# Variable-width Variable-packed SIMD / Simple-V / Parallelism Extension Proposal
-* TODO 23may2018: CSR-CAM-ify regfile tables
-* TODO 23may2018: zero-mark predication CSR
-* TODO 28may2018: sort out VSETVL: CSR length to be removed?
-* TODO 09jun2018: Chennai Presentation more up-to-date
-* TODO 09jun2019: elwidth only 4 values (dflt, dflt/2, 8, 16)
-* TODO 09jun2019: extra register banks (future option)
-* TODO 09jun2019: new Reg CSR table (incl. packed=Y/N)
-
-
Key insight: Simple-V is intended as an abstraction layer to provide
a consistent "API" to parallelisation of existing *and future* operations.
*Actual* internal hardware-level parallelism is *not* required, such
*Actual* parallelism, if added independently of Simple-V in the form
of Out-of-order restructuring (including parallel ALU lanes) or VLIW
-implementations, or SIMD, or anything else, would then benefit *if*
-Simple-V was added on top.
+implementations, or SIMD, or anything else, would then benefit from
+the uniformity of a consistent API.
[[!toc ]]
Compressed Branch instructions are likewise re-interpreted as predicated
2-register operations, with the result going into rs3. All the bits of
-the immediatee are re-interpreted for different purposes, to extend the
+the immediate are re-interpreted for different purposes, to extend the
number of comparator operations to beyond the original specification,
but also to cater for floating-point comparisons as well as integer ones.
in advance, accordingly: other strategies are explored in the Appendix
Section "Virtual Memory Page Faults".
+## Vectorised Copy/Move (and conversion) instructions
+
+There is a series of 2-operand instructions involving copying (and
+alteration): C.MV, FMV, FNEG, FABS, FCVT, FSGNJ. These operations all
+follow the same pattern, as it is *both* the source *and* destination
+predication masks that are taken into account. This is different from
+the three-operand arithmetic instructions, where the predication mask
+is taken from the *destination* register, and applied uniformly to the
+elements of the source register(s), element-for-element.
+
+### C.MV Instruction <a name="c_mv"></a>
+
+There is no MV instruction in RV however there is a C.MV instruction.
+It is used for copying integer-to-integer registers (vectorised FMV
+is used for copying floating-point).
+
+If either the source or the destination register are marked as vectors
+C.MV is reinterpreted to be a vectorised (multi-register) predicated
+move operation. The actual instruction's format does not change:
+
+[[!table data="""
+15 12 | 11 7 | 6 2 | 1 0 |
+funct4 | rd | rs | op |
+4 | 5 | 5 | 2 |
+C.MV | dest | src | C0 |
+"""]]
+
+A simplified version of the pseudocode for this operation is as follows:
+
+ function op_mv(rd, rs) # MV not VMV!
+ rd = int_vec[rd].isvector ? int_vec[rd].regidx : rd;
+ rs = int_vec[rs].isvector ? int_vec[rs].regidx : rs;
+ ps = get_pred_val(FALSE, rs); # predication on src
+ pd = get_pred_val(FALSE, rd); # ... AND on dest
+ for (int i = 0, int j = 0; i < VL && j < VL;):
+ if (int_vec[rs].isvec) while (!(ps & 1<<i)) i++;
+ if (int_vec[rd].isvec) while (!(pd & 1<<j)) j++;
+ ireg[rd+j] <= ireg[rs+i];
+ if (int_vec[rs].isvec) i++;
+ if (int_vec[rd].isvec) j++;
+
+Note that:
+
+* elwidth (SIMD) is not covered above
+* ending the loop early in scalar cases (VINSERT, VEXTRACT) is also
+ not covered
+
+There are several different instructions from RVV that are covered by
+this one opcode:
+
+[[!table data="""
+src | dest | predication | op |
+scalar | vector | none | VSPLAT |
+scalar | vector | destination | sparse VSPLAT |
+scalar | vector | 1-bit dest | VINSERT |
+vector | scalar | 1-bit? src | VEXTRACT |
+vector | vector | none | VCOPY |
+vector | vector | src | Vector Gather |
+vector | vector | dest | Vector Scatter |
+vector | vector | src & dest | Gather/Scatter |
+vector | vector | src == dest | sparse VCOPY |
+"""]]
+
+Also, VMERGE may be implemented as back-to-back (macro-op fused) C.MV
+operations with inversion on the src and dest predication for one of the
+two C.MV operations.
+
+Note that in the instance where the Compressed Extension is not implemented,
+MV may be used, but that is a pseudo-operation mapping to addi rd, x0, rs.
