3 # New instructions for CR/INT predication
9 * main bugreport for crweirds
10 <https://bugs.libre-soc.org/show_bug.cgi?id=533>
11 * <https://bugs.libre-soc.org/show_bug.cgi?id=527>
12 * <https://bugs.libre-soc.org/show_bug.cgi?id=569>
13 * <https://bugs.libre-soc.org/show_bug.cgi?id=558#c47>
17 Condition Registers are conceptually perfect for use as predicate masks,
18 the only problem being that typical Vector ISAs have quite comprehensive
19 mask-based instructions: set-before-first, popcount and much more.
20 In fact many Vector ISAs can use Vectors *as* masks, consequently the
21 entire Vector ISA is usually available for use in creating masks (one
22 exception being AVX512 which has a dedicated Mask regfile and opcodes).
23 Duplication of such operations (popcount etc) is not practical for SV
24 given the strategy of leveraging pre-existing Scalar instructions in a
27 With the scalar OpenPOWER v3.0B ISA having already popcnt, cntlz and
28 others normally seen in Vector Mask operations it makes sense to allow
29 *both* scalar integers *and* CR-Vectors to be predicate masks. That in
30 turn means that much more comprehensive interaction between CRs and scalar
31 Integers is required, because with the CR Predication Modes designating
32 CR *Fields* (not CR bits) as Predicate Elements, fast transfers between
33 CR *Fields* and the Integer Register File is needed.
35 The opportunity is therefore taken to also augment CR logical arithmetic
36 as well, using a mask-based paradigm that takes into consideration
37 multiple bits of each CR Field (eq/lt/gt/ov). By contrast v3.0B Scalar
38 CR instructions (crand, crxor) only allow a single bit calculation, and
39 both mtcr and mfcr are CR-orientated rather than CR *Field* orientated.
41 Also strangely there is no v3.0 instruction for directly moving CR Fields,
42 only CR *bits*, so that is corrected here with `mcrfm`. The opportunity
43 is taken to allow inversion of CR Field bits, when copied.
47 * CR-based instructions that perform simple AND/OR from any four bits
48 of a CR field to create a single bit value (0/1) in an integer register
49 * Inverse of the same, taking a single bit value (0/1) from an integer
50 register to selectively target any four bits of a given CR Field
51 * CR-to-CR version of the same, allowing multiple bits to be AND/OR/XORed
53 * Optional Vectorisation of the same when SVP64 is implemented
57 * To provide a merged version of what is currently a multi-sequence of
58 CR operations (crand, cror, crxor) with mfcr and mtcrf, reducing
60 * To provide a vectorised version of the same, suitable for advanced
65 * mtcrweird when RA=0 is a means to set or clear arbitrary CR bits
66 using immediates embedded within the instruction.
68 (Twin) Predication interactions:
70 * INT twin predication with zeroing is a way to copy an integer into
71 CRs without necessarily needing the INT register (RA). if it is, it is
72 effectively ANDed (or negate-and-ANDed) with the INT Predicate
73 * CR twin predication with zeroing is likewise a way to interact with
76 this gets particularly powerful if data-dependent predication is also
77 enabled. further explanation is below.
