5 * <https://bugs.libre-soc.org/show_bug.cgi?id=238>
6 * <https://ftp.libre-soc.org/VLE_314-68105.pdf> VLE Encoding
8 This one is a conundrum. OpenPOWER ISA was never designed with 16
9 bit in mind. VLE was added 10 years ago but only by way of marking
10 an entire 64k page as "VLE". With VLE not maintained it is not
11 fully compatible with current PowerISA.
13 Here, in order to embed 16 bit into a predominantly 32 bit stream the
14 overhead of using an entire 16 bits just to switch into Compressed mode
15 is itself a significant overhead. The situation is made worse by 5 bits
16 being taken up by Major Opcode space, leaving only 11 bits to allocate
17 to actual instructions.
19 In addition we would like to add SV-C32 which is a Vectorised version
20 of 16 bit Compressed, and ideally have a variant that adds the 27-bit
21 prefix format from SV-P64, as well.
23 Potential ways to reduce pressure on the 16 bit space are:
25 * To provide "paging". This involves bank-switching to alternative optimised encodings for specific workloads
26 * To enter "16 bit mode" for durations specified at the start
27 * To reserve one bit of every 16 bit instruction to indicate that the 16 bit mode is to continue to be sustained
29 This latter would be useful in the Vector context to have an alternative
30 meaning: as the bit which determines whether the instruction is 11-bit
31 prefixed or 27-bit prefixed:
33 0 1 2 3 4 5 6 7 8 9 a b c d e f |
34 |major op | 11 bit vector prefix|
35 |16 bit opcode alt vec. mode ^ |
36 | extra vector prefix if alt set|
38 Using a major opcode to enter 16 bit mode, leaves 11 bits to find
39 something to use them for:
41 0 1 2 3 4 5 6 7 8 9 a b c d e f |
42 |major op | what to do here 1 |
43 |16 bit stay in 16bit mode 1 |
44 |16 bit stay in 16bit mode 1 |
45 |16 bit exit 16bit mode 0 |
47 One possibility is that the 11 bits are used for bank selection, with
48 some room for additional context such as altering the registers used
49 for the 16 bit operations (bank selection of which scalar regs)
51 Another is to use the 11 bits for only the utmost commonly used
52 instructions. That being the case then even one of those 11 bits would
53 also need to be dedicated to saying if 16 bit mode is to be continued.
54 10 bits remain for actual opcodes!
56 # Opcode Allocation Ideas
58 * one bit from the 16-bit mode is used to indicate that 32-bit mode
59 is to be dropped into for only one single instruction
60 <https://bugs.libre-soc.org/show_bug.cgi?id=238#c2>
62 ## Opcodes exploration (Attempt 1)
64 Switching between different encoding modes is controlled by M (alone)
65 in 10-bit mode, and M and N in 16-bit mode.
67 * M in 10-bit mode if zero indicates that following instructions are
68 standard OpenPOWER ISA 32-bit encoded (including, redundantly,
69 further 10/16-bit instructions)
70 * M in 10-bit mode if 1 indicates that following instructions are
71 in 16-bit encoding mode
75 * 0b01 (M=1, N=0): stay in 16-bit mode
76 * 0b00: leave 16-bit mode permanently (return to standard OpenPOWER ISA)
77 * 0b10: leave 16-bit mode for one cycle (return to standard OpenPOWER ISA)
78 * 0b11: free to be used for something completely different.
80 The current "top" idea for 0b11 is to use it for a new encoding format
81 of predominantly "immediates-based" 16-bit instructions (branch-conditional,
84 The Compressed Major Opcode is in bits 5-7.
86 * M+N mode-switching is not available for C-Major 0b001 or 0b111
90 only available in 16-bit mode, and only available when M=1 and N=1
92 | 0 | 1 | 2 3 4 | | 567.8 | 9ab | c d e | f |
93 | 1 | o2 | RT | | 010.1 | RB|0 | offs | addi.
94 | 1 | o2 | RT | | 011.1 | RB|0 | offs | addis.
95 | 1 | o2 | 0 | | 100.1 | RB | offs | cmpdi
96 | 1 | o2 | 1 | | 100.1 | RB | offs | cmpwi
97 | 1 | o2 | 0 | | 101.1 | RA | offs | ldi
98 | 1 | o2 | 1 | | 101.1 | RA | offs | lwi
99 | 1 | o2 | 0 | | 110.1 | RA | offs | flwi
100 | 1 | o2 | 1 | | 110.1 | RA | offs | fldi
102 * Note that bc is included (below)
103 * immediate is constructed from offs (LSBs) and o2 (MSB)
104 * for loads, offset is aligned. 8byte: o2||offs||0b000 4byte: 0b00
105 * RB|0 if RB is zero, addi. becomes "li"
109 10 bit mode may be expanded by 16 bit mode later, adding capabilities
110 that do not fit in the extreme limited space.
