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Sylpheed/tools/ppc-manual/vmx/vmulesb.md
sim dedcf37867 docs(ppc-manual): quote Canary and our own decoder, not the retired xenia-rs 
The generator had not been able to run correctly since the manual moved into
`tools/ppc-manual/`: it computed the repository root as `HERE.parent.parent`,
which now names `tools/`, so the XML, Canary's emitters and xenia-rs all stopped
resolving — silently, because both scrapers skipped what they could not find.
Every page's references had been pointing at paths that exist nowhere.

What each source contributed, measured on the 350 pages before this change:

  Operation (pseudocode)  251 pages: fixed boilerplate "derives from the xenia-rs
                          interpreter"; 99 carry real hand-written seeds
  C translation           337 pages: the same kind of boilerplate
  xenia-rs snapshot       336 pages: the interpreter arm, pasted in — the only
                          per-instruction semantics on unseeded pages
  links                   xenia-rs opcode/decoder/interpreter + Canary emitter

Now:

  * semantics come from **Xenia Canary**, the reference emulator, read through
    `git show` at a pinned upstream commit (`origin/canary_experimental`,
    f21ebd49e9). Not our checkout: it carries instrumentation and lacked
    upstream's `mcrf` fix, so it would have published probes and a wrong `mcrf`.
    Each page embeds the emitter (`InstrEmit_<mnem>`), and for the 128 pure
    one-line delegations also the helper that holds the semantics.
  * decode references point at `crates/sylpheed-ppc` — the decoder that
    produces `sylpheed.db` — as in-repo relative links.
  * the boilerplate now says what is true, and the C translation guide maps
    Canary's actual HIR calls, checked against `ppc_hir_builder.h` (including
    that `UpdateCR(n, v)` truncates to 32 bits).
  * `rust_scraper.py` -> `decoder_scraper.py` (interpreter half dropped);
    missing sources are now errors, not empty results.

Verified:

  consistency checks        455 XML entries, 350 families, 598 index keys
  hand-written tails        386/386 byte-identical after regeneration
  xenia-rs in generated     0
  pages with a snapshot     349/350 (was 336) — `dcbi` has no Canary emitter at all
  in-repo decoder links     910/910 resolve to a line holding the identifier
  emitter boundaries        brace counter == column-0 `}` rule on 521/521;
                            preprocessor model unit-tested (#if 0/#else/#elif)
  idempotency               re-run: 0 pages updated, 0 working-tree changes

Hand-written notes (outside the generated regions) are not rewritten here:

  * 110 links into `../../xenia-rs/...` were dead; they now point at the file in
    the archived repository (git.mc02.dev/fabi/xenia-rs @ 8401d4d). Line anchors
    were dropped because the notes predate that commit — 0 of 441 old line
    ranges match it — and a precise-looking wrong anchor is worse than none. The
    link text, which carries the author's line numbers, is unchanged.
  * 140 prose claims about xenia-rs's behaviour remain. 23 are verified to hold
    for Canary too (the 32-bit CR0 truncation, OE left unimplemented); the other
    114 need checking one by one, and some invert — e.g. `divdx` notes a correct
    64-bit CR0 update in xenia-rs where Canary's `UpdateCR` truncates. Left for
    a deliberate pass rather than a blind substitution.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
2026-09-16 20:34:34 +02:00

5.4 KiB
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vmulesb — Vector Multiply Even Signed Byte

Category: VMX (Altivec) · Form: VX · Opcode: 0x10000308

Assembler Mnemonics

Mnemonic XML entry Flags Description
vmulesb vmulesb — Vector Multiply Even Signed Byte

Syntax

vmulesb [VD], [VA], [VB]

Encoding

vmulesb — form VX

  • Opcode word: 0x10000308
  • Primary opcode (bits 0–5): 4
  • Extended opcode: 776
  • Synchronising: no
Bits Field Meaning
0–5 OPCD primary opcode (4)
6–10 VRT/VD destination vector register
11–15 VRA/VA source A vector register
16–20 VRB/VB source B vector register
21–31 XO extended opcode (11 bits)

Operands

Field Role Description
VA vmulesb: read Source A vector register.
VB vmulesb: read Source B vector register.
VD vmulesb: write Destination vector register.

Register Effects

vmulesb

  • Reads (always): VA, VB
  • Reads (conditional): none
  • Writes (always): VD
  • Writes (conditional): none

Status-Register Effects

No condition-register or status-register effects.

Operation (pseudocode)

; No hand-written pseudocode for this instruction yet.
; The authoritative semantics are the Canary emitter snapshot under
; Implementation References; about half of Canary's emitters open
; with the PPC-style definition as a comment (`RD <- (RA) + (RB)`).
; Every side effect is also enumerated in the Register Effects and
; Status-Register Effects tables above.

C Translation Example

/* No hand-written C yet. Translate the Canary emitter snapshot   */
/* under Implementation References; its HIR maps directly:        */
/*   f.LoadGPR(n) / f.StoreGPR(n, v)  -> r[n] / r[n] = v          */
/*   f.LoadFPR / StoreFPR, f.LoadVR / StoreVR -> f[n], v[n]        */
/*   f.Load(ea, T), f.Store(ea, v) -> raw read / write; emitters   */
/*     wrap them in f.ByteSwap for the big-endian guest value      */
/*   f.UpdateCR(n, v)  -> CR field n from v's LOW 32 BITS vs 0     */
/*   f.LoadCA / f.StoreCA -> xer.CA;  f.StoreSAT -> vscr.SAT       */
/*   i.XO.RA, i.D.DS, ... -> the bit-fields listed under Operands  */
/* The Register Effects and Status-Register Effects tables above  */
/* enumerate every side effect a faithful translation must emit.  */

Implementation References

vmulesb

Canary emitter (frozen snapshot @ f21ebd49e9) 
int InstrEmit_vmulesb(PPCHIRBuilder& f, const InstrData& i) {
  XEINSTRNOTIMPLEMENTED();
  return 1;
}

Special Cases & Edge Conditions

  • Even-byte signed multiply, half-word result. Per output half lane: 
    VD[i] = int16(int8(VA[2*i]) * int8(VB[2*i]))           ; for i = 0..7
    Only the eight even-indexed byte lanes (lanes 0, 2, 4, …, 14 in big-endian) are read from each source. Each int8 × int8 product is widened to int16, producing eight half-word results that fill all of VD.
  • No saturation, no VSCR[SAT]. The full 16-bit product of two signed bytes always fits in int16 (range -127*-128 = +16256 .. +127*+127 = +16129 is well within ±32767), so no clipping is needed — even at the bit-pattern extremes (-128) * (-128) = +16384 is representable.
  • Pairs with vmulosb (odd-byte sibling). Together they consume all 16 bytes; two instructions are needed for a "multiply every lane" 16×16-bit byte multiply.
  • Big-endian byte indexing. Even byte indices 0, 2, 4, …, 14 correspond to the high-order halves of each half-word slot.
  • No XER, no exceptions.
  • Aliasing legal.
  • No VMX128 sibling.
  • Common usage. Signed-coefficient byte multiply for image filters; first half of a 16-byte signed multiply when paired with vmulosb.

Related Instructions

  • vmulosb — odd-byte sibling (lanes 1, 3, …, 15).
  • vmuleub, vmuloub — same split, unsigned.
  • vmulesh, vmulosh — same family at half width (→ word results).
  • vmladduhm — per-lane modulo multiply-add (low half only).
  • vpkshus — saturating pack down from int16 halves to uint8 bytes (combine with vmule* for "scale + clamp" pipelines).

IBM Reference

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