81 Please see [[svp64/appendix]] regarding CR bit ordering and for
82 the definition of `CR{n}`
84 # Instruction form and pseudocode
86 **DRAFT** Instruction format (use of MAJOR 19 not approved by
89 |0-5|6-10 |11|12-15|16-18|19-20|21-25 |26-30 |31|name |
90 |---|---- |--|-----|-----|-----|----- |----- |--|---- |
91 |19 |RT | |mask |BFA | |XO[0:4]|XO[5:9]|/ | |
92 |19 | | | | | |1 //// |00011 | |rsvd |
93 |19 |RT |M |mask |BFA | 0 0 |1 mode |00011 |Rc|crrweird |
94 |19 |RT |M |mask |BFA | 0 1 |1 mode |00011 |Rc|mfcrweird |
95 |19 |RA |M |mask |BF | 0 0 |0 mode |00011 |1 |mtcrrweird |
96 |19 |RA |M |mask |BF | 0 1 |0 mode |00011 |0 |mtcrweird |
97 |19 |BT |M |mask |BFA | 0 1 |0 mode |00011 |1 |crweirder |
98 |19 |BF //|M |mask |BFA | 1 1 |0 mode |00011 |0 |crweird |
99 |19 |BF //|M |mask |BFA | 1 1 |0 mode |00011 |1 |mcrfm |
103 mode is encoded in XO and is 4 bits
107 crrweird: RT, BFA, M, mask.mode
110 n0 = mask[0] & (mode[0] == creg[0])
111 n1 = mask[1] & (mode[1] == creg[1])
112 n2 = mask[2] & (mode[2] == creg[2])
113 n3 = mask[3] & (mode[3] == creg[3])
114 result = n0|n1|n2|n3 if M else n0&n1&n2&n3
115 RT[63] = result # MSB0 numbering, 63 is LSB
119 When used with SVP64 Prefixing this is a [[openpower/sv/normal]]
120 SVP64 type operation and as such can use Rc=1 and RC1 Data-dependent
123 Also as noted below, element-width override bits normally used
124 on the source is instead used to allow multiple results to be packed
125 sequentially into the destination. *Destination elwidth overrides still apply*.
129 mode is encoded in XO and is 4 bits
133 mfcrrweird: RT, BFA, mask.mode
136 n0 = mask[0] & (mode[0] == creg[0])
137 n1 = mask[1] & (mode[1] == creg[1])
138 n2 = mask[2] & (mode[2] == creg[2])
139 n3 = mask[3] & (mode[3] == creg[3])
140 result = n0||n1||n2||n3
141 RT[60:63] = result # MSB0 numbering, 63 is LSB
145 When used with SVP64 Prefixing this is a [[openpower/sv/normal]]
146 SVP64 type operation and as such can use Rc=1 and RC1 Data-dependent
149 Also as noted below, element-width override bits normally used
150 on the source is instead used to allow multiple results to be packed
151 into the destination. *Destination elwidth overrides still apply*
155 mode is encoded in XO and is 4 bits
159 mtcrrweird: BF, RA, M, mask.mode
161 n0 = mask[0] & (mode[0] == RA[63])
162 n1 = mask[1] & (mode[1] == RA[62])
163 n2 = mask[2] & (mode[2] == RA[61])
164 n3 = mask[3] & (mode[3] == RA[60])
165 result = n0 || n1 || n2 || n3
167 result |= CR{BF} & ~mask
170 When used with SVP64 Prefixing this is a [[openpower/sv/normal]]
171 SVP64 type operation and as such can use RC1 Data-dependent
178 mtcrweird: BF, RA, M, mask.mode
181 lsb = reg[63] # MSB0 numbering
182 n0 = mask[0] & (mode[0] == lsb)
183 n1 = mask[1] & (mode[1] == lsb)
184 n2 = mask[2] & (mode[2] == lsb)
185 n3 = mask[3] & (mode[3] == lsb)
186 result = n0 || n1 || n2 || n3
188 result |= CR{BF} & ~mask
191 Note that when M=1 this operation is a Read-Modify-Write on the CR Field
192 BF. Masked-out bits of the 4-bit CR Field BF will not be changed when
193 M=1. Correspondingly when M=0 this operation is an overwrite: no read
194 of BF is required because the masked-out bits of the BF CR Field are
197 When used with SVP64 Prefixing this is a [[openpower/sv/cr_ops]] SVP64
198 type operation that has 3-bit Data-dependent and 3-bit Predicate-result
199 capability (BF is 3 bits)
203 bit 19=1, bit 20=0, bit 30=0
205 crweird: BF, BFA, M, mask.mode
208 n0 = mask[0] & (mode[0] == creg[0])
209 n1 = mask[1] & (mode[1] == creg[1])
210 n2 = mask[2] & (mode[2] == creg[2])
211 n3 = mask[3] & (mode[3] == creg[3])
212 result = n0 || n1 || n2 || n3
214 result |= CR{BF} & ~mask
217 Note that when M=1 this operation is a Read-Modify-Write on the CR Field
218 BF. Masked-out bits of the 4-bit CR Field BF will not be changed when
219 M=1. Correspondingly when M=0 this operation is an overwrite: no read
220 of BF is required because the masked-out bits of the BF CR Field are
223 When used with SVP64 Prefixing this is a [[openpower/sv/cr_ops]] SVP64
224 type operation that has 3-bit Data-dependent and 3-bit Predicate-result
225 capability (BF is 3 bits)
227 **mcrfm** - Move CR Field, masked.
