[[!tag standards]] # New instructions for CR/INT predication **DRAFT STATUS** See: * main bugreport for crweirds * * * Rationale: Condition Registers are conceptually perfect for use as predicate masks, the only problem being that typical Vector ISAs have quite comprehensive mask-based instructions: set-before-first, popcount and much more. In fact many Vector ISAs can use Vectors *as* masks, consequently the entire Vector ISA is usually available for use in creating masks (one exception being AVX512 which has a dedicated Mask regfile and opcodes). Duplication of such operations (popcount etc) is not practical for SV given the strategy of leveraging pre-existing Scalar instructions in a minimalist way. With the scalar OpenPOWER v3.0B ISA having already popcnt, cntlz and others normally seen in Vector Mask operations it makes sense to allow *both* scalar integers *and* CR-Vectors to be predicate masks. That in turn means that much more comprehensive interaction between CRs and scalar Integers is required, because with the CR Predication Modes designating CR *Fields* (not CR bits) as Predicate Elements, fast transfers between CR *Fields* and the Integer Register File is needed. The opportunity is therefore taken to also augment CR logical arithmetic as well, using a mask-based paradigm that takes into consideration multiple bits of each CR Field (eq/lt/gt/ov). By contrast v3.0B Scalar CR instructions (crand, crxor) only allow a single bit calculation, and both mtcr and mfcr are CR-orientated rather than CR *Field* orientated. Also strangely there is no v3.0 instruction for directly moving CR Fields, only CR *bits*, so that is corrected here with `mcrfm`. The opportunity is taken to allow inversion of CR Field bits, when copied. Basic concept: * CR-based instructions that perform simple AND/OR from any four bits of a CR field to create a single bit value (0/1) in an integer register * Inverse of the same, taking a single bit value (0/1) from an integer register to selectively target any four bits of a given CR Field * CR-to-CR version of the same, allowing multiple bits to be AND/OR/XORed in one hit. * Optional Vectorisation of the same when SVP64 is implemented Purpose: * To provide a merged version of what is currently a multi-sequence of CR operations (crand, cror, crxor) with mfcr and mtcrf, reducing instruction count. * To provide a vectorised version of the same, suitable for advanced predication Useful side-effects: * mtcrweird when RA=0 is a means to set or clear multiple arbitrary CR Field bits simultaneously, using immediates embedded within the instruction. * With SVP64 on the weird instructions there is bit-for-bit interaction between GPR predicate masks (r3, r10, r31) and the source or destination GPR, in ways that are not possible with other SVP64 instructions because normal SVP64 is bit-per-element. On these weird instructions the element in effect *is* a bit. * `mfcrweird` mitigates a need to add `conflictd`, part of [[sv/vector_ops]], as well as allowing more complex comparisons. # Bit ordering. Please see [[svp64/appendix]] regarding CR bit ordering and for the definition of `CR{n}` # Instruction form and pseudocode **DRAFT** Instruction format (use of MAJOR 19 not approved by OPF ISA WG): |0-5|6-10 |11|12-15|16-18|19-20|21-25 |26-30 |31|name | |---|---- |--|-----|-----|-----|----- |----- |--|---- | |19 |RT | |fmsk |BFA | |XO[0:4]|XO[5:9]|/ | | |19 | | | | | |1 //// |00011 | |rsvd | |19 |RT |M |fmsk |BFA | 0 0 |0 fmap |00011 |Rc|crrweird | |19 |RT |M |fmsk |BFA | 0 1 |0 fmap |00011 |Rc|mfcrweird | |19 |RA |M |fmsk |BF | 1 0 |0 fmap |00011 |0 |mtcrrweird | |19 |RA |M |fmsk |BF | 1 0 |0 fmap |00011 |1 |mtcrweird | |19 |BT |M |fmsk |BFA | 1 1 |0 fmap |00011 |0 |crweirder | |19 |BF //|M |fmsk |BFA | 1 1 |0 fmap |00011 |1 |mcrfm | **crrweird** fmap is encoded in XO and is 4 bits crrweird: RT,BFA,M,fmsk,fmap creg = CR{BFA} n0 = fmsk[0] & (fmap[0] == creg[0]) n1 = fmsk[1] & (fmap[1] == creg[1]) n2 = fmsk[2] & (fmap[2] == creg[2]) n3 = fmsk[3] & (fmap[3] == creg[3]) n = (n0||n1||n2||n3) & fmsk result = (n != 0) if M else (n == fmsk) RT[63] = result # MSB0 numbering, 63 is LSB If Rc: CR0 = analyse(RT) When used with SVP64 Prefixing this is a [[sv/normal]] SVP64 type operation and as such can use Rc=1 and RC1 Data-dependent Mode capability Also as noted below, element-width override bits normally used on the source is instead used to allow multiple results to be packed sequentially into the destination. *Destination elwidth overrides still apply*. **mfcrrweird** fmap is encoded in XO and is 4 bits mfcrrweird: RT,BFA,fmsk,fmap creg = CR{BFA} n0 = fmsk[0] & (fmap[0] == creg[0]) n1 = fmsk[1] & (fmap[1] == creg[1]) n2 = fmsk[2] & (fmap[2] == creg[2]) n3 = fmsk[3] & (fmap[3] == creg[3]) result = n0||n1||n2||n3 RT[60:63] = result # MSB0 numbering, 63 is LSB If Rc: CR0 = analyse(RT) When used with SVP64 Prefixing this is a [[sv/normal]] SVP64 type operation and as such can use Rc=1 and RC1 Data-dependent Mode capability. Also as noted below, element-width override bits normally used on the source is instead used to allow multiple results to be packed into the destination. *Destination elwidth overrides still apply* **mtcrrweird** fmap is encoded in XO and is 4 bits mtcrrweird: BF,RA,M,fmsk,fmap a = (RA|0) n0 = fmsk[0] & (fmap[0] == a[63]) n1 = fmsk[1] & (fmap[1] == a[62]) n2 = fmsk[2] & (fmap[2] == a[61]) n3 = fmsk[3] & (fmap[3] == a[60]) result = n0 || n1 || n2 || n3 if M: result |= CR{BF} & ~fmsk CR{BF} = result When used with SVP64 Prefixing this is a [[sv/normal]] SVP64 type operation and as such can use RC1 Data-dependent Mode capability **mtcrweird** mtcrweird: BF,RA,M,fmsk,fmap reg = (RA|0) lsb = reg[63] # MSB0 numbering n0 = fmsk[0] & (fmap[0] == lsb) n1 = fmsk[1] & (fmap[1] == lsb) n2 = fmsk[2] & (fmap[2] == lsb) n3 = fmsk[3] & (fmap[3] == lsb) result = n0 || n1 || n2 || n3 if M: result |= CR{BF} & ~fmsk CR{BF} = result Note that when M=1 this operation is a Read-Modify-Write on the CR Field BF. Masked-out bits of the 4-bit CR Field BF will not be changed when M=1. Correspondingly when M=0 this operation is an overwrite: no read of BF is required because the masked-out bits of the BF CR Field are set to zero. When used with SVP64 Prefixing this is a [[sv/cr_ops]] SVP64 type operation that has 3-bit Data-dependent and 3-bit Predicate-result capability (BF is 3 bits) **mcrfm** - Move CR Field, masked. This instruction copies, sets, or inverts parts of a CR Field into another CR Field. `mcrf` copies only one bit of the CR from any arbitrary bit to any other arbitrary bit, whereas `mcrfm` copies an entire 4-bit CR Field (or masked parts thereof). Unlike `mcrf` the bits of the CR Field may not change position: the EQ bit from the source may only go into the EQ bit of the destination (optionally inverted, set, or cleared). mcrfm: BF,BFA,M,fmsk,fmap result = fmsk & CR{BFA} if M: result |= CR{BF} & ~fmsk result ^= fmap CR{BF} = result When M=1 this operation is a Read-Modify-Write on the CR Field BF. Masked-out bits of the 4-bit CR Field BF will not be changed when M=1. Correspondingly when M=0 this operation is an overwrite: no read of BF is required because the masked-out bits of the BF CR Field are set to zero. When used with SVP64 Prefixing this is a [[sv/cr_ops]] SVP64 type operation that has 3-bit Data-dependent and 3-bit Predicate-result capability (BF is 3 bits) *Programmer's note: `fmap` being XORed onto the result provides