Sylpheed/tools/ppc-manual/vmx/vslo.md

vslo — Vector Shift Left by Octet

Category: VMX (Altivec) · Form: VX · Opcode: 0x1000040c

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
vslo	vslo	—	Vector Shift Left by Octet
vslo128	vslo128	—	Vector128 Shift Left Octet

Syntax

vslo [VD], [VA], [VB]
vslo128 [VD], [VA], [VB]

Encoding

vslo — form VX

• Opcode word: 0x1000040c
• Primary opcode (bits 0–5): 4
• Extended opcode: 1036
• 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)

vslo128 — form VX128

• Opcode word: 0x14000390
• Primary opcode (bits 0–5): 5
• Extended opcode: 912
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode (4 or 5)
6–10	VD128l	destination low 5 bits
11–15	VA128l	source A low 5 bits
16–20	VB128l	source B low 5 bits
21	VA128H	source A high bit
22	—	reserved
23–25	VC	optional VC / XO sub-field
26	VA128h	source A middle bit
27	—	reserved
28–29	VD128h	destination high 2 bits
30–31	VB128h	source B high 2 bits

Operands

Field	Role	Description
VA	vslo: read; vslo128: read	Source A vector register.
VB	vslo: read; vslo128: read	Source B vector register.
VD	vslo: write; vslo128: write	Destination vector register.

Register Effects

vslo

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

vslo128

• 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

vslo

• Canary XML: tools/ppc-instructions.xml — search for mnem="vslo"
• Canary emitter: src/xenia/cpu/ppc/ppc_emit_altivec.cc:1482
• Sylpheed opcode: crates/sylpheed-ppc/src/opcode.rs:445
• Sylpheed decoder: crates/sylpheed-ppc/src/decoder.rs:638

Canary emitter (frozen snapshot @ f21ebd49e9)

int InstrEmit_vslo(PPCHIRBuilder& f, const InstrData& i) {
  return InstrEmit_vslo_(f, i.VX.VD, i.VX.VA, i.VX.VB);
}

// ── delegates to (src/xenia/cpu/ppc/ppc_emit_altivec.cc:1471) ──
int InstrEmit_vslo_(PPCHIRBuilder& f, uint32_t vd, uint32_t va, uint32_t vb) {
  // (VD) <- (VA) << (VB.b[F] & 0x78) (by octet)
  // TODO(benvanik): flag for shift-by-octet as optimization.
  Value* sh = f.Shr(
      f.And(f.Extract(f.LoadVR(vb), 15, INT8_TYPE), f.LoadConstantInt8(0x78)),
      3);
  Value* v = f.Permute(f.LoadVectorShl(sh), f.LoadVR(va), f.LoadZeroVec128(),
                       INT8_TYPE);
  f.StoreVR(vd, v);
  return 0;
}

vslo128

• Canary XML: tools/ppc-instructions.xml — search for mnem="vslo128"
• Canary emitter: src/xenia/cpu/ppc/ppc_emit_altivec.cc:1485
• Sylpheed opcode: crates/sylpheed-ppc/src/opcode.rs:446
• Sylpheed decoder: crates/sylpheed-ppc/src/decoder.rs:746

Canary emitter (frozen snapshot @ f21ebd49e9)

int InstrEmit_vslo128(PPCHIRBuilder& f, const InstrData& i) {
  return InstrEmit_vslo_(f, VX128_VD128, VX128_VA128, VX128_VB128);
}

// ── delegates to (src/xenia/cpu/ppc/ppc_emit_altivec.cc:1471) ──
int InstrEmit_vslo_(PPCHIRBuilder& f, uint32_t vd, uint32_t va, uint32_t vb) {
  // (VD) <- (VA) << (VB.b[F] & 0x78) (by octet)
  // TODO(benvanik): flag for shift-by-octet as optimization.
  Value* sh = f.Shr(
      f.And(f.Extract(f.LoadVR(vb), 15, INT8_TYPE), f.LoadConstantInt8(0x78)),
      3);
  Value* v = f.Permute(f.LoadVectorShl(sh), f.LoadVR(va), f.LoadZeroVec128(),
                       INT8_TYPE);
  f.StoreVR(vd, v);
  return 0;
}

Special Cases & Edge Conditions

• Whole-register shift-left by octets (bytes). VA is shifted left by N bytes, where N = (VB.b[15] >> 3) & 0xF — bits 1..4 of the last byte of VB. Right end is zero-filled. N saturates at 15 because only 4 bits are honoured.
• Shift count constraint. The ISA mandates a uniform 4-bit count across all of VB; Canary reads only byte 15 ((VB.b[15] & 0x78) >> 3). Splat with vspltb before invoking when the count is derived dynamically.
• Pair with vsl for full bit-level shifts. vslo contributes the byte-granular part; vsl contributes the 0..7 residual bits.
• Big-endian. "Left" = toward MSB = toward VD.b[0].
• No flags, no VSCR.
• VMX128 sibling vslo128.

Related Instructions

• vsl — the bit-level whole-register shift-left.
• vsro — shift-right by octets.
• vsldoi — static-immediate variant.
• vslb, vslh, vslw — per-lane shifts.

IBM Reference

• AIX 7.3 — vslo (Vector Shift Left by Octet)
• IBM AltiVec Technology Programmer's Interface Manual, Chapter 4 — Integer Shift / Rotate