+[[!tag standards]]
+
# New instructions for CR/INT predication
+**DRAFT STATUS**
+
See:
-* <https://bugs.libre-soc.org/show_bug.cgi?id=533>
+* main bugreport for crweirds
+ <https://bugs.libre-soc.org/show_bug.cgi?id=533>
+* <https://bugs.libre-soc.org/show_bug.cgi?id=527>
+* <https://bugs.libre-soc.org/show_bug.cgi?id=569>
+* <https://bugs.libre-soc.org/show_bug.cgi?id=558#c47>
+
+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 available for use in creating masks. This is not practical for SV given the premise to minimise adding of instructions.
+
+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.
+
+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 (eq/lt/gt/ov). By contrast
+v3.0B Scalar CR instructions (crand, crxor) only allow a single bit calculation.
Basic concept:
-* CR-based instructions that perform simple AND/OR/XOR from all four bits
- of a CR to create a single bit value (0/1) in an integer register
+* CR-based instructions that perform simple AND/OR/XOR 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 all four bits of a given CR
+ 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.
-* Vectorisation of the same
+* Optional Vectorisation of the same when SVP64 is implemented
Purpose:
Side-effects:
-* mtcrweird when RA=0 is a means to set or clear arbitrary CR bits from immediates
+* mtcrweird when RA=0 is a means to set or clear arbitrary CR bits
+ using immediates embedded within the instruction.
(Twin) Predication interactions:
* INT twin predication with zeroing is a way to copy an integer into CRs without necessarily needing the INT register (RA). if it is, it is effectively ANDed (or negate-and-ANDed) with the INT Predicate
* CR twin predication with zeroing is likewise a way to interact with the incoming integer
-this gets particularly powerful if data-dependent predication is also enabled.
+this gets particularly powerful if data-dependent predication is also enabled. further explanation is below.
+
+# Bit ordering.
+
+IBM chose MSB0 for the OpenPOWER v3.0B specification. This makes things slightly hair-raising and the relationship between the CR and the CR Field
+numbers is not clearly defined. To make it clear we define a new
+term, `CR{n}`.
+`CR{n}` refers to `CR0` when `n=0` and consequently, for CR0-7, is defined, in v3.0B pseudocode, as:
+
+ CR{7-n} = CR[32+n*4:35+n*4]
+
+Also note that for SVP64 the relationship for the sequential
+numbering of elements is to the CR **fields** within
+the CR Register, not to individual bits within the CR register.
# Instruction form and pseudocode
- | 0-5 | 6-10 | 11 | 12-15 | 16-18 | 19-20 | 21-30 | 31 |
- | 19 | RT | 0 | mask | BB | 0 / | XO | / |
- | 19 | RA | 1 | mask | BB | 0 / | XO | / |
- | 19 | BT // | 0 | mask | BB | 1 / | XO | / |
- | 19 | BFT | 1 | mask | BB | 1 / | XO | / |
+**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 | |mask |BFA | |XO[0:4]|XO[5:9]|/ | |
+|19 |RT |M |mask |BFA | 0 0 |XO[0:4]|0 mode |Rc|crrweird |
+|19 |RA |M |mask |BF | 0 1 |XO[0:4]|0 mode |/ |mtcrweird |
+|19 |BFT//|M |mask |BFA | 1 0 |XO[0:4]|0 mode |/ |crweirder |
+|19 |BF |M |mask |BFA | 1 1 |XO[0:4]|0 mode |/ |crweird |
+
+**crrweird**
mode is encoded in XO and is 4 bits
-bit 11=0, bit 19=0
+bit 19=0, bit 20=0
- crrweird: RT, BB, mask.mode
+ crrweird: RT, BFA, M, mask.mode
- creg = CRfile[32+BB*4:36+BB*4]
+ creg = CR{BFA}
n0 = mask[0] & (mode[0] == creg[0])
n1 = mask[1] & (mode[1] == creg[1])
n2 = mask[2] & (mode[2] == creg[2])
n3 = mask[3] & (mode[3] == creg[3])
- RT[0] = n0|n1|n2|n3
+ result = n0|n1|n2|n3 if M else n0&n1&n2&n3
+ RT[63] = result # MSB0 numbering, 63 is LSB
