1 # SVP64 Branch Conditional behaviour
5 * <https://bugs.libre-soc.org/show_bug.cgi?id=664>
6 * <http://lists.libre-soc.org/pipermail/libre-soc-dev/2021-August/003416.html>
7 * [[openpower/isa/branch]]
9 Scalar 3.0B Branch Conditional operations, `bc`, `bctar` etc. test a Condition Register.
10 When doing so in a Vector Context, it is quite reasonable and logical to test a *Vector* of
11 CR Fields. In 3D Shader binaries, which are inherently parallelised
12 and predicated, testing all or some results and branching based on
13 multiple tests is extremely common, and a fundamental part of
15 Therefore, `sv.bc` and other Vector-aware Branch Conditional instructions are worth
18 The `BI` field of Branch Conditional operations is five bits,
19 in scalar v3.0B this would select one bit of the 32 bit CR.
20 In SVP64 there are 16 32 bit CRs, containing 128 4-bit CR Fields.
21 Therefore, the 2 LSBs of `BI` select the bit from the CR Field
22 (EQ LT GT SO), and the
23 top 3 bits are extended to either scalar or vector and to
24 select CR Fields 0..127 as specified
25 in SVP64 [[sv/svp64/appendix]]
27 When considering an "array" of branches, there are two useful modes:
29 * Branch takes place on the first CR test to succeed
30 (a Great Big OR of all condition tests)
31 * Branch takes place only if **all** CR tests succeed:
32 a Great Big AND of all condition tests
33 (including those where the predicate is masked out
34 and the corresponding CR Field is considered to be
37 In SVP64 Horizontal-First Mode, the first failure
38 in ALL mode (Great Big AND) results in early exit: no more updates to
39 CTR occur (if requested); no branch occurs, and LR is
40 not updated (if requested). Likewise
41 for non-ALL mode (Great Big Or) on first success early
42 exit also occurs, however this time with the Branch proceeding.
43 In both cases the testing of the Vector of CRs should be
44 done in linear sequential order (or in REMAP re-sequenced order):
45 such that tests that are sequentially beyond the exit point are *not*
46 carried out. (*Note: is standard practice in Programming
47 languages to exit early from conditional tests*)
49 In Vertical-First Mode, the `ALL` bit should
50 not be used. If set, behaviour is `UNDEFINED`.
51 (*The reason is that Vertical-First hints may permit
52 multiple elements up to hint length to be executed
53 in parallel, however the number is entirely up to
54 implementors. Attempting to test an arbitrary
55 indeterminate number of Conditional tests is impossible
56 to define, and efforts to enforce such defined behaviour
57 interfere with Vertical-First mode parallel
58 opportunistic behaviour.*)
61 the whole CR Field, part of which is
62 selected by `BI` (top 3 bits), is updated based on
63 incrementing srcstep and dststep, and performing the
64 same tests as [[sv/svstep]]. Following the step
65 update, which involved writing to the exact
66 CR Field about to be tested, the Branch
67 Conditional instruction proceeds as normal (reading
68 and testing the CR bit just updated, if the relevant
69 `BO` bit is set). Note that the SVSTATE fields
70 are still updated, and the CR field still updated,
71 even if the `BO` bits do not require CR testing.
73 Predication in both INT and CR modes may be applied to
74 `sv.bc` and other SVP64 Branch Conditional operations,
75 exactly as they may be applied to other SVP64 operations.
76 When `sz` is zero, any masked-out Branch-element operations
77 are not executed, exactly like all other SVP64
80 However when `sz` is non-zero, this normally requests insertion
81 of a zero in place of the input data, when the relevant predicate
82 mask bit is zero. This would mean that a zero is inserted in
83 place of `CR[BI+32]` for testing against `BO`, which may not
84 be desirable in all circumstances. Therefore, an extra field
85 is provided `SNZ`, which, if set, will insert a **one** in
86 place of a masked-out element instead of a zero.
88 (*Note: Both options are provided because it is useful to
89 deliberately cause the Branch-Conditional Vector testing
90 to fail at a specific point, controlled by the Predicate
91 mask. This is particularly useful in `VLSET` mode, which
92 will truncate SVSTATE.VL at the point of the first failed
95 SVP64 RM `MODE` for Branch Conditional:
97 | 0-1 | 2 | 3 4 | description |
98 | --- | --- |---------|-------------------------- |
99 | 00 | SNZ | ALL sz | normal mode |
100 | 01 | VLI | ALL sz | VLSET mode |
101 | 10 | SNZ | ALL sz | svstep mode |
102 | 11 | VLI | ALL sz | svstep VLSET mode, in Horizontal-First |
103 | 11 | VLI | SNZ sz | svstep VLSET mode, in Vertical-First |
107 * **sz** if predication is enabled will put 4 copies of `SNZ` in place of the src CR Field when the predicate bit is zero. otherwise the element is ignored or skipped, depending on context.
108 * **ALL** when set, all branch conditional tests must pass in order for
109 the branch to succeed.
110 * **VLI** In VLSET mode, VL is set equal (truncated) to the first branch
111 which succeeds. If VLI (Vector Length Inclusive) is clear, VL is truncated
112 to *exclude* the current element, otherwise it is included. SVSTATE.MVL is not changed.
