e6d43a23ac
Bundles state that lives OUTSIDE the xenia-rs repo so a fresh clone on
another machine can be brought up to identical configuration via
migration/setup.sh:
- claude-memory/ ~/.claude/projects/-home-fabi-RE-Project-Sylpheed/memory/
(103 files, 1.1 MB - MEMORY.md + every
project_xenia_rs_*.md from audits
addis_signext through audit-058)
- project-root/dot-claude/ <project-root>/.claude/settings.json
(Stop hook + permissions)
- project-root/ppc-manual/ <project-root>/ppc-manual/
(PowerPC reference docs, 397 files, 3.7 MB)
- project-root/run-canary.sh <project-root>/run-canary.sh
- README.md Human-readable setup checklist
- setup.sh Idempotent installer (also reclones
xenia-canary at pinned HEAD 6de80dffe)
- MANIFEST.md Per-file mapping + per-file-not-bundled
restoration recipe
Excluded from bundle (not shippable via git):
- Sylpheed ISO (7.8 GB; copyright; manual copy required)
- sylpheed.db (395 MB; regenerable from XEX via analysis tooling)
- target/ build artifacts (rebuild on target)
- audit-runs probe firehoses (.log/.stdout/.stderr ~11 GB; rerun if needed)
- audit-runs memory dumps (.bin ~4.5 GB; rerun audit-026/027/029 if needed)
- xenia-canary checkout (setup.sh reclones from
git.mc02.dev/fabi/Xenia-Canary.git at HEAD 6de80dffe)
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
122 lines
4.9 KiB
Markdown
122 lines
4.9 KiB
Markdown
# `cntlzwx` — Count Leading Zeros Word
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> **Category:** [Integer ALU](../categories/alu.md) · **Form:** [X](../forms/X.md) · **Opcode:** `0x7c000034`
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<!-- GENERATED: BEGIN -->
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## Assembler Mnemonics
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| Mnemonic | XML entry | Flags | Description |
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| --- | --- | --- | --- |
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| `cntlzw` | `cntlzwx` | — | Count Leading Zeros Word |
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| `cntlzw.` | `cntlzwx` | Rc=1 | Count Leading Zeros Word |
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## Syntax
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```asm
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cntlzw[Rc] [RA], [RS]
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```
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## Encoding
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### `cntlzwx` — form `X`
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- **Opcode word:** `0x7c000034`
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- **Primary opcode (bits 0–5):** `31`
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- **Extended opcode:** `26`
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- **Synchronising:** no
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| Bits | Field | Meaning |
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| --- | --- | --- |
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| 0–5 | `OPCD` | primary opcode |
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| 6–10 | `RT/FRT/VRT` | destination |
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| 11–15 | `RA/FRA/VRA` | source A |
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| 16–20 | `RB/FRB/VRB` | source B |
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| 21–30 | `XO` | extended opcode (10 bits) |
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| 31 | `Rc` | record-form flag |
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## Operands
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| Field | Role | Description |
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| --- | --- | --- |
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| `RS` | cntlzwx: read | Source GPR (alias for RD in some stores). |
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| `RA` | cntlzwx: write | Source GPR (`r0`–`r31`). |
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| `CR` | cntlzwx: write (conditional) | Condition-register update. When `Rc=1`, CR field 0 (or CR6 for vector compares, CR1 for FPU) is updated from the result. |
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## Register Effects
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### `cntlzwx`
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- **Reads (always):** `RS`
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- **Reads (conditional):** _none_
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- **Writes (always):** `RA`
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- **Writes (conditional):** `CR`
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## Status-Register Effects
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- `cntlzwx`: **CR0** ← signed-compare(result, 0) with `SO ← XER[SO]`, when `Rc=1`.
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## Operation (pseudocode)
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```
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n <- number_of_leading_zero_bits((RS)[32:63]) ; n in 0..32
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RA <- zero_extend(n)
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```
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## C Translation Example
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```c
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/* C translation: the xenia-rs interpreter arm below in */
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/* Implementation References is the authoritative semantic */
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/* snapshot. Translate it line-by-line: */
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/* - ctx.gpr[N] -> r[N] (or f[]/v[] for FPRs/VRs) */
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/* - mem.read_u*/write_u* -> mem_read_u*_be / mem_write_u*_be */
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/* - ctx.update_cr_signed(fld, v) -> update_cr_signed(fld, v) */
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/* - ctx.xer_ca / xer_ov / xer_so -> xer.CA / xer.OV / xer.SO */
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/* The Register Effects and Status-Register Effects tables above */
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/* enumerate every side effect a faithful translation must emit. */
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```
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## Implementation References
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**`cntlzwx`**
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- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="cntlzwx"`](../../xenia-canary/tools/ppc-instructions.xml)
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- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_alu.cc:689`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_alu.cc#L689)
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- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:15`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L15)
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- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:758`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L758)
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- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:608-614`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L608-L614)
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<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>
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```rust
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PpcOpcode::cntlzwx => {
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// Result is 0..=32, fits in u32 with bit 31 always zero, so the
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// CR0 view is benign — use the catch-all 32-bit form for consistency.
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ctx.gpr[instr.ra()] = (ctx.gpr[instr.rs()] as u32).leading_zeros() as u64;
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if instr.rc_bit() { ctx.update_cr_signed(0, ctx.gpr[instr.ra()] as u32 as i32 as i64); }
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ctx.pc += 4;
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}
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```
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</details>
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<!-- GENERATED: END -->
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## Special Cases & Edge Conditions
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- **Operates only on the low 32 bits.** `RS[0:31]` is ignored; the count is taken on `RS[32:63]`. Result range is `0..=32`.
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- **`RA = 32` when the low 32 bits are zero**, regardless of the high 32 bits. A common pitfall: `cntlzw` after computing a 64-bit value can give a counter-intuitive result when the leading 1-bit lives in the high half.
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- **`RA = 0` when bit 32 (the sign bit of the low word) is set.** This makes `cntlzw RA, RS; cmpwi RA, 0` a one-instruction-pair "is the low half negative" test, though `srawi RA, RS, 31` is more idiomatic.
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- **High 32 bits of the result are zero.** `RA[0:31] = 0`, `RA[32:63] = count`.
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- **`Rc=1` CR0 update is small-positive-only.** Result fits in 6 bits; xenia's `as i32 as i64` truncation in [`interpreter.rs:404`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L404) is harmless. CR0 will be `EQ` only when `RS[32]` (sign bit of low word) is 1, otherwise `GT`.
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- **Useful for `floor(log2)` of a 32-bit value.** `31 - cntlzw(x)` for nonzero `x`.
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## Related Instructions
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- [`cntlzdx`](cntlzdx.md) — 64-bit version (counts the full register).
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- [`slwx`](slwx.md), [`srwx`](srwx.md), [`srawx`](srawx.md) — 32-bit shifts often paired with `cntlzw` for normalisation.
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- [`rlwinmx`](rlwinmx.md) — to mask off bits before counting.
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- [`cmpi`](cmpi.md), [`cmpl`](cmpl.md) — alternative ways to detect zero or sign.
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## IBM Reference
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- [AIX 7.3 — `cntlzw` (Count Leading Zeros Word)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-cntlzw-count-leading-zeros-word-instruction)
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