considerable flexibility. individual bits of BFA may be copied inverted to BF by ensuring that `fmsk` and `fmap` have the same bit set. Also, individual bits in BF may be set to 1 by ensuring that the required bit of `fmsk` is set to zero and the same bit in `fmap` is set to 1* **crweirder** crweirder: BT,BFA,fmsk,fmap creg = CR{BFA} n0 = fmsk[0] & (fmap[0] == creg[0]) n1 = fmsk[1] & (fmap[1] == creg[1]) n2 = fmsk[2] & (fmap[2] == creg[2]) n3 = fmsk[3] & (fmap[3] == creg[3]) bf = BT[2:4] # select CR field bit = BT[0:1] # select bit of CR field n = (n0||n1||n2||n3) & fmsk result = (n != 0) if M else (n == fmsk) CR{bf}[bit] = result When used with SVP64 Prefixing this is a [[sv/cr_ops]] SVP64 type operation that has 5-bit Data-dependent and 5-bit Predicate-result capability (BT is 5 bits) **Example Pseudo-ops:** mtcri BF, fmap mtcrweird BF, r0, 0, 0b1111,~fmap mtcrset BF, fmsk mtcrweird BF, r0, 1, fmsk,0b0000 mtcrclr BF, fmsk mtcrweird BF, r0, 1, fmsk,0b1111 # Vectorised versions involving GPRs The name "weird" refers to a minor violation of SV rules when it comes to deriving the Vectorised versions of these instructions. Normally the progression of the SV for-loop would move on to the next register. Instead however in the scalar case these instructions **remain in the same register** and insert or transfer between **bits** of the scalar integer source or destination. The reason is that when using CR Fields as predicate masks and there is a need to transfer into a GPR, again for use as a predicate mask, the CR Field bits need to be efficiently packed into that one GPR (r3, r10 or r31). Further useful violation of the normal SV Elwidth override rules allows for packing (or unpacking) of multiple CR test results into (or out of) an Integer Element. Note that the CR (source operand) elwidth field is utilised to determine the bit- packing size (1/2/4/8 with remaining bits within the Integer element set to zero) whilst the INT (dest operand) elwidth field still sets the Integer element size as usual (8/16/32/default) **crrweird: RT, BB, fmsk.fmap** for i in range(VL): if BB.isvec: creg = CR{BB+i} else: creg = CR{BB} n0 = fmsk[0] & (fmap[0] == creg[0]) n1 = fmsk[1] & (fmap[1] == creg[1]) n2 = fmsk[2] & (fmap[2] == creg[2]) n3 = fmsk[3] & (fmap[3] == creg[3]) # OR or AND to a single bit n = (n0||n1||n2||n3) & fmsk result = (n != 0) if M else (n == fmsk) if RT.isvec: # TODO: RT.elwidth override to be also added here # note, yes, really, the CR's elwidth field determines # the bit-packing into the INT! if BB.elwidth == 0b00: # pack 1 result into 64-bit registers iregs[RT+i][0..62] = 0 iregs[RT+i][63] = result # sets LSB to result if BB.elwidth == 0b01: # pack 2 results sequentially into INT registers iregs[RT+i//2][0..61] = 0 iregs[RT+i//2][63-(i%2)] = result if BB.elwidth == 0b10: # pack 4 results sequentially into INT registers iregs[RT+i//4][0..59] = 0 iregs[RT+i//4][63-(i%4)] = result if BB.elwidth == 0b11: # pack 8 results sequentially into INT registers iregs[RT+i//8][0..55] = 0 iregs[RT+i//8][63-(i%8)] = result else: iregs[RT][63-i] = result # results also in scalar INT Note that: * in the scalar case the CR-Vector assessment is stored bit-wise starting at the LSB of the destination scalar INT * in the INT-vector case the results are packed into LSBs of the INT Elements, the packing arrangement depending on both elwidth override settings. **mfcrrweird: RT, BFA, fmsk.fmap** Unlike `crrweird` the results are 4-bit wide, so the packing will begin to spill over to other destination elements. 