions whose full
+    operand grammar wasn't targeted (e.g. `T2RPNTLVWZ0*`, `TCONJT*`,
+    `TTMMULTF32PS`).
+- APX `scc`-suffix condition-code families (`CCMPscc`, `CTESTscc`,
+  `CMPccXADD`, `SETccZU`) are not expanded (see above).
+- Memory/vsib operands only exercise base-register-free addressing
+  forms (see above) — true base+index+scale+disp combinations aren't
+  covered by generated tests.
+- ~70+ distinct operand-type tokens have generator support (see the
+  `%fixed`/`%gen` tables at the top of the script); any new/renamed
+  token introduced by a future `insns.dat` change that isn't in those
+  tables will show up in the "unsupported operand-type tokens" summary
+  and cause the affected templates (not necessarily the whole mnemonic)
+  to be skipped.
+
+Extending coverage for any of the above is a matter of adding entries to
+the `%fixed`/`%gen` operand-generator tables or the `@cc_suffixes`
+expansion loop — no changes to the parser or driver loop are needed.
+
+## Validation
+
+Full-suite validation after the last generation run (existing `travis/`
+tests + `travis/insns/`):
+
+```sh
+python3 tools/travis/nasm-t.py --nasm=./nasm --directory=./travis run
+```
+
+4225 tests total, 4224 PASS, 1 pre-existing SKIP (`time`, an
+intentional/known skip unrelated to this tool), 0 FAIL, 0 ABORT.