SV Context Propagation

Context Propagation is for a future version of SV. It requires one Major opcode in some cases.

The purpose of Context Propagation is a hardware compression algorithm for 64-bit prefix-suffix ISAs. The prefix is separated from the suffix and, on the reasonable assumption that the exact same prefix will need to be applied to multiple suffixes, a bit-level FIFO is given to indicate when a particular prefix shall be applied to future instructions.

In this way, with the suffixes being only 32 bit and multiple 32-bit instructions having the exact same prefix applied to them, the ISA is much more compact.

Put another way: svp64 context is 24 bits long, and Swizzle is 12. These are enormous and not sustainable as far as power consumption is concerned. Also, there is repetition of the same contexts to different instructions. An idea therefore is to add a level of indirection that allows these contexts to be applied to multiple instructions.

The basic principle is to have a suite of 40 indices in a shift register that indicate one of seven Contexts shall be applied to upcoming 32 bit v3.0B instructions. The Least Significant Index in the shift register is the one that is applied. One of those indices is 0b000 which indicates "no prefix applied". Effectively this is a bit-level FIFO.

A special instruction in an svp64 context takes a copy of the RM[0..23] bits, alongside a 21 bit suite that indicates up to 20 32 bit instructions will have that RM applied to them, as well as an index to associate with the RM. If there are already indices set within the shift register then the new entries are placed after the end of the highest-indexed one.

0.5 6.8 9.10 11.31 name
OP MMM ?-Form
OP idx 000 imm

Four different types of contexts are available so far: svp64 RM, setvl, Remap and swizzle. Their format is as follows when stored in SPRs:

0..3 4..7 8........31 name
0000 0000 RM[0:23] svp64 RM
0000 0001 setvl[0:23] setvl VL
0001 0 mask swiz1 swiz2 swizzle
0010 brev sh0-4 ms0-5 Remap
0011 brev sh0-4 ms0-4 SubVL Remap

There are 4 64 bit SPRs used for storing Context, and the data is stored as follows:

  • 7 32 bit contexts are stored, each indexed from 0b001 to 0b111, 2 per 64 bit SPR and 1 in the 4th.
  • Starting from bit 32 of the 4th SPR, in batches of 40 bits the Shift Registers (bit-level FIFOs) are stored.
             0            31 32         63
    SVCTX0   context 0       context 1
    SVCTX1   context 2       context 3
    SVCTX2   context 4       context 5
    SVCTX3   context 6       FIFO0[0..31]
    SVCTX4   FIFO0[32:39]   FIFO1[0:39] FIFO2[0:15]
    SVCTX5   FIFO2[16:39]   FIFO3[0:39] FIFO4[0:7]
    SVCTX5   FIFO4[8:39]    FIFO5[0:39] FIFO5[0:15]
    SVCTX6   FIFO5[16:39]   FIFO6[0:39] FIFO7[0:7
    SVCTX7   FIFO7[16:39]

When each LSB is nonzero in any one of the seven Shift Registers the corresponding Contexts are looked up and merged (ORed) together. Contexts for different purposes however may not be mixed: an illegal instruction is raised if this occurs.

The reason for merging the contexts is so that different aspects may be applied. For example some RM contexts may indicate that predication is to be applied to an instruction whilst another context may contain the svp64 Mode. Combining the two allows the predication aspect to be merged and shared, making for better packing.

These changes occur on a precise schedule: compilers should not have difficulties statically allocating the Context Propagation, as long as certain conventions are followed, such as avoidance of allowing the context to propagate through branches used by more than one incoming path, and variable-length loops.

Loops, clearly, because if the setup of the shift registers does not precisely match the number of instructions, the meaning of those instructions will change as the bits in the shift registers run out! However if the loops are of fixed static size, with no conditional early exit, and small enough (40 instructions maximum) then it is perfectly reasonable to insert repeated patterns into the shift registers, enough to cover all the loops. Ordinarily however the use of the Context Propagation instructions should be inside the loop and it is the responsibility of the compiler and assembler writer to ensure that the shift registers reach zero before any loop jump-back point.

