V-Extension to Simple-V Comparative Analysis

This section covers the ways in which Simple-V is comparable to, or more flexible than, V-Extension (V2.3-draft). Also covered is one major weak-point (register files are fixed size, where V is arbitrary length), and how best to deal with that, should V be adapted to be on top of Simple-V.

The first stages of this section go over each of the sections of V2.3-draft V where appropriate

17.3 Shape Encoding

Simple-V's proposed means of expressing whether a register (from the standard integer or the standard floating-point file) is a scalar or a vector is to simply set the vector length to 1. The instruction would however have to specify which register file (integer or FP) that the vector-length was to be applied to.

Extended shapes (2-D etc) would not be part of Simple-V at all.

17.4 Representation Encoding

Simple-V would not have representation-encoding. This is part of polymorphism, which is considered too complex to implement (TODO: confirm?)

17.5 Element Bitwidth

This is directly equivalent to Simple-V's "Packed", and implies that integer (or floating-point) are divided down into vector-indexable chunks of size Bitwidth.

In this way it becomes possible to have ADD effectively and implicitly turn into ADDb (8-bit add), ADDw (16-bit add) and so on, and where vector-length has been set to greater than 1, it becomes a "Packed" (SIMD) instruction.

It remains to be decided what should be done when RV32 / RV64 ADD (sized) opcodes are used. One useful idea would be, on an RV64 system where a 32-bit-sized ADD was performed, to simply use the least significant 32-bits of the register (exactly as is currently done) but at the same time to respect the packed bitwidth as well.

The extended encoding (Table 17.6) would not be part of Simple-V.

17.6 Base Vector Extension Supported Types

TODO: analyse. probably exactly the same.

17.7 Maximum Vector Element Width

No equivalent in Simple-V

17.8 Vector Configuration Registers

TODO: analyse.

17.9 Legal Vector Unit Configurations

TODO: analyse

17.10 Vector Unit CSRs

TODO: analyse

Ok so this is an aspect of Simple-V that I hadn't thought through, yet (proposal / idea only a few days old!).  in V2.3-Draft ISA Section 17.10 the CSRs are listed.  I note that there's some general-purpose CSRs (including a global/active vector-length) and 16 vcfgN CSRs.  i don't precisely know what those are for.

 In the Simple-V proposal, every register in both the integer register-file and the floating-point register-file would have at least a 2-bit "data-width" CSR and probably something like an 8-bit "vector-length" CSR (less in RV32E, by exactly one bit).

 What I don't know is whether that would be considered perfectly reasonable or completely insane.  If it turns out that the proposed Simple-V CSRs can indeed be stored in SRAM then I would imagine that adding somewhere in the region of 10 bits per register would be... okay?  I really don't honestly know.

 Would these proposed 10-or-so-bit per-register Simple-V CSRs need to be multi-ported? No I don't believe they would.

17.11 Maximum Vector Length (MVL)

Basically implicitly this is set to the maximum size of the register file multiplied by the number of 8-bit packed ints that can fit into a register (4 for RV32, 8 for RV64 and 16 for RV128).

!7.12 Vector Instruction Formats

No equivalent in Simple-V because all instructions of all Extensions are implicitly parallelised (and packed).

17.13 Polymorphic Vector Instructions

Polymorphism (implicit type-casting) is deliberately not supported in Simple-V.

17.14 Rapid Configuration Instructions

TODO: analyse if this is useful to have an equivalent in Simple-V

17.15 Vector-Type-Change Instructions

TODO: analyse if this is useful to have an equivalent in Simple-V

17.16 Vector Length

Has a direct corresponding equivalent.

17.17 Predicated Execution

Predicated Execution is another name for "masking" or "tagging". Masked (or tagged) implies that there is a bit field which is indexed, and each bit associated with the corresponding indexed offset register within the "Vector". If the tag / mask bit is 1, when a parallel operation is issued, the indexed element of the vector has the operation carried out. However if the tag / mask bit is zero, that particular indexed element of the vector does not have the requested operation carried out.