229 bit 19=1, bit 20=0, bit 30=1
231 mcrfm: BF, BFA, M, mask.mode
233 result = mask & CR{BFA}
235 result |= CR{BF} & ~mask
239 Note that when M=1 this operation is a Read-Modify-Write on the CR Field
240 BF. Masked-out bits of the 4-bit CR Field BF will not be changed when
241 M=1. Correspondingly when M=0 this operation is an overwrite: no read
242 of BF is required because the masked-out bits of the BF CR Field are
245 When used with SVP64 Prefixing this is a [[openpower/sv/cr_ops]] SVP64
246 type operation that has 3-bit Data-dependent and 3-bit Predicate-result
247 capability (BF is 3 bits)
249 *Programmer's note: `mode` being XORed onto the result provides
250 considerable flexibility. individual bits of BFA may be copied inverted
251 to BF by ensuring that `mask` and `mode` have the same bit set. Also,
252 individual bits in BF may be set to 1 by ensuring that the required bit of
253 `mask` is set to zero and the same bit in `mode` is set to 1*
259 crweirder: BT, BFA, mask.mode
262 n0 = mask[0] & (mode[0] == creg[0])
263 n1 = mask[1] & (mode[1] == creg[1])
264 n2 = mask[2] & (mode[2] == creg[2])
265 n3 = mask[3] & (mode[3] == creg[3])
266 BF = BT[2:4] # select CR
267 bit = BT[0:1] # select bit of CR
268 result = n0|n1|n2|n3 if M else n0&n1&n2&n3
271 When used with SVP64 Prefixing this is a [[openpower/sv/cr_ops]] SVP64
272 type operation that has 5-bit Data-dependent and 5-bit Predicate-result
273 capability (BFT is 5 bits)
275 **Example Pseudo-ops:**
277 mtcri BF, mode mtcrweird BF, r0, 0, 0b1111.~mode
278 mtcrset BF, mask mtcrweird BF, r0, 1, mask.0b0000
279 mtcrclr BF, mask mtcrweird BF, r0, 1, mask.0b1111
281 # Vectorised versions
283 The name "weird" refers to a minor violation of SV rules when it comes
284 to deriving the Vectorised versions of these instructions.
286 Normally the progression of the SV for-loop would move on to the
287 next register. Instead however in the scalar case these instructions
288 **remain in the same register** and insert or transfer between **bits**
289 of the scalar integer source or destination.
291 Further useful violation of the normal SV Elwidth override rules allows
292 for packing (or unpacking) of multiple CR test results into (or out of)
293 an Integer Element. Note that the CR (source operand) elwidth field is
294 utilised to determine the bit- packing size (1/2/4/8 with remaining
295 bits within the Integer element set to zero) whilst the INT (dest
296 operand) elwidth field still sets the Integer element size as usual
299 **crrweird: RT, BB, mask.mode**
306 n0 = mask[0] & (mode[0] == creg[0])
307 n1 = mask[1] & (mode[1] == creg[1])
308 n2 = mask[2] & (mode[2] == creg[2])
309 n3 = mask[3] & (mode[3] == creg[3])
310 # OR or AND to a single bit
311 result = n0|n1|n2|n3 if M else n0&n1&n2&n3
313 # TODO: RT.elwidth override to be also added here
314 # note, yes, really, the CR's elwidth field determines
315 # the bit-packing into the INT!
