xenia-rs/migration/project-root/ppc-manual/vmx/lvsl.md

lvsl — Load Vector for Shift Left Indexed

Category: VMX (Altivec) · Form: X · Opcode: 0x7c00000c

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
lvsl	lvsl	—	Load Vector for Shift Left Indexed
lvsl128	lvsl128	—	Load Vector for Shift Left Indexed 128

Syntax

lvsl [VD], [RA0], [RB]
lvsl128 [VD], [RA0], [RB]

Encoding

lvsl — form X

• Opcode word: 0x7c00000c
• Primary opcode (bits 0–5): 31
• Extended opcode: 6
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode
6–10	RT/FRT/VRT	destination
11–15	RA/FRA/VRA	source A
16–20	RB/FRB/VRB	source B
21–30	XO	extended opcode (10 bits)
31	Rc	record-form flag

lvsl128 — form VX128_1

• Opcode word: 0x10000003
• Primary opcode (bits 0–5): 4
• Extended opcode: 3
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode (4)
6–10	VD128l	destination low 5 bits
11–15	RA	address register
16–20	RB	offset register
21–27	XO	extended opcode
28–29	VD128h	destination high 2 bits
30–31	—	reserved

Operands

Field	Role	Description
RA0	lvsl: read; lvsl128: read	Source GPR; when the encoded register number is 0 the operand is the literal 64-bit zero, not r0.
RB	lvsl: read; lvsl128: read	Source GPR.
VD	lvsl: write; lvsl128: write	Destination vector register.

Register Effects

lvsl

• Reads (always): RA0, RB
• Reads (conditional): none
• Writes (always): VD
• Writes (conditional): none

lvsl128

• Reads (always): RA0, RB
• Reads (conditional): none
• Writes (always): VD
• Writes (conditional): none

Status-Register Effects

No condition-register or status-register effects.

Operation (pseudocode)

addr_lo <- ((RA|0) + (RB))[60:63]
for i in 0..15: VD[i] <- addr_lo + i

C Translation Example

/* lvsl VD, RA, RB — load-shift-left permute control                */
uint64_t base = (insn.RA == 0) ? 0 : r[insn.RA];
uint8_t  sh   = (uint8_t)((base + r[insn.RB]) & 0xF);
for (int i = 0; i < 16; ++i) v[insn.VD].b[i] = sh + i;

Implementation References

lvsl

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="lvsl"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_altivec.cc:111
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:46
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:751
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:2520-2529

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::lvsl | PpcOpcode::lvsl128 => {
            let ea = if instr.ra() == 0 { 0u64 } else { ctx.gpr[instr.ra()] };
            let ea = ea.wrapping_add(ctx.gpr[instr.rb()]);
            let sh = (ea & 0xF) as u8;
            let mut r = [0u8; 16];
            for i in 0..16 { r[i] = sh + i as u8; }
            let vd = if matches!(instr.opcode, PpcOpcode::lvsl128) { instr.vd128() } else { instr.rd() };
            ctx.vr[vd] = xenia_types::Vec128::from_bytes(r);
            ctx.pc += 4;
        }

lvsl128

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="lvsl128"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_altivec.cc:114
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:46
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:412
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:2520-2529

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::lvsl | PpcOpcode::lvsl128 => {
            let ea = if instr.ra() == 0 { 0u64 } else { ctx.gpr[instr.ra()] };
            let ea = ea.wrapping_add(ctx.gpr[instr.rb()]);
            let sh = (ea & 0xF) as u8;
            let mut r = [0u8; 16];
            for i in 0..16 { r[i] = sh + i as u8; }
            let vd = if matches!(instr.opcode, PpcOpcode::lvsl128) { instr.vd128() } else { instr.rd() };
            ctx.vr[vd] = xenia_types::Vec128::from_bytes(r);
            ctx.pc += 4;
        }

Extended Pseudocode

; lvsl VD, RA, RB — load vector for shift left (generates a permute mask)
EA     <- (RA|0) + (RB)                          ; full 64-bit EA; only the low 4 bits matter
sh     <- EA[60:63]                              ; bits 60..63 of EA (the misalignment)
for i in 0..15:
    VD[i] <- sh + i                              ; bytes 0..15 of VD = {sh, sh+1, …, sh+15}

Special Cases & Edge Conditions

• No memory is actually read. Despite the name, lvsl / lvsr do not touch memory. They consume the effective address only to extract the low four bits (the alignment offset) and materialise a 16-byte permute control vector in VD. They are pure "address → permute-mask" converters.
• Big-endian byte indexing. VD[0] is the most-significant byte of the 128-bit register (lane 0). When EA & 0xF == 0 the output is {0, 1, 2, …, 15}, i.e. the identity permute. When EA & 0xF == 3 the output is {3, 4, …, 18} — modulo nothing, the values do exceed 15. That's intentional: fed into vperm (vperm VD, VA, VB, VC), byte selectors 0..15 index into VA and 16..31 index into VB. A stream of lvsl + two aligned lvx loads of consecutive 16-byte blocks + vperm reconstructs the unaligned 16-byte vector at EA.
• Pair with lvsr for the opposite direction. lvsl shifts "left" (toward the low index / high address byte); lvsr shifts "right". Which one to pick depends on which aligned block you're starting from — see the idiom below.
• Standard unaligned-load idiom.

  lvx   vAL, r0, rA           ; aligned block at EA & ~0xF
  lvx   vAH, r0, rA + 16      ; next aligned block
  lvsl  vC,  r0, rA           ; permute mask from misalignment
  vperm vD,  vAL, vAH, vC     ; the unaligned 16 bytes starting at EA
  

• RA0 semantics. When RA = 0 the base is the literal zero, so lvsl vD, 0, rB derives the mask from rB & 0xF.
• VMX128 sibling (lvsl128). Same semantics; only the VD register is encoded with the 7-bit VMX128 register-fusion (VD128l ‖ VD128h) so vD may be v0..v127.
• No flags, no side effects beyond writing VD. Trivial to move and schedule.

Related Instructions

• lvsr — the mirror: VD[i] = 16 − sh + i.
• vperm — consumes the mask to perform arbitrary byte-level permutation across two vectors.
• lvx, lvlx, lvrx — the actual memory loads used alongside the mask.
• vsldoi — static-offset shift-double; when the shift is compile-time known, this is cheaper than the lvsl/vperm pair.

IBM Reference

• AIX 7.3 — lvsl (Load Vector for Shift Left Indexed)
• IBM AltiVec Technology Programmer's Interface Manual — unaligned-load idiom