In V2.3-draft V, there is a significant (not recommended) difference: the zero-tagged elements are set to zero. This loses a significant advantage of mask / tagging, particularly if the entire mask register is itself a general-purpose register, as that general-purpose register can be inverted, shifted, and'ed, or'ed and so on. In other words it becomes possible, especially if Carry/Overflow from each vector operation is also accessible, to do conditional (step-by-step) vector operations including things like turn vectors into 1024-bit or greater operands with very few instructions, by treating the "carry" from one instruction as a way to do "Conditional add of 1 to the register next door". If V2.3-draft V sets zero-tagged elements to zero, such extremely powerful techniques are simply not possible.

It is noted that there is no mention of an equivalent to BEXT (element skipping) which would be particularly fascinating and powerful to have. In this mode, the "mask" would skip elements where its mask bit was zero in either the source or the destination operand.

Lots to be discussed.

17.18 Vector Load/Store Instructions

The Vector Load/Store instructions as proposed in V are extremely powerful and can be used for reordering and regular restructuring.

Vector Load:

if (unit-strided) stride = elsize;
else stride = areg[as2]; // constant-strided
for (int i=0; i<vl; ++i)
  if ([!]preg[p][i])
    for (int j=0; j<seglen+1; j++)
      vreg[vd+j][i] = mem[areg[as1] + (i*(seglen+1)+j)*stride];

Store:

if (unit-strided) stride = elsize;
else stride = areg[as2]; // constant-strided
for (int i=0; i<vl; ++i)
  if ([!]preg[p][i])
    for (int j=0; j<seglen+1; j++)
      mem[areg[base] + (i*(seglen+1)+j)*stride] = vreg[vd+j][i];

Indexed Load:

for (int i=0; i<vl; ++i)
  if ([!]preg[p][i])
    for (int j=0; j<seglen+1; j++)
      vreg[vd+j][i] = mem[sreg[base] + vreg[vs2][i] + j*elsize];

Indexed Store:

for (int i=0; i<vl; ++i)
if ([!]preg[p][i])
  for (int j=0; j<seglen+1; j++)
    mem[sreg[base] + vreg[vs2][i] + j*elsize] = vreg[vd+j][i];

Keeping these instructions as-is for Simple-V is highly recommended. However: one of the goals of this Extension is to retro-fit (re-use) existing RV Load/Store:

   imm[11:0]       

 rs1    

   rd       

opcode 

        12         

  5     

3     

    5       

  7 

   offset[11:0]    

base    

  dest      

LOAD 

rs1    

opcode 

  7        

5     

 5     

3     

   5       

  7 

The RV32 instruction opcodes as follows:

rs1 

rs1 

rs1 

rs1 

rs1 

rs1 

Conversion on LOAD as follows:

• rd or rs1 are CSR-vectorised indicating "Vector Mode"
• rd equivalent to vd
• rs1 equivalent to rs1
• imm[4:0] from RV format (11..7]) is same
• imm[9:5] from RV format (29..25] is rs2 (rs2=00000 for VLD)
• imm[11:10] from RV format (31..30] is opcode (VLD, VLDS, VLDX)
• width from RV format (14..12) is same (width and zero/sign extend)

Similar conversion on STORE as follows:

Notes:

• Predication CSR-marking register is not explicitly shown in instruction
• In both LOAD and STORE, it is possible now to rs2 (or rs3) as a vector.
• That in turn means that Indexed Load need not have an explicit opcode
• That in turn means that bit 30 may indicate "stride" and bit 31 is free

Revised LOAD:

Where in turn the pseudo-code may now combine the two:

if (unit-strided) stride = elsize;
else stride = areg[as2]; // constant-strided
for (int i=0; i<vl; ++i)
  if ([!]preg[p][i])
    for (int j=0; j<seglen+1; j++)
    {
      if CSRvectorised[rs2])
         offs = vreg[rs2][i]
      else
         offs = i*(seglen+1)*stride;
      vreg[vd+j][i] = mem[sreg[base] + offs + j*stride];
    }

Notes:

• j is multiplied by stride, not elsize, including in the rs2 vectorised case.
• There may be more sophisticated variants involving the 31st bit, however it would be nice to reserve that bit for post-increment of address registers *

