# 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 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.
+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/XOR from any four bits
+* 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
(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
+* 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. further explanation is below.
+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. Our desire initially is therefore to follow the logical progression from the defined behaviour of `mtcr` and `mfcr` etc.
-In [[isa/sprset]] we see the pseudocode for `mtcrf` for example:
-
- mtcrf FXM,RS
-
- do n = 0 to 7
- if FXM[n] = 1 then
- CR[4*n+32:4*n+35] <- (RS)[4*n+32:4*n+35]
-
-This places (according to a mask schedule) `CR0` into MSB0-numbered bits 32-35 of the target Integer register `RS`, these bits of `RS` being the 31st down to the 28th. Unfortunately, even when not Vectorised, this inserts CR numbering inversions on each batch of 8 CRs, massively complicating matters. Predication when using CRs would have to be morphed to this (unacceptably complex) behaviour:
-
- for i in range(VL):
- if INTpredmode:
- predbit = (r3)[63-i] # IBM MSB0 spec sigh
- else:
- # completely incomprehensible vertical numbering
- n = (7-(i%8)) | (i & ~0x7) # total mess
- CRpredicate = CR{n} # select CR0, CR1, ....
- predbit = CRpredicate[offs] # select eq..ov bit
-
-Which is nowhere close to matching the straightforward obvious case:
+Please see [[svp64/appendix]] regarding CR bit ordering and for
+the definition of `CR{n}`
- for i in range(VL):
- if INTpredmode:
- predbit = (r3)[63-i] # IBM MSB0 spec sigh
- else:
- CRpredicate = CR{i} # start at CR0, work up
- predbit = CRpredicate[offs]
+# Instruction form and pseudocode
-In other words unless we do something about this, when we transfer bits from an Integer Predicate into a Vector of CRs, our numbering of CRs, when enumerating them in a CR Vector, would be **CR7** CR6 CR5.... CR0 **CR15** CR14 CR13... CR8 **CR23** CR22 etc. **not** the more natural and obvious CR0 CR1 ... CR23.
+**DRAFT** Instruction format (use of MAJOR 19 not approved by
+OPF ISA WG):
-Therefore the instructions below need to **redefine** the relationship so that CR numbers (CR0, CR1) sequentially match the arithmetically-ordered bits of Integer registers. By `arithmetic` this is deduced from the fact that the instruction `addi r3, r0, 1` will result in the **LSB** (numbered 63 in IBM MSB0 order) of r3 being set to 1 and all other bits set to zero. We therefore refer, below, to this LSB as "Arithmetic bit 0", and it is this bit which is used - defined - as being the first bit used in Integer predication (on element 0).
+|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 | | | | | |1 //// |00011 | |rsvd |
+|19 |RT |M |mask |BFA | 0 0 |1 mode |00011 |Rc|crrweird |
+|19 |RT |M |mask |BFA | 0 1 |1 mode |00011 |Rc|mfcrweird |
+|19 |RA |M |mask |BF | 0 0 |0 mode |00011 |1 |mtcrrweird |
+|19 |RA |M |mask |BF | 0 1 |0 mode |00011 |0 |mtcrweird |
+|19 |BT |M |mask |BFA | 0 1 |0 mode |00011 |1 |crweirder |
+|19 |BF //|M |mask |BFA | 1 1 |0 mode |00011 |0 |crweird |
+|19 |BF //|M |mask |BFA | 1 1 |0 mode |00011 |1 |mcrfm |
-Below is some pseudocode that, given a CR offset `offs` to represent `CR.eq` thru to `CR.ov` respectively, will copy the INT predicate bits in the correct order into the first 8 CRs:
+**crrweird**
- do n = 0 to 7
- CR[4*n+32+offs] <- (RS)[63-n]
+mode is encoded in XO and is 4 bits
-Assuming that `offs` is set to `CR.eq` this results in:
+bit 19=0, bit 20=0
-* Arithmetic bit 0 (the LSB, numbered 63 in IBM MSB0 terminology)
- of RS being inserted into CR0.eq
-* Arithmetic bit 1 of RS being inserted into CR1.eq
-* ...
-* Arithmetic bit 7 of RS being inserted into CR7.eq
+ crrweird: RT, BFA, M, mask.mode
-To clarify, then: all instructions below do **NOT** follow the IBM convention, they follow the natural sequence CR0 CR1 instead, using `CR{fieldnum}` to refer to the individual CR Fields. However it is critically important to note that the offsets **in** a CR field
-(`CR.eq` for example) continue to follow the v3.0B definition and convention.
+ 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])
+ 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
-# Instruction form and pseudocode
+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*
-Note that `CR{n}` refers to `CR0` when `n=0` and consequently, for CR0-7, is defined, in v3.0B pseudocode, as:
+When the destination elwidth is default (0b00) the following packing occurs
+into destination elements:
- CR{7-n} = CR[32+n*4:35+n*4]
+- SVRM bits 6:7 equal to 0b00 - one result element packed into one bit of each
+ destination element (in the LSB)
+- SVRM bits 6:7 equal to 0b01 - two result elements packed into two bits of
+ destination element (in the bottom two LSBs)
+- SVRM bits 6:7 equal to 0b10 - four result elements packed into four bits of
+ destination element (in the bottom four LSBs)
+- SVRM bits 6:7 equal to 0b11 - eight result elements packed into four bits of
+ destination element (in the bottom four LSBs)
-Instruction format:
+When for example the destination elwidth is 8-bit (0b11) then the destination
+element widths are 8-bit, and the result elements (grouped up to 8) still fit
+neatly into each 8-bit destination element.
