# setvl: Set Vector Length

Add the following section to the Simple-V Chapter

## setvl

SVL-Form

0-5 6-10 11-15 16-22 23 24 25 26-30 31 FORM
PO RT RA SVi ms vs vf XO Rc SVL-Form
• setvl RT,RA,SVi,vf,vs,ms (Rc=0)
• setvl. RT,RA,SVi,vf,vs,ms (Rc=1)

Pseudo-code:

overflow <- 0b0    # sets CR.SO if set and if Rc=1
VLimm <- SVi + 1
# set or get MVL
if ms = 1 then MVL <- VLimm[0:6]
else           MVL <- SVSTATE[0:6]
# set or get VL
if vs = 0                then VL <- SVSTATE[7:13]
else if _RA != 0         then
if (RA) >u 0b1111111 then
VL <- 0b1111111
overflow <- 0b1
else                      VL <- (RA)[57:63]
else if _RT = 0          then VL <- VLimm[0:6]
else if CTR >u 0b1111111 then
VL <- 0b1111111
overflow <- 0b1
else                          VL <- CTR[57:63]
# limit VL to within MVL
if VL >u MVL then
overflow <- 0b1
VL <- MVL
SVSTATE[0:6] <- MVL
SVSTATE[7:13] <- VL
if _RT != 0 then
GPR(_RT) <- [0]*57 || VL
# MAXVL is a static "state-reset" opportunity so VF is only set then.
if ms = 1 then
SVSTATE[63] <- vf   # set Vertical-First mode
SVSTATE[62] <- 0b0  # clear persist bit

Special Registers Altered:

CR0                     (if Rc=1)
SVSTATE
• SVi - bits 16-22 - an immediate operand for setting MVL and/or VL
• ms - bit 23 - allows for setting of MVL
• vs - bit 24 - allows for setting of VL
• vf - bit 25 - sets "Vertical First Mode".

Note that in immediate setting mode VL and MVL start from one but that this is compensated for in the assembly notation. i.e. that an immediate value of 1 in assembler notation actually places the value 0b0000000 in the SVi field bits: on execution the setvl instruction adds one to the decoded SVi field bits, resulting in VL/MVL being set to 1. In future this will allow VL to be set to values ranging from 1 to 128 with only 7 bits instead of 8. 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 SVSTATE SPR.

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

setvli   VL=8   : setvl  r0, r0, VL=8, vf=0, vs=1, ms=0
setvli.  VL=8   : setvl. r0, r0, VL=8, vf=0, vs=1, ms=0
setmvli  MVL=8  : setvl  r0, r0, MVL=8, vf=0, vs=0, ms=1
setmvli. MVL=8  : setvl. r0, r0, MVL=8, vf=0, vs=0, ms=1

Additional pseudo-op for obtaining VL without modifying it (or any state):

getvl  r5      : setvl  r5, r0, vf=0, vs=0, ms=0
getvl. r5      : setvl. r5, r0, vf=0, vs=0, ms=0

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. Doing so would require two instructions.

Use of setvl results in changes to the SVSTATE SPR. see sprs

Selecting sources for VL

There is considerable opcode pressure, consequently to set MVL and VL from different sources is as follows:

condition effect
vs=1, RA=0, RT!=0 VL,RT set to MIN(MVL, CTR)
vs=1, RA=0, RT=0 VL set to MIN(MVL, SVi+1)
vs=1, RA!=0, RT=0 VL set to MIN(MVL, RA)
vs=1, RA!=0, RT!=0 VL,RT set to MIN(MVL, RA)

The reasoning here is that the opportunity to set RT equal to the immediate SVi+1 is sacrificed in favour of setting from CTR.

Unusual Rc=1 behaviour

Normally, the return result from an instruction is in RT. With it being possible for RT=0 to mean that CTR mode is to be read, some different semantics are needed.

CR Field 0, when Rc=1, may be set even if RT=0. The reason is that overflow may occur: VL, if set either from an immediate or from CTR, may not exceed MAXVL, and if it is, CR0.SO must be set.

In reality it is VL being set. Therefore, rather than CR0 testing RT when Rc=1, CR0.EQ is set if VL=0, CR0.GE is set if VL is non-zero.

SUBVL

Sub-vector elements are not be considered "Vertical". The vec2/3/4 is to be considered as if the "single element". Caveats exist for mv.swizzle and mv.vec when Pack/Unpack is enabled, due to the order in which VL and SUBVL loops are applied being swapped (outer-inner becomes inner-outer)

## Examples

### Core concept loop

This example illustrates the Cray-style Loop concept. However where most Cray Vectors have a Max Vector Length hard-coded into the architecture, Simple-V allows MVL to be set, but only as a static immediate, so that compilers may embed the register resource allocation statically at compile-time.

loop:
setvl a3, a0, MVL=8    #  update a3 with vl
# (# of elements this iteration)
# set MVL to 8 and
# set a3=VL=MIN(a0,MVL)
# do vector operations at up to 8 length (MVL=8)
# ...
sub. a0, a0, a3   # Decrement count by vl, set CR0.eq
bnez a0, loop    # Any more?

### Loop using Rc=1

In this example, the setvl. instruction enabled Rc=1, which sets CR0.eq when VL becomes zero. Testing of r4 (cmpi) is thus redundant saving one instruction.

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