17.19 Vector Register Gather

TODO

TODO, sort

However, there are also several features that go beyond simply attaching VL to a scalar operation and are crucial to being able to vectorize a lot of code. To name a few: - Conditional execution (i.e., predicated operations) - Inter-lane data movement (e.g. SLIDE, SELECT) - Reductions (e.g., VADD with a scalar destination)

Ok so the Conditional and also the Reductions is one of the reasons why as part of SimpleV / variable-SIMD / parallelism (gah gotta think of a decent name) i proposed that it be implemented as "if you say r0 is to be a vector / SIMD that means operations actually take place on r0,r1,r2... r(N-1)".

Consequently any parallel operation could be paused (or... more specifically: vectors disabled by resetting it back to a default / scalar / vector-length=1) yet the results would actually be in the main register file (integer or float) and so anything that wasn't possible to easily do in "simple" parallel terms could be done out of parallel "mode" instead.

I do appreciate that the above does imply that there is a limit to the length that SimpleV (whatever) can be parallelised, namely that you run out of registers! my thought there was, "leave space for the main V-Ext proposal to extend it to the length that V currently supports". Honestly i had not thought through precisely how that would work.

Inter-lane (SELECT) i saw 17.19 in V2.3-Draft p117, I liked that, it reminds me of the discussion with Clifford on bit-manipulation (gather-scatter except not Bit Gather Scatter, data gather scatter): if applied "globally and outside of V and P" SLIDE and SELECT might become an extremely powerful way to do fast memory copy and reordering [2[.

However I haven't quite got my head round how that would work: i am used to the concept of register "tags" (the modern term is "masks") and i think if "masks" were applied to a Simple-V-enhanced LOAD / STORE you would get the exact same thing as SELECT.

SLIDE you could do simply by setting say r0 vector-length to say 16 (meaning that if referred to in any operation it would be an implicit parallel operation on all registers r0 through r15), and temporarily set say.... r7 vector-length to say... 5. Do a LOAD on r7 and it would implicitly mean "load from memory into r7 through r11". Then you go back and do an operation on r0 and ta-daa, you're actually doing an operation on a SLID {SLIDED?) vector.

The advantage of Simple-V (whatever) over V would be that you could actually do operations in the middle of vectors (not just SLIDEs) simply by (as above) setting r0 vector-length to 16 and r7 vector-length to 5. There would be nothing preventing you from doing an ADD on r0 (which meant do an ADD on r0 through r15) followed immediately in the next instruction with no setup cost a MUL on r7 (which actually meant "do a parallel MUL on r7 through r11").

btw it's worth mentioning that you'd get scalar-vector and vector-scalar implicitly by having one of the source register be vector-length 1 (the default) and one being N > 1. but without having special opcodes to do it. i believe (or more like "logically infer or deduce" as i haven't got access to the spec) that that would result in a further opcode reduction when comparing [draft] V-Ext to [proposed] Simple-V.

Also, Reduction might be possible by specifying that the destination be a scalar (vector-length=1) whilst the source be a vector. However... it would be an awful lot of work to go through every single instruction in every Extension, working out which ones could be parallelised (ADD, MUL, XOR) and those that definitely could not (DIV, SUB). Is that worth the effort? maybe. Would it result in huge complexity? probably. Could an implementor just go "I ain't doing that as parallel! let's make it virtual-parallelism (sequential reduction) instead"? absolutely. So, now that I think it through, Simple-V (whatever) covers Reduction as well. huh, that's a surprise.

• Vector-length speculation (making it possible to vectorize some loops with unknown trip count) - I don't think this part of the proposal is written down yet.

Now that is an interesting concept. A little scary, i imagine, with the possibility of putting a processor into a hard infinite execution loop...

Also, note the vector ISA consumes relatively little opcode space (all the arithmetic fits in 7/8ths of a major opcode). This is mainly because data type and size is a function of runtime configuration, rather than of opcode.

yes. i love that aspect of V, i am a huge fan of polymorphism [1] which is why i am keen to advocate that the same runtime principle be extended to the rest of the RISC-V ISA [3]

Yikes that's a lot. I'm going to need to pull this into the wiki to make sure it's not lost.