-|0-5|6-10 |11|12-15|16-18|19-20|21-25 |26-30 |31|name |
-|---|---- |--|-----|-----|-----|----- |----- |--|---- |
-|19 |RT | |mask |BB | |XO[0:4]|XO[5:9]|/ | |
-|19 |RT |0 |mask |BB | 0 M |XO[0:4]|0 mode |Rc|crrweird |
-|19 |RA |1 |mask |BT | 0 / |XO[0:4]|0 mode |/ |mtcrweird |
-|19 |BT //|0 |mask |BB | 1 / |XO[0:4]|0 mode |/ |crweird |
-|19 |BFT |1 |mask |BB | 1 M |XO[0:4]|0 mode |/ |crweirder |
-
-**crrweird**
+**mfcrrweird**
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
+ mfcrrweird: RT, BFA, mask.mode
- creg = CR{BB}
+ 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])
- result = n0|n1|n2|n3 if M else n0&n1&n2&n3
- RT[63] = result # MSB0 numbering, 63 is LSB
+ 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 [[openpower/sv/normal]] SVP64 type operation and as
-such can use Rc=1 and RC1 Data-dependent Mode capability
+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.
+
+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*
+
+Unlike `crrweird` however, 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.
+
+**mtcrrweird**
+
+mode is encoded in XO and is 4 bits
+
+bit 19=0, bit 20=0
+
+ mtcrrweird: BF, RA, M, mask.mode
+
+ n0 = mask[0] & (mode[0] == RA[63])
+ n1 = mask[1] & (mode[1] == RA[62])
+ n2 = mask[2] & (mode[2] == RA[61])
+ n3 = mask[3] & (mode[3] == RA[60])
+ result = n0 || n1 || n2 || n3
+ if M:
+ result |= CR{BF} & ~mask
+ CR{BF} = result
+
+When used with SVP64 Prefixing this is a [[openpower/sv/normal]]
+SVP64 type operation and as such can use RC1 Data-dependent
+Mode capability
**mtcrweird**
-bit 11=1, bit 19=0
+bit 19=0, bit 20=1
- mtcrweird: BT, RA, mask.mode
+ mtcrweird: BF, RA, M, mask.mode
reg = (RA|0)
lsb = reg[63] # MSB0 numbering
n1 = mask[1] & (mode[1] == lsb)
n2 = mask[2] & (mode[2] == lsb)
n3 = mask[3] & (mode[3] == lsb)
- CR{BT} = n0 || n1 || n2 || n3
+ result = n0 || n1 || n2 || n3
+ if M:
+ result |= CR{BF} & ~mask
+ CR{BF} = result
-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
-(BT is 3 bits)
+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)
**crweird**
-bit 11=0, bit 19=1
+bit 19=1, bit 20=0, bit 30=0
- crweird: BT, BB, mask.mode
+ crweird: BF, BFA, M, mask.mode
- creg = CR{BB}
+ 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])
- CR{BT} = 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)
+
+**mcrfm** - Move CR Field, masked.
+
+bit 19=1, bit 20=0, bit 30=1
-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
-(BT is 3 bits)
+ mcrfm: BF, BFA, M, mask.mode
+
+ result = mask & CR{BFA}
+ if M:
+ result |= CR{BF} & ~mask
+ result ^= mode
+ 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)
+
+*Programmer's note: `mode` being XORed onto the result provides
+considerable flexibility. individual bits of BFA may be copied inverted
+to BF by ensuring that `mask` and `mode` have the same bit set. Also,
+individual bits in BF may be set to 1 by ensuring that the required bit of
+`mask` is set to zero and the same bit in `mode` is set to 1*
**crweirder**
-bit 11=1, bit 19=1
+bit 19=1, bit 20=1
- crweirder: BFT, BB, mask.mode
+ crweirder: BT, BFA, mask.mode
- creg = CR{BB}
+ 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])
- BF = BFT[2:4] # select CR
- bit = BFT[0:1] # select bit of CR
+ 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)
+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 BB, mode mtcrweird r0, BB, 0b1111.~mode
- mtcrset BB, mask mtcrweird r0, BB, mask.0b0000
- mtcrclr BB, mask mtcrweird r0, BB, mask.0b1111
+ 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.
+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.
-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
* 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 result is stored in the
- LSB of each element in the result vector
-
-Note that element width overrides are respected on the INT src or destination register, however that it is the CR element-width
-override that is used to indicate how many bits of CR results
-are packed/extracted into/from each INT register
+* 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)
+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.
+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
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.
+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):
* 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.
+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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