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>
120 lines
4.8 KiB
Markdown
120 lines
4.8 KiB
Markdown
# `cntlzdx` — Count Leading Zeros Doubleword
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> **Category:** [Integer ALU](../categories/alu.md) · **Form:** [X](../forms/X.md) · **Opcode:** `0x7c000074`
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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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| `cntlzd` | `cntlzdx` | — | Count Leading Zeros Doubleword |
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| `cntlzd.` | `cntlzdx` | Rc=1 | Count Leading Zeros Doubleword |
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## Syntax
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```asm
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cntlzd [RA], [RS]
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```
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## Encoding
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### `cntlzdx` — form `X`
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- **Opcode word:** `0x7c000074`
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- **Primary opcode (bits 0–5):** `31`
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- **Extended opcode:** `58`
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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` | cntlzdx: read | Source GPR (alias for RD in some stores). |
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| `RA` | cntlzdx: write | Source GPR (`r0`–`r31`). |
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| `CR` | cntlzdx: 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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### `cntlzdx`
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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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- `cntlzdx`: **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)) ; n in 0..64
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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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**`cntlzdx`**
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- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="cntlzdx"`](../../xenia-canary/tools/ppc-instructions.xml)
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- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_alu.cc:674`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_alu.cc#L674)
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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:767`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L767)
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- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:615-619`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L615-L619)
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<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>
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```rust
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PpcOpcode::cntlzdx => {
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ctx.gpr[instr.ra()] = ctx.gpr[instr.rs()].leading_zeros() as u64;
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if instr.rc_bit() { ctx.update_cr_signed(0, ctx.gpr[instr.ra()] 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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- **Result range is `0..=64`.** When `RS == 0`, every bit is a leading zero and `RA = 64`. When the high bit (`RS[0]`) is set, `RA = 0`. Any intermediate value yields the count of high-order zeros before the first 1-bit.
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- **Counts across the full 64 bits.** Use [`cntlzwx`](cntlzwx.md) when you only want to count the low 32 bits.
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- **Useful as `floor(log2(x)) = 63 − cntlzd(x)`** for nonzero `x`. Frequently used in fast normalization, priority encoders, and bit-vector operations.
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- **`RB` field unused.** This is X-form but only `RS` is read; `RB` is a placeholder slot.
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- **`Rc=1` quirk.** `update_cr_signed(0, RA as i64)` is correct in xenia-rs because the result fits in 7 bits and is non-negative. The CR0 result will always be `EQ` (when `RS != 0` and `RA != 0`? — actually `EQ` only when `RS[0] == 1`, i.e. `RA == 0`) or `GT` (when `RS != 0` so `RA > 0`); never `LT`. `EQ` corresponds to "high bit set in `RS`", a useful one-instruction sign test for negative-as-signed values.
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- **No `XER` side effects.** Counts neither overflow nor carry.
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## Related Instructions
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- [`cntlzwx`](cntlzwx.md) — 32-bit version (counts only `RS[32:63]`).
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- [`sldx`](sldx.md), [`srdx`](srdx.md) — pair with `cntlzd` for normalisation.
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- [`rldiclx`](rldiclx.md) — for extracting individual bit positions once the leading-zero count is known.
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- [`cmpi`](cmpi.md) / [`cmp`](cmp.md) — alternative for testing the high bit.
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## IBM Reference
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- [AIX 7.3 — `cntlzd` (Count Leading Zeros Doubleword)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-cntlzd-count-leading-zeros-double-word-instruction)
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