+ If Rc:
+ CR0 = analyse(RT)
+
+When used with SVP64 Prefixing this is a [[openpower/sv/normal]] SVP64 type operation and as
+such can use Rc=1 and RC1 Data-dependent Mode capability
-bit 11=1, bit 19=0
+**mtcrweird**
- mtcrweird: RA, BB, mask.mode
+bit 19=0, bit 20=1
+
+ mtcrweird: BF, RA, M, mask.mode
reg = (RA|0)
- n0 = mask[0] & (mode[0] == reg[0])
- n1 = mask[1] & (mode[1] == reg[0])
- n2 = mask[2] & (mode[2] == reg[0])
- n3 = mask[3] & (mode[3] == reg[0])
- CRfile[32+BB*4:36+BB*4] = n0 || n1 || n2 || n3
+ lsb = reg[63] # MSB0 numbering
+ n0 = mask[0] & (mode[0] == lsb)
+ n1 = mask[1] & (mode[1] == lsb)
+ n2 = mask[2] & (mode[2] == lsb)
+ n3 = mask[3] & (mode[3] == lsb)
+ result = n0 || n1 || n2 || n3
+ if M:
+ result |= CR{BF} & ~mask
+ 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 [[openpower/sv/cr_ops]] SVP64 type operation that has
+3-bit Data-dependent and 3-bit Predicate-result capability
+(BF is 3 bits)
-bit 11=0, bit 19=1
+**crweird**
- crweird: BT, BB, mask.mode
+bit 19=1, bit 20=0
- creg = CRfile[32+BB*4:36+BB*4]
+ crweird: BF, BFA, M, mask.mode
+
+ creg = CR{BFA}
n0 = mask[0] & (mode[0] == creg[0])
n1 = mask[1] & (mode[1] == creg[1])
n2 = mask[2] & (mode[2] == creg[2])
n3 = mask[3] & (mode[3] == creg[3])
- CRfile[32+BT*4:36+BT*4] = n0 || n1 || n2 || n3
+ result = n0 || n1 || n2 || n3
+ if M:
+ result |= CR{BF} & ~mask
+ 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 [[openpower/sv/cr_ops]] SVP64 type operation that has
+3-bit Data-dependent and 3-bit Predicate-result capability
+(BF is 3 bits)
-bit 11=1, bit 19=1
+**crweirder**
- crweirder: BFT, BB, mask.mode
+bit 19=1, bit 20=1
- creg = CRfile[32+BB*4:36+BB*4]
+ crweirder: BT, BFA, mask.mode
+
+ creg = CR{BFA}
n0 = mask[0] & (mode[0] == creg[0])
n1 = mask[1] & (mode[1] == creg[1])
n2 = mask[2] & (mode[2] == creg[2])
n3 = mask[3] & (mode[3] == creg[3])
- CRfile[32+BFT] = n0|n1|n2|n3
+ BF = BT[2:4] # select CR
+ bit = BT[0:1] # select bit of CR
+ result = n0|n1|n2|n3 if M else n0&n1&n2&n3
+ CR{BF}[bit] = result
+
+When used with SVP64 Prefixing this is a [[openpower/sv/cr_ops]] SVP64 type operation that has
+5-bit Data-dependent and 5-bit Predicate-result capability
+(BFT is 5 bits)
+
+**Example Pseudo-ops:**
+
+ mtcri BF, mode mtcrweird BF, r0, 0, 0b1111.~mode
+ mtcrset BF, mask mtcrweird BF, r0, 1, mask.0b0000
+ mtcrclr BF, mask mtcrweird BF, r0, 1, mask.0b1111
+
+# Vectorised versions
+
+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.
+
+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, mask.mode
+
+ for i in range(VL):
+ if BB.isvec:
+ creg = CR{BB+i}
+ else:
+ creg = CR{BB}
+ n0 = mask[0] & (mode[0] == creg[0])
+ n1 = mask[1] & (mode[1] == creg[1])
+ n2 = mask[2] & (mode[2] == creg[2])
+ n3 = mask[3] & (mode[3] == creg[3])
+ # OR or AND to a single bit
+ result = n0|n1|n2|n3 if M else n0&n1&n2&n3
+ 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.
+
+# 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.
+mask is 0b0011, mode 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.
-Pseudo-op:
+Now the second element, CR Field 9 (CR9):
- mtcri BB, mode mtcrweird r0, BB, 0b1111.~mode
- mtcrset BB, mask mtcrweird r0, BB, mask.0b0000
- mtcrclr BB, mask mtcrweird r0, BB, mask.0b1111
+* 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. mask is 0b0011 and mode 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 mask.
+It should be clear that this instruction uses bits of the integer
+predicate to decide whether to set CR Fields to `(mask & ~mode)`
+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.
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