114 svstep mode will run an increment of SVSTATE srcstep and dststep
115 (which is still useful in Horizontal First Mode). Unlike `svstep.` however
116 which updates only CR0 with the testing of REMAP loop progress,
117 the CR Field is taken from the branch `BI` field, and updated
118 prior to proceeding to each element branch conditional testing.
120 Note that, interestingly, due to the useful side-effects of `VLSET` mode
121 and `svstep` mode it is actually useful to use Branch Conditional even
122 to perform no actual branch operation, i.e to point to the instruction
125 In particular, svstep mode is still useful for Horizontal-First Mode
126 particularly in combination with REMAP. All "loop end" conditions
127 will be tested on a per-element basis and placed into a Vector of
128 CRs starting from the point specified by the Branch `BI` field.
129 This Vector of CR Fields may then be subsequently used as a Predicate
130 Mask, and, furthermore, if VLSET mode was requested, VL will have
131 been set to the length of one of the loop endpoints, again as specified
132 by the bit from the Branch `BI` field.
134 Also, the unconditional bit `BO[0]` is still relevant when Predication
135 is applied to the Branch because in `ALL` mode all nonmasked bits have
136 to be tested. Even when svstep mode or VLSET mode are not used, CTR
137 may still be decremented by the total number of nonmasked elements.
138 In short, Vectorised Branch becomes an extremely powerful tool.
140 Available options to combine:
142 * `BO[0]` to make an unconditional branch would seem irrelevant if
143 it were not for predication and for side-effects.
144 * `BO[1]` to select whether the CR bit being tested is zero or nonzero
145 * `R30` and `~R30` and other predicate mask options including CR and
146 inverted CR bit testing
147 * `sz` and `SNZ` to insert either zeros or ones in place of masked-out
149 * `ALL` or `ANY` behaviour corresponding to `AND` of all tests and
150 `OR` of all tests, respectively.
152 Pseudocode for Rc in sv.bc
155 # Use bit 30, disable AA
160 Pseudocode for Rc in sv.bclr
166 Pseudocode for Horizontal-First Mode:
169 cond_ok = not SVRMmode.ALL
170 for srcstep in range(VL):
171 new_srcstep, CRbits = SVSTATE_NEXT(srcstep)
172 # select predicate bit or zero/one
173 if predicate[srcstep]:
174 # get SVP64 extended CR field 0..127
175 SVCRf = SVP64EXTRA(BI>>2)
176 if Rc = 1 then # CR0 Vectorised
177 CR{0+srcstep} = CRbits
178 testbit = CRbits[BI & 0b11]
179 # testbit = CR[BI+32+srcstep*4]
180 else if not SVRMmode.sz:
183 testbit = SVRMmode.SNZ
184 # actual element test here
185 el_cond_ok <- BO[0] | ¬(testbit ^ BO[1])
188 cond_ok &= el_cond_ok
190 cond_ok |= el_cond_ok
191 # test for VL to be set (and exit)
192 if ~el_cond_ok and VLSET
194 SVSTATE.VL = srcstep+1
207 Pseudocode for Vertical-First Mode:
210 new_srcstep, CRbits = SVSTATE_NEXT(srcstep)
211 # select predicate bit or zero/one
212 if predicate[srcstep]:
213 # get SVP64 extended CR field 0..127
214 SVCRf = SVP64EXTRA(BI>>2)
215 if Rc = 1 then # CR0 vectorised
216 CR{0+srcstep} = CRbits
217 testbit = CRbits[BI & 0b11]
218 else if not SVRMmode.sz:
219 SVSTATE.srcstep = new_srcstep
220 exit # no branch testing
222 testbit = SVRMmode.SNZ
223 # actual element test here
224 cond_ok <- BO[0] | ¬(testbit ^ BO[1])
225 # test for VL to be set (and exit)
226 if ~cond_ok and VLSET
228 SVSTATE.VL = new_srcstep+1
230 SVSTATE.VL = new_srcstep
231 SVSTATE.srcstep = new_srcstep
234 # Example Shader code
246 which compiles to something like:
251 pred loop_pred = a > 2;
252 while(loop_pred.any()) {
253 pred if_pred = loop_pred & (b < 5);
258 pred else_pred = loop_pred & ~if_pred;
259 if(else_pred.any()) {
266 which will end up as:
269 sv.cmpi CR60.v a.v, 2 # vector compare a into CR60 vector
270 sv.crweird r30, CR60.GT # transfer GT vector to r30
272 sv.cmpi CR80.v, b.v, 5 # vector compare b into CR64 Vector
273 sv.bc/m=r30/~ALL/sz CR80.v.LT skip_f # skip when none
274 # only calculate loop_pred & pred_b because needed in f()
275 sv.crand CR80.v.SO, CR60.v.GT, CR80.V.LT # if = loop & pred_b
278 # illustrate inversion of pred_b. invert r30, test ALL
279 # rather than SOME, but masked-out zero test would FAIL,
280 # therefore masked-out instead is tested against 1 not 0
281 sv.bc/m=~r30/ALL/SNZ CR80.v.LT skip_g
282 # else = loop & ~pred_b, need this because used in g()
283 sv.crternari(A&~B) CR80.v.SO, CR60.v.GT, CR80.V.LT
286 # conditionally call h(r30) if any loop pred set
287 sv.bclr/m=r30/~ALL/sz BO[1]=1 h()
288 sv.bc/m=r30/~ALL/sz BO[1]=1 while_loop