112 | 16-bit mode | | 10-bit mode |
113 | 0 | 1 | 234 | | 567.8 | 9 ab | c de | f |
114 | 0 | 0 000 | | 000.0 | 0 00 | 0 00 | 0 | illeg
115 | 0 | 0 000 | | 000.1 | 0 00 | 0 00 | 0 | nop
116 | N | offs2 | | 000.LK | offs!=0 | M | b, bl
117 | 1 | offs2 | | 000.LK | BI | BO1 oo | 1 | bc, bcl
118 | N | BO3 BI3 | | 001.0 | LK BI | BO | M | bclr, bclrl
122 * bc only available when N,M=0b11
123 * offs2 extends offset in MSBs
124 * BI3 extends BI in MSBs to allow selection of full CR
126 * bc offset constructed from oo as LSBs and offs2 as MSBs
127 * bc BI allows selection of all bits from CR0 or CR1
128 * bc CR check is always active (as if BO0=1) therefore BO1 inverts
132 * illegal (all zeros) covers part of branch (offs=0,M=0,LK=0)
133 * nop also covers part of branch (offs=0,M=0,LK=1)
134 * bc **not available** in 10-bit mode
135 * BO[0] enables CR check, BO[1] inverts check
136 * BI refers to CR0 only (4 bits of)
137 * no Branch Conditional with immediate
138 * no Absolute Address
139 * CTR mode allowed with BO[2] for b only.
140 * offs is to 2 byte (signed) aligned
141 * all branches to 2 byte aligned
145 | 16-bit mode | | 10-bit mode |
146 | 0 | 1 | 2 3 4 | | 567.8 | 9 a b | c d e | f |
147 | RB2 | RA2 | RT | | 001.1 | 1 RA | 0 RB | M | fld
148 | RA2 | RT2 | RB | | 001.1 | 1 RA | 1 RT | M | fst
149 | | | RT | | 111.0 | RA | RB | M | ld
150 | | | RB | | 111.1 | RA | RT | M | st
152 * elwidth overrides can set different widths
157 * RA2 extends RA to 3 bits (MSB)
158 * RT2 extends RT to 3 bits (MSB)
162 * RA and RB are only 2 bit (0-3)
163 * for LD, RT is implicitly RB: "ld RT=RB, RA(RB)"
164 * for ST, there is no offset: "st RT, RA(0)"
168 | 16-bit mode | | 10-bit mode |
169 | 0 | 1 | 2 3 4 | | 567.8 | 9ab | c d e | f |
170 | N | | RT | | 010.0 | RB | RA!=0 | M | add
171 | N | | RT | | 010.1 | RB | RA | M | mul
172 | N | | RT!=0 | | 011.0 | RB | RA!=0 | M | sub.
173 | N | 0 | 000 | | 011.0 | RB | RA!=0 | M | cmpw
174 | N | 1 | 000 | | 011.0 | RB | RA!=0 | M | cmpl
175 | N | | RT | | 011.0 | RB | 000 | M | neg.
179 * sub. default CR target is CR0
180 * for (RA|0) when RA=0 the input is a zero immediate,
181 meaning that sub. becomes neg.
182 * RT is implicitly RB: "add RT(=RB), RA, RB"
186 | 16-bit mode | | 10-bit mode |
187 | 0 | 1 | 2 3 4 | | 567.8 | 9ab | c d e | f |
188 | N | 0 | RT | | 100.0 | RB | RA!=0 | M | and
189 | N | 0 | RT | | 100.1 | RB | RA!=0 | M | nand
190 | N | 0 | RT | | 101.0 | RB | RA!=0 | M | or
191 | N | 0 | RT | | 101.1 | RB | RA!=0 | M | nor
192 | N | 0 | RT | | 100.0 | RB | 0 0 0 | M | extsw
193 | N | 0 | RT | | 100.1 | RB | 0 0 0 | M | cntlz
194 | N | 0 | RT | | 101.0 | RB | 0 0 0 | M | popcnt
195 | N | 0 | RT | | 101.1 | RB | 0 0 0 | M | not
199 | 0 | 1 | 2 3 4 | | 567.8 | 9ab | c d e | f |
200 | N | 1 | RT | | 100.0 | RB | RA!=0 | M |
201 | N | 1 | RT | | 100.1 | RB | RA!=0 | M |
202 | N | 1 | RT | | 101.0 | RB | RA!=0 | M | xor
203 | N | 1 | RT | | 101.1 | RB | RA!=0 | M | eqv (xnor)
204 | N | 1 | RT | | 100.0 | RB | 0 0 0 | M | extsb
205 | N | 1 | RT | | 100.1 | RB | 0 0 0 | M | cnttz
206 | N | 1 | RT | | 101.0 | RB | 0 0 0 | M |
207 | N | 1 | RT | | 101.1 | RB | 0 0 0 | M | extsh
211 * for (RA|0) when RA=0 the input is a zero immediate,
212 meaning that nor becomes not
213 * cntlz, popcnt, exts **not available** in 10-bit mode
214 * RT is implicitly RB: "and RT(=RB), RA, RB"
218 Note here that elwidth overrides (SV Prefix) can be used to select FP16/32/64
220 | 16-bit mode | | 10-bit mode |
221 | 0 | 1 | 2 3 4 | | 567.8 | 9ab | c d e | f |
222 | N | | RT | | 011.1 | RB | RA!=0 | M | fsub.