+Note that the behaviour is **different** from C.MV because with addi the
+predication mask to use is taken **only** from rd and is applied against
+all elements: rs[i] = rd[i].
+
+### FMV, FNEG and FABS Instructions
+
+These are identical in form to C.MV, except covering floating-point
+register copying. The same double-predication rules also apply.
+However when elwidth is not set to default the instruction is implicitly
+and automatic converted to a (vectorised) floating-point type conversion
+operation of the appropriate size covering the source and destination
+register bitwidths.
+
+(Note that FMV, FNEG and FABS are all actually pseudo-instructions)
+
+### FVCT Instructions
+
+These are again identical in form to C.MV, except that they cover
+floating-point to integer and integer to floating-point. When element
+width in each vector is set to default, the instructions behave exactly
+as they are defined for standard RV (scalar) operations, except vectorised
+in exactly the same fashion as outlined in C.MV.
+
+However when the source or destination element width is not set to default,
+the opcode's explicit element widths are *over-ridden* to new definitions,
+and the opcode's element width is taken as indicative of the SIMD width
+(if applicable i.e. if packed SIMD is requested) instead.
+
+For example FCVT.S.L would normally be used to convert a 64-bit
+integer in register rs1 to a 64-bit floating-point number in rd.
+If however the source rs1 is set to be a vector, where elwidth is set to
+default/2 and "packed SIMD" is enabled, then the first 32 bits of
+rs1 are converted to a floating-point number to be stored in rd's
+first element and the higher 32-bits *also* converted to floating-point
+and stored in the second. The 32 bit size comes from the fact that
+FCVT.S.L's integer width is 64 bit, and with elwidth on rs1 set to
+divide that by two it means that rs1 element width is to be taken as 32.
+
+Similar rules apply to the destination register.
+
# Exceptions
> What does an ADD of two different-sized vectors do in simple-V?
* Throw an exception. Whether that actually results in spawning threads
as part of the trap-handling remains to be seen.
+# Under consideration <a name="issues"></a>
+
+From the Chennai 2018 slides the following issues were raised.
+Efforts to analyse and answer these questions are below.
+
+* Should future extra bank be included now?
+* How many Register and Predication CSRs should there be?
+ (and how many in RV32E)
+* How many in M-Mode (for doing context-switch)?
+* Should use of registers be allowed to "wrap" (x30 x31 x1 x2)?
+* Can CLIP be done as a CSR (mode, like elwidth)
+* SIMD saturation (etc.) also set as a mode?
+* Include src1/src2 predication on Comparison Ops?
+ (same arrangement as C.MV, with same flexibility/power)
+* 8/16-bit ops is it worthwhile adding a "start offset"?
+ (a bit like misaligned addressing... for registers)
+ or just use predication to skip start?
+
+## Future (extra) bank be included (made mandatory)
+
+The implications of expanding the *standard* register file from
+32 entries per bank to 64 per bank is quite an extensive architectural
+change. Also it has implications for context-switching.
+
+Therefore, on balance, it is not recommended and certainly should
+not be made a *mandatory* requirement for the use of SV. SV's design
+ethos is to be minimally-disruptive for implementors to shoe-horn
+into an existing design.
+
+## How large should the Register and Predication CSR key-value stores be?
+
+This is something that definitely needs actual evaluation and for
+code to be run and the results analysed. At the time of writing
+(12jul2018) that is too early to tell. An approximate best-guess
+however would be 16 entries.
+
+RV32E however is a special case, given that it is highly unlikely
+(but not outside the realm of possibility) that it would be used
+for performance reasons but instead for reducing instruction count.
+The number of CSR entries therefore has to be considered extremely
+carefully.
+
+## How many CSR entries in M-Mode or S-Mode (for context-switching)?
+
+The minimum required CSR entries would be 1 for each register-bank:
+one for integer and one for floating-point. However, as shown
+in the "Context Switch Example" section, for optimal efficiency
+(minimal instructions in a low-latency situation) the CSRs for
+the context-switch should be set up *and left alone*.
+
+This means that it is not really a good idea to touch the CSRs
+used for context-switching in the M-Mode (or S-Mode) trap, so
+if there is ever demonstrated a need for vectors then there would
+need to be *at least* one more free. However just one does not make
+much sense (as it one only covers scalar-vector ops) so it is more
+likely that at least two extra would be needed.
+
+This *in addition* - in the RV32E case - if an RV32E implementation
+happens also to support U/S/M modes. This would be considered quite
+rare but not outside of the realm of possibility.