Pseudocode:

The internal data structures need not precisely match the SPRs. Here are some internal datastructures:

bit sreg[7][40] # seven 40 bit shift registers
bit context[7][24]   # seven contexts
int sregoffs[7] # indicator where last bits were placed

The Context Propagation instruction then inserts bits into the selected stream:

count = 20-count_trailing_zeros(imm)
context[idx] = new_context
start = sregoffs[idx]
sreg[idx][start:start+count] = imm[0:count]
sregoffs[idx] += count

With each shift register being maintained independently the new bits are dropped in where the last ones end. To get which one is to be applied is as follows:

apply_context
for i in range(7):
    if sreg[i][0]:
        apply_context |= context[i]
    sreg[i] = sreg[i] >> 1
    sregoffs[i] -= 1

Note that it is the LSB that says which context is to be applied.

Swizzle Propagation

Swizzle Contexts follow the same schedule except that there is a mask for specifying to which registers the swizzle is to be applied, and there is only 17 bit suite to indicate the instructions to which the swizzle applies.

The bits in the svp64 RM field are interpreted as a pair of 12 bit swizzles

0.5 6.8 9.11 12.14 15.31 name
OP MMM mask ?-Form
OP idx 001 mask imm

Note however that it is only svp64 encoded instructions to which swizzle applies, so Swizzle Shift Registers only activate (and shift down) on svp64 instructions. This includes Context-propagated ones!

The mask is encoded as follows:

  • bit 0 indicates that src1 is swizzled
  • bit 1 indicates that src2 is swizzled
  • bit 2 indicates that src3 is swizzled

When the compiler creates Swizzle Contexts it is important to recall that the Contexts will be ORed together. Thus one Context may specify a mask whilst the other Context specifies the swizzles: ORing different mask contexts with different swizzle Contexts allows more combinations than would normally fit into seven Contexts.

More than one bit is permitted to be set in the mask: swiz1 is applied to the first src operand specified by the mask, and swiz2 is applied to the second.

2D/3D Matrix Remap

remap allows up to four Vectors (all four arguments of fma for example) to be algorithmically arbitrarily remapped via 1D, 2D or 3D reshaping. The amount of information needed to do so is however quite large: consequently it is only practical to apply indirectly, via Context propagation.

Vectors may be remapped such that Matrix multiply of any arbitrary size is performed in one Vectorised fma instruction as long as the total number of elements is less than 64 (maximum for VL).

Additionally, in a fashion known as "Structure Packing" in NEON and RVV, it may be used to perform "zipping" and "unzipping" of elements in a regular fashion of any arbitrary size and depth: RGB or Audio channel data may be split into separate contiguous lanes of registers, for example.

There are four possible Shapes. Unlike swizzle contexts this one requires he external remap Shape SPRs because the state information is too large to fit into the Context itself. Thus the Remap Context says which Shapes apply to which registers.

The instruction format is the same as RM and thus uses 21 bits of immediate, 29 of which are dropped into the indexed Shift Register

0.5 6.8 9.10 11.14 15.31 name
OP MM ?-Form
OP idx 10 brev imm Remap
OP idx 11 brev imm SUBVL Remap

SUBVL Remap applies the remapping even into the SUBVL Elements, for a total of VL\*SUBVL Elements. swizzle may be applied on top as a second phase after SUBVL Remap.

brev field, which also applied down to SUBVL elements (not to the whole vec2/3/4, that would be handled by swizzle reordering):

  • bit 0 indicates that dest elements are byte-reversed
  • bit 1 indicates that src1 elements are byte-reversed
  • bit 2 indicates that src2 elements are byte-reversed
  • bit 3 indicates that src3 elements are byte-reversed

Again it is the 24 bit RM that is interpreted differently:

0 2 4 6 8 10.14 15..23
mi0 mi1 mi2 mo0 mo1 en0-4 rsv

si0-2 and so0-1 each select SVSHAPE0-3 to apply to a given register. si0-2 apply to RA, RB, RC respectively, as input registers, and likewise so0-1 apply to output registers. en0-4 indicate whether the SVSHAPE is actively applied or not.

setvl

Fitting into 22 bits with 2 reserved and 2 for future expansion of SV Vector Length is a total of 24 bits which is exactly the same size as SVP64 RM

0.5 6.10 11..18 19..20 21 22.23
RT RA SVi // vs ms Rc rsvd