OpenPOWER SV setvl/setvli

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Use of setvl results in changes to the MVL, VL and STATE SPRs. see sprs

Behaviour and Rationale

SV's Vector Engine is based on Cray-style Variable-length Vectorisation, just like RVV. However unlike RVV, SV sits on top of the standard Scalar regfiles: there is no separate Vector register numbering. Therefore, also unlike RVV, SV does not have hard-coded "Lanes". The relevant parameter in RVV is "MAXVL" and this is architecturally hard-coded into RVV systems, anywhere from 1 to tens of thousands of Lanes in supercomputers.

SV is more like how MMX used to sit on top of the x86 FP regfile. Therefore when Vector operations are performed, the question has to be asked, "well, how much of the regfile do you want to allocate to this operation?" because if it is too small an amount performance may be affected, and if too large then other registers would overlap and cause data corruption, or even if allocated correctly would require spill to memory.

The answer effectively needs to be parameterised. Hence: MAXVL (MVL) is set from an immediate, so that the compiler may decide, statically, a guaranteed resource allocation according to the needs of the application.

Other than being able to set MVL, SV's VL (Vector Length) works just like RVV's VL, with one minor twist. RVV permits the setvl instruction to set VL to an arbitrary value. Given that RVV only works on Vector Loops, this is fine and part of its value and design. However, SV sits on top of the standard register files. When MVL=VL=2, a Vector Add on r3 will perform two Scalar Adds: one on r3 and one on r4.

Thus there is the opportunity to set VL to an explicit value (within the limits of MVL) with the reasonable expectation that if two operations are requested (by setting VL=2) then two operations are guaranteed. This avoids the need for a loop (with not-insignificant use of the regfiles for counters), simply two instructions:

setvli r0, MVL=64, VL=64
ld r0.v, 0(r30) # load 64 registers from memory

Page Faults etc. aside this is guaranteed 100% without fail to perform 64 unit-strided LDs starting from the address pointed to by r30 and put the contents into r0 through r63. Thus it becomes a "LOAD-MULTI". Twin Predication could even be used to only load relevant registers from the stack. This only works if VL is set to the requested value (caveat being, limited to not exceed MVL)

Format

(Allocation of opcode TBD pending OPF ISA WG approval)

0.5 6.10 11.15 16.20 21..24.25 26...30 31 name
19 RT RA XO[0:4] XO[5:9] Rc XL-Form
19 RT RA imm i // vs ms NNNNN Rc setvl

Note that imm spans 7 bits (16 to 22), and that bit 22 is reserved and must be zero. Setting bit 22 causes an illegal exception.

Note that in immediate setting mode VL and MVL start from one i.e. that an immediate value of zero will result in VL/MVL being set to 1. 0b111111 results in VL/MVL being set to 64. This is because setting VL/MVL to 1 results in "scalar identity" behaviour, where setting VL/MVL to 0 would result in all Vector operations becoming nop. If this is truly desired (nop behaviour) then setting VL and MVL to zero is to be done via the SV SPRs

Note that setmvli is a pseudo-op, based on RA/RT=0, and setvli likewise

setvli VL=8    : setvl r5, r0, VL=8
setmvli MVL=8  : setvl r0, r0, MVL=8

Note that whilst it is possible to set both MVL and VL from the same immediate, it is not possible to set them to different immediates in the same instruction. That would require two instructions.

Pseudocode

// instruction fields:
rd = get_rt_field();         // bits 6..10
ra = get_ra_field();         // bits 11..15
vs = get_vs_field();         // bit 24
ms = get_ms_field();         // bit 25
Rc = get_Rc_field();         // bit 31
// add one. MVL/VL=1..64 not 0..63
vlimmed = get_immed_field()+1; //  16..22

// set VL (or not).
// 3 options: from SPR, from immed, from ra
if vs {
   // VL to be sourced from fields/regs
   if ra != 0 {
       VL = GPR[ra]  
   } else {
       VL = vlimmed
   }
} else {
   // VL not to change (except if MVL is reduced)
   // read from SPRs
   VL = SPR[SV_VL]
}

// set MVL (or not).
// 2 options: from SPR, from immed
if ms {
   MVL = vlimmed
} else {
   // MVL not to change, read from SPRs
   MVL = SPR[SV_MVL]
}

// calculate (limit) VL
VL = min(VL, MVL)

// store VL, MVL
SPR[SV_VL] = VL
SPR[SV_MVL] = MVL

// write rd
if rt != 0 {
    // rt is not zero
    regs[rt] = VL;
}
// write CR?
if Rc {
    // update CR from VL (not rt)
    CR0.eq = (VL == 0)
    ...
    ...
}

Examples

Core concept loop

loop:
setvl a3, a0, MVL=8    #  update a3 with vl
                       # (# of elements this iteration)
                       # set MVL to 8
# do vector operations at up to 8 length (MVL=8)
# ...
sub a0, a0, a3   # Decrement count by vl
bnez a0, loop    # Any more?

Loop using Rc=1

my_fn:
  li r3, 1000
  b test
loop:
  sub r3, r3, r4
  ...
test:
  setvli. r4, r3, MVL=64
  bne cr0, loop
end:
  blr