223 | N | 0 | RT | | 110.0 | RB | RA!=0 | M | fadd
224 | N | 0 | RT | | 110.1 | RB | RA!=0 | M | fmul
225 | N | 0 | RT | | 011.1 | RB | 0 0 0 | M | fneg.
226 | N | 0 | RT | | 110.0 | RB | 0 0 0 | M |
227 | N | 0 | RT | | 110.1 | RB | 0 0 0 | M |
231 | 0 | 1 | 2 3 4 | | 567.8 | 9ab | c d e | f |
232 | N | 1 | RT | | 011.1 | RB | RA!=0 | M |
233 | N | 1 | RT | | 110.0 | RB | RA!=0 | M |
234 | N | 1 | RT | | 110.1 | RB | RA!=0 | M | fdiv
235 | N | 1 | RT | | 011.1 | RB | 0 0 0 | M | fabs.
236 | N | 1 | RT | | 110.0 | RB | 0 0 0 | M | fmr.
237 | N | 1 | RT | | 110.1 | RB | 0 0 0 | M |
241 * fsub. fneg. and fmr. default target is CR1
242 * fmr. is **not available** in 10-bit mode
243 * fdiv is **not available** in 10-bit mode
247 * fmr. copies RB to RT (and sets CR1)
249 ### Condition Register
251 | 16-bit mode | | 10-bit mode |
252 | 0 1 2 3 | 4 | | 567.8 | 9 ab | cde | f |
253 | 0 0 0 0 | BF2 | | 001.1 | 0 BF | BFA | M | mcrf
254 | 0 0 0 1 | BA2 | | 001.1 | 0 BA | BB | M | crnor
255 | 0 1 0 0 | BA2 | | 001.1 | 0 BA | BB | M | crandc
256 | 0 1 1 0 | BA2 | | 001.1 | 0 BA | BB | M | crxor
257 | 0 1 1 1 | BA2 | | 001.1 | 0 BA | BB | M | crnand
258 | 1 0 0 0 | BA2 | | 001.1 | 0 BA | BB | M | crand
259 | 1 0 0 1 | BA2 | | 001.1 | 0 BA | BB | M | creqv
260 | 1 1 0 1 | BA2 | | 001.1 | 0 BA | BB | M | crorc
261 | 1 1 1 0 | BA2 | | 001.1 | 0 BA | BB | M | cror
265 * mcrf BF is only 2 bits which means the destination is only CR0-CR3
266 * CR operations: **not available** in 10-bit mode
270 * mcrf BF2 extends BF (in MSB) to 3 bits
271 * CR operations: destination register is same as BA.
272 * CR operations: only possible on CR0 and CR1
276 * CR operations: greatly extended reach/range (useful for predicates)
280 cbank: Selection of Compressed-encoding "Bank". Different "banks" give different
282 Example: CBank=0b001 is heavily optimised to A/Video
285 | 16-bit mode | | 10-bit mode |
286 | 0 1 | 2 3 4 | | 567.8 | 9 a b | c d e | f |
287 | Bank2 | | 010.0 | CBank | 0 0 0 | M | cbank
289 **not available** in 10-bit mode:
291 | 0 1 2 3 | 4 | | 567.8 | 9 ab | c d e | f |
292 | 1 1 1 1 | 0 | | 001.1 | 0 00 | RT | M | mtlr
293 | 1 1 1 1 | 0 | | 001.1 | 0 01 | RT | M | mtctr
294 | 1 1 1 1 | 0 | | 001.1 | 0 11 | RT | M | mtcr
295 | 1 1 1 1 | 1 | | 001.1 | 0 00 | RA | M | mflr
296 | 1 1 1 1 | 1 | | 001.1 | 0 01 | RA | M | mfctr
297 | 1 1 1 1 | 1 | | 001.1 | 0 11 | RA | M | mfcr
301 | 0 1 2 3 | 4 | | 567.8 | 9 ab | c d e | f |
302 | 0 0 1 0 | | | 001.1 | 0 | 0 | M |
303 | 0 0 1 1 | | | 001.1 | 0 | 0 | M |
304 | 0 1 0 1 | | | 001.1 | 0 | 0 | M |
305 | 1 0 1 0 | | | 001.1 | 0 | 0 | M |
306 | 1 0 1 1 | | | 001.1 | 0 | 0 | M |
307 | 1 1 0 0 | | | 001.1 | 0 | 0 | M |
308 | 1 1 1 1 | 0 | | 001.1 | 0 10 | 0 | M |
309 | 1 1 1 1 | 1 | | 001.1 | 0 10 | 0 | M |