[1] inherent data type conversion: 25 years ago i designed a hypothetical hyper-hyper-hyper-escape-code-sequencing ISA based around 2-bit (escape-extended) opcodes and 2-bit (escape-extended) operands that only required a fixed 8-bit instruction length. that relied heavily on polymorphism and runtime size configurations as well. At the time I thought it would have meant one HELL of a lot of CSRs... but then I met RISC-V and was cured instantly of that delusion^{Wmisapprehension}

[2] Interestingly if you then also add in the other aspect of Simple-V (the data-size, which is effectively functionally orthogonal / identical to "Packed" of Packed-SIMD), masked and packed and vectored LOAD / STORE operations become byte / half-word / word augmenters of B-Ext's proposed "BGS" i.e. where B-Ext's BGS dealt with bits, masked-packed-vectored LOAD / STORE would deal with 8 / 16 / 32 bits at a time. Where it would get really REALLY interesting would be masked-packed-vectored B-Ext BGS instructions. I can't even get my head fully round that, which is a good sign that the combination would be really powerful

[3] ok sadly maybe not the polymorphism, it's too complicated and I think would be much too hard for implementors to easily "slide in" to an existing non-Simple-V implementation.  i say that despite really really wanting IEEE 704 FP Half-precision to end up somewhere in RISC-V in some fashion, for optimising 3D Graphics.  sigh.

TODO: analyse, auto-increment on unit-stride and constant-stride

so i thought about that for a day or so, and wondered if it would be possible to propose a variant of zero-overhead loop that included auto-incrementing the two address registers a2 and a3, as well as providing a means to interact between the zero-overhead loop and the vsetvl instruction. a sort-of pseudo-assembly of that would look like:

# a2 to be auto-incremented by t0 times 4
zero-overhead-set-auto-increment a2, t0, 4
# a2 to be auto-incremented by t0 times 4
zero-overhead-set-auto-increment a3, t0, 4
zero-overhead-set-loop-terminator-condition a0 zero
zero-overhead-set-start-end stripmine, stripmine+endoffset
stripmine:
vsetvl t0,a0
vlw v0, a2
vlw v1, a3
vfma v1, a1, v0, v1
vsw v1, a3
sub a0, a0, t0
stripmine+endoffset:

the question is: would something like this even be desirable? it's a variant of auto-increment [1]. last time i saw any hint of auto-increment register opcodes was in the 1980s... 68000 if i recall correctly... yep see [1]

[1] http://fourier.eng.hmc.edu/e85_old/lectures/instruction/node6.html

Reply:

Another option for auto-increment is for vector-memory-access instructions to support post-increment addressing for unit-stride and constant-stride modes. This can be implemented by the scalar unit passing the operation to the vector unit while itself executing an appropriate multiply-and-add to produce the incremented address. This does not require additional ports on the scalar register file, unlike scalar post-increment addressing modes.

TODO: instructions V-Ext duplication analysis

TODO: sort

I suspect that the "hardware loop" in question is actually a zero-overhead loop unit that diverts execution from address X to address Y if a certain condition is met.

 not quite.  The zero-overhead loop unit interestingly would be at an [independent] level above vector-length.  The distinctions are as follows:

• Vector-length issues virtual instructions where the register operands are specifically altered (to cover a range of registers), whereas zero-overhead loops specifically do NOT alter the operands in ANY way.

• Vector-length-driven "virtual" instructions are driven by one and only one instruction (whether it be a LOAD, STORE, or pure one/two/three-operand opcode) whereas zero-overhead loop units specifically apply to multiple instructions.

Where vector-length-driven "virtual" instructions might get conceptually blurred with zero-overhead loops is LOAD / STORE.  In the case of LOAD / STORE, to actually be useful, vector-length-driven LOAD / STORE should increment the LOAD / STORE memory address to correspondingly match the increment in the register bank.  example:

• set vector-length for r0 to 4
• issue RV32 LOAD from addr 0x1230 to r0

translates effectively to:

• RV32 LOAD from addr 0x1230 to r0
• ...
• ...
• RV32 LOAD from addr 0x123B to r3