8 results per destination at 4-bits each still fits into destination elwidth at 32-bit, but for 16-bit and 8-bit obviously this does not fit, and must split across to the next element When for example destination elwidth is 16-bit (0b10) the following packing occurs: - SVRM bits 6:7 equal to 0b00 - one 4-bit result element packed into the first 4-bits of the 16-bit destination element (in the first 4 LSBs) - SVRM bits 6:7 equal to 0b01 - two 4-bit result elements packed into the first 8-bits of the 16-bit destination element (in the first 8 LSBs) - SVRM bits 6:7 equal to 0b10 - four 4-bit result elements packed into each 16-bit destination element - SVRM bits 6:7 equal to 0b11 - eight 4-bit result elements, the first four of which are packed into the first 16-bit destination element, the second four of which are packed into the second 16-bit destination element. Pseudocode example: note that dest elwidth overrides affect the packing of results. BB.elwidth in effect requests how many 4-bit result elements would like to be packed, but RT.elwidth determines the limit. Any parts of the destination elements not containing results are set to zero. for i in range(VL): if BB.isvec: creg = CR{BB+i} else: creg = CR{BB} n0 = fmsk[0] & (fmap[0] == creg[0]) n1 = fmsk[1] & (fmap[1] == creg[1]) n2 = fmsk[2] & (fmap[2] == creg[2]) n3 = fmsk[3] & (fmap[3] == creg[3]) result = n0||n1||n2||n3 # 4-bit result if RT.isvec: # RT.elwidth override can affect the packing bwid = {0b00:64, 0b01:8, 0b10:16, 0b11:32}[RT.elwidth] t4, t8 = min(4, bwid//2), min(8, bwid//2) # yes, really, the CR's elwidth field determines # the bit-packing into the INT! if BB.elwidth == 0b00: # pack 1 result into 64-bit registers idx, boff = i, 0 if BB.elwidth == 0b01: # pack 2 results sequentially into INT registers idx, boff = i//2, i%2 if BB.elwidth == 0b10: # pack 4 results sequentially into INT registers idx, boff = i//t4, i%t4 if BB.elwidth == 0b11: # pack 8 results sequentially into INT registers idx, boff = i//t8, i%t8 else: # exceeding VL=16 is UNDEFINED idx, boff = 0, i iregs[RT+idx][60-boff*4:63-boff*4] = result # v3.1 setbc instructions There are additional setb conditional instructions in v3.1 (p129) RT = (CR[BI] == 1) ? 1 : 0 which also negate that, and also return -1 / 0. these are similar to crweird but not the same purpose. most notable is that crweird acts on CR fields rather than the entire 32 bit CR. # Predication Examples Take the following example: r10 = 0b00010 sv.mtcrweird/dm=r10/dz cr8.v, 0, 0b0011.0000 Here, RA is zero, so the source input is zero. The destination is CR Field 8, and the destination predicate mask indicates to target the first two elements. Destination predicate zeroing is enabled, and the destination predicate is only set in the 2nd bit. fmsk is 0b0011, fmap is all zeros. Let us first consider what should go into element 0 (CR Field 8): * The destination predicate bit is zero, and zeroing is enabled. * Therefore, what is in the source is irrelevant: the result must be zero. * Therefore all four bits of CR Field 8 are therefore set to zero. Now the second element, CR Field 9 (CR9): * Bit 2 of the destination predicate, r10, is 1. Therefore the computation of the result is relevant. * RA is zero therefore bit 2 is zero. fmsk is 0b0011 and fmap is 0b0000 * When calculating n0 thru n3 we get n0=1, n1=2, n2=0, n3=0 * Therefore, CR9 is set (using LSB0 ordering) to 0b0011, i.e. to fmsk. It should be clear that this instruction uses bits of the integer predicate to decide whether to set CR Fields to `(fmsk & ~fmap)` or to zero. Thus, in effect, it is the integer predicate that has been copied into the CR Fields. By using twin predication, zeroing, and inversion (sm=~r3, dm=r10) for example, it becomes possible to combine two Integers together in order to set bits in CR Fields. Likewise there are dozens of ways that CR Predicates can be used, on the same sv.mtcrweird instruction.