316 if BB.elwidth == 0b00:
317 # pack 1 result into 64-bit registers
318 iregs[RT+i][0..62] = 0
319 iregs[RT+i][63] = result # sets LSB to result
320 if BB.elwidth == 0b01:
321 # pack 2 results sequentially into INT registers
322 iregs[RT+i//2][0..61] = 0
323 iregs[RT+i//2][63-(i%2)] = result
324 if BB.elwidth == 0b10:
325 # pack 4 results sequentially into INT registers
326 iregs[RT+i//4][0..59] = 0
327 iregs[RT+i//4][63-(i%4)] = result
328 if BB.elwidth == 0b11:
329 # pack 8 results sequentially into INT registers
330 iregs[RT+i//8][0..55] = 0
331 iregs[RT+i//8][63-(i%8)] = result
333 iregs[RT][63-i] = result # results also in scalar INT
337 * in the scalar case the CR-Vector assessment
338 is stored bit-wise starting at the LSB of the
339 destination scalar INT
340 * in the INT-vector case the results are packed into LSBs
341 of the INT Elements, the packing arrangement depending on both
342 elwidth override settings.
344 **mfcrrweird: RT, BFA, mask.mode**
346 Unlike `crrweird` the results are 4-bit wide, so the packing
347 will begin to spill over to other destination elements. 8 results per
348 destination at 4-bits each still fits into destination elwidth at 32-bit,
349 but for 16-bit and 8-bit obviously this does not fit, and must split
350 across to the next element
352 When for example destination elwidth is 16-bit (0b10) the following packing
355 - SVRM bits 6:7 equal to 0b00 - one 4-bit result element packed into the
356 first 4-bits of the 16-bit destination element (in the first 4 LSBs)
357 - SVRM bits 6:7 equal to 0b01 - two 4-bit result elements packed into the
358 first 8-bits of the 16-bit destination element (in the first 8 LSBs)
359 - SVRM bits 6:7 equal to 0b10 - four 4-bit result elements packed into each
360 16-bit destination element
361 - SVRM bits 6:7 equal to 0b11 - eight 4-bit result elements, the first four
362 of which are packed into the first 16-bit destination element, the
363 second four of which are packed into the second 16-bit destination element.
365 Pseudocode example (dest elwidth overrides not included):
372 n0 = mask[0] & (mode[0] == creg[0])
373 n1 = mask[1] & (mode[1] == creg[1])
374 n2 = mask[2] & (mode[2] == creg[2])
375 n3 = mask[3] & (mode[3] == creg[3])
376 result = n0||n1||n2||n3 # 4-bit result
377 RT[60:63] = result # MSB0 numbering, 63 is LSB
383 # v3.1 setbc instructions
385 There are additional setb conditional instructions in v3.1 (p129)
387 RT = (CR[BI] == 1) ? 1 : 0
389 which also negate that, and also return -1 / 0. these are similar to
390 crweird but not the same purpose. most notable is that crweird acts on
391 CR fields rather than the entire 32 bit CR.
393 # Predication Examples
395 Take the following example:
398 sv.mtcrweird/dm=r10/dz cr8.v, 0, 0b0011.0000
400 Here, RA is zero, so the source input is zero. The destination is CR Field
401 8, and the destination predicate mask indicates to target the first two
402 elements. Destination predicate zeroing is enabled, and the destination
403 predicate is only set in the 2nd bit. mask is 0b0011, mode is all zeros.
405 Let us first consider what should go into element 0 (CR Field 8):
407 * The destination predicate bit is zero, and zeroing is enabled.
408 * Therefore, what is in the source is irrelevant: the result must
410 * Therefore all four bits of CR Field 8 are therefore set to zero.
412 Now the second element, CR Field 9 (CR9):
414 * Bit 2 of the destination predicate, r10, is 1. Therefore the computation
415 of the result is relevant.
416 * RA is zero therefore bit 2 is zero. mask is 0b0011 and mode is 0b0000
417 * When calculating n0 thru n3 we get n0=1, n1=2, n2=0, n3=0
418 * Therefore, CR9 is set (using LSB0 ordering) to 0b0011, i.e. to mask.
420 It should be clear that this instruction uses bits of the integer
421 predicate to decide whether to set CR Fields to `(mask & ~mode)` or
422 to zero. Thus, in effect, it is the integer predicate that has been
423 copied into the CR Fields.
425 By using twin predication, zeroing, and inversion (sm=~r3, dm=r10) for
426 example, it becomes possible to combine two Integers together in order
427 to set bits in CR Fields. Likewise there are dozens of ways that CR
428 Predicates can be used, on the same sv.mtcrweird instruction.