+
+Conclusion: all needs careful analysis and future work.
+
+## Should use of registers be allowed to "wrap" (x30 x31 x1 x2)?
+
+On balance it's a neat idea however it does seem to be one where the
+benefits are not really clear. It would however obviate the need for
+an exception to be raised if the VL runs out of registers to put
+things in (gets to x31, tries a non-existent x32 and fails), however
+the "fly in the ointment" is that x0 is hard-coded to "zero". The
+increment therefore would need to be double-stepped to skip over x0.
+Some microarchitectures could run into difficulties (SIMD-like ones
+in particular) so it needs a lot more thought.
+
+## Can CLIP be done as a CSR (mode, like elwidth)
+
+RVV appears to be going this way. At the time of writing (12jun2018)
+it's noted that in V2.3-Draft V0.4 RVV Chapter, RVV intends to do
+clip by way of exactly this method: setting a "clip mode" in a CSR.
+
+No details are given however the most sensible thing to have would be
+to extend the 16-bit Register CSR table to 24-bit (or 32-bit) and have
+extra bits specifying the type of clipping to be carried out, on
+a per-register basis. Other bits may be used for other purposes
+(see SIMD saturation below)
+
+## SIMD saturation (etc.) also set as a mode?
+
+Similar to "CLIP" as an extension to the CSR key-value store, "saturate"
+may also need extra details (what the saturation maximum is for example).
+
+## Include src1/src2 predication on Comparison Ops?
+
+In the C.MV (and other ops - see "C.MV Instruction"), the decision
+was taken, unlike in ADD (etc.) which are 3-operand ops, to use
+*both* the src *and* dest predication masks to give an extremely
+powerful and flexible instruction that covers a huge number of
+"traditional" vector opcodes.
+
+The natural question therefore to ask is: where else could this
+flexibility be deployed? What about comparison operations?
+
+Unfortunately, C.MV is basically "regs[dest] = regs[src]" whilst
+predicated comparison operations are actually a *three* operand
+instruction:
+
+ regs[pred] |= 1<< (cmp(regs[src1], regs[src2]) ? 1 : 0)
+
+Therefore at first glance it does not make sense to use src1 and src2
+predication masks, as it breaks the rule of 3-operand instructions
+to use the *destination* predication register.
+
+In this case however, the destination *is* a predication register
+as opposed to being a predication mask that is applied *to* the
+(vectorised) operation, element-at-a-time on src1 and src2.
+
+Thus the question is directly inter-related to whether the modification
+of the predication mask should *itself* be predicated.
+
+It is quite complex, in other words, and needs careful consideration.
+
+## 8/16-bit ops is it worthwhile adding a "start offset"?
+
+The idea here is to make it possible, particularly in a "Packed SIMD"
+case, to be able to avoid doing unaligned Load/Store operations
+by specifying that operations, instead of being carried out
+element-for-element, are offset by a fixed amount *even* in 8 and 16-bit
+element Packed SIMD cases.
+
+For example rather than take 2 32-bit registers divided into 4 8-bit
+elements and have them ADDed element-for-element as follows:
+
+ r3[0] = add r4[0], r6[0]
+ r3[1] = add r4[1], r6[1]
+ r3[2] = add r4[2], r6[2]
+ r3[3] = add r4[3], r6[3]
+
+an offset of 1 would result in four operations as follows, instead:
+
+ r3[0] = add r4[1], r6[0]
+ r3[1] = add r4[2], r6[1]
+ r3[2] = add r4[3], r6[2]
+ r3[3] = add r5[0], r6[3]
+
+In non-packed-SIMD mode there is no benefit at all, as a vector may
+be created using a different CSR that has the offset built-in. So this
+leaves just the packed-SIMD case to consider.
+
+Two ways in which this could be implemented / emulated (without special
+hardware):
+
+* bit-manipulation that shuffles the data along by one byte (or one word)
+ either prior to or as part of the operation requiring the offset.
+* just use an unaligned Load/Store sequence, even if there are performance
+ penalties for doing so.
+
+The question then is whether the performance hit is worth the extra hardware
+involving byte-shuffling/shifting the data by an arbitrary offset. On
+balance given that there are two reasonable instruction-based options, the
+hardware-offset option should be left out for the initial version of SV,
+with the option to consider it in an "advanced" version of the specification.
+
# Impementing V on top of Simple-V
With Simple-V converting the original RVV draft concept-for-concept
projects / libreriscv.git / blobdiff