Links:
+* [[discussion]]
* <https://github.com/riscv/riscv-v-spec/blob/master/v-spec.adoc#vector-register-gather-instructions>
* <http://0x80.pl/notesen/2016-10-23-avx512-conflict-detection.html> conflictd example
* <https://lists.libre-soc.org/pipermail/libre-soc-dev/2022-May/004884.html>
* [[simple_v_extension/specification/bitmanip]] previous version,
contains pseudocode for sof, sif, sbf
-The core OpenPOWER ISA was designed as scalar: SV provides a level of abstraction to add variable-length element-independent parallelism. However, certain classes of instructions only make sense in a Vector context: AVX512 conflictd for example. This section includes such examples. Many of them are from the RISC-V Vector ISA (with thanks to the efforts of RVV's contributors)
+The core OpenPOWER ISA was designed as scalar: SV provides a level of abstraction to add variable-length element-independent parallelism.
+Therefore there are not that many cases where *actual* Vector
+instructions are needed. If they are, they are more "assistance"
+functions. Two traditional Vector instructions were initially
+considered (conflictd and vmiota) however they may be synthesised
+from existing SVP64 instructions and have been moved to [[discussion]]
Notes:
* Instructions suited to 3D GPU workloads (dotproduct, crossproduct, normalise) are out of scope: this document is for more general-purpose instructions that underpin and are critical to general-purpose Vector workloads (including GPU and VPU)
* Instructions related to the adaptation of CRs for use as predicate masks are covered separately, by crweird operations. See [[sv/cr_int_predication]].
-# Vector
-
-Both of these instructions may be synthesised from SVP64 Vector
-instructions. conflictd is an O(N^2) instruction based on
-`sv.cmpi` and iota is an O(N) instruction based on `sv.addi`
-with the appropriate predication
-
-## conflictd
-
-This is based on the AVX512 conflict detection instruction. Internally the logic is used to detect address conflicts in multi-issue LD/ST operations. Two arrays of values are given: the indices are compared and duplicates reported in a triangular fashion. the instruction may be used for histograms (computed in parallel)
-
- input = [100, 100, 3, 100, 5, 100, 100, 3]
- conflict result = [
- 0b00000000, // Note: first element always zero
- 0b00000001, // 100 is present on #0
- 0b00000000,
- 0b00000011, // 100 is present on #0 and #1
- 0b00000000,
- 0b00001011, // 100 is present on #0, #1, #3
- 0b00011011, // .. and #4
- 0b00000100 // 3 is present on #2
- ]
-
-Pseudocode:
-
- for i in range(VL):
- for j in range(1, i):
- if src1[i] == src2[j]:
- result[j] |= 1<<i
-
-Idea 1: implement this as a Triangular Schedule, Vertical-First Mode,
- using `mfcrweird` and `cmpi`. first triangular schedule on src1,
-secpnd on src2.
-
-Idea 2: implement using outer loop on varying setvl Horizontal-First
-with `1<<r3` predicate mask for src2 as scalar, creates CR field vector, transfer into INT with mfcrweird then OR into the
-result.
-
- li r3, 1
- li result, 0
- for i in range(target):
- setvl target
- sv.addi/sm=1<<r3 t0, src1.v, 0 # copy src1[i]
- sv.cmpi src2.v, t0 # compare src2 vector to scalar
- sv.mfcrweird t1, cr0.v, eq # copy CR eq result bits to t1
- srr t1, t1, i # shift up by i before ORing
- or result, result, t1
- srr r3, r3, 1 # shift r3 predicate up by one
-
-See [[sv/cr_int_predication]] for full details on the crweird instructions:
-the primary important aspect here is that a Vector of CR Field's EQ bits is
-transferred into a single GPR. The secondary important aspect is that VL
-is being adjusted in each loop, testing successively more of the input
-vector against a given scalar, each time.
-
-To investigate:
-
-* <https://stackoverflow.com/questions/39266476/how-to-speed-up-this-histogram-of-lut-lookups>
-* <https://stackoverflow.com/questions/39913707/how-do-the-conflict-detection-instructions-make-it-easier-to-vectorize-loops>
-
-## iota
-
-Based on RVV vmiota. vmiota may be viewed as a cumulative variant of popcount, generating multiple results. successive iterations include more and more bits of the bitstream being tested.
-
-When masked, only the bits not masked out are included in the count process.
-
- viota RT/v, RA, RB
-
-Note that when RA=0 this indicates to test against all 1s, resulting in the instruction generating a vector sequence [0, 1, 2... VL-1]. This will be equivalent to RVV vid.m which is a pseudo-op, here (RA=0).
-
-Example
-
- 7 6 5 4 3 2 1 0 Element number
-
- 1 0 0 1 0 0 0 1 v2 contents
- viota.m v4, v2 # Unmasked
- 2 2 2 1 1 1 1 0 v4 result
-
- 1 1 1 0 1 0 1 1 v0 contents
- 1 0 0 1 0 0 0 1 v2 contents
- 2 3 4 5 6 7 8 9 v4 contents
- viota.m v4, v2, v0.t # Masked
- 1 1 1 5 1 7 1 0 v4 results
-
- def iota(RT, RA, RB):
- mask = RB ? iregs[RB] : 0b111111...1
- val = RA ? iregs[RA] : 0b111111...1
- for i in range(VL):
- if RA.scalar:
- testmask = (1<<i)-1 # only count below
- to_test = val & testmask & mask
- iregs[RT+i] = popcount(to_test)
-
-a Vector CR-based version of the same, due to CRs being used for predication. This would use the same testing mechanism as branch: BO[0:2]
-where bit 2 is inv, bits 0:1 select the bit of the CR.
-
- def test_CR_bit(CR, BO):
- return CR[BO[0:1]] == BO[2]
-
- def iotacr(RT, BA, BO):
- mask = get_src_predicate()
- count = 0
- for i in range(VL):
- if mask & (1<<i) == 0:
- count = 0 # reset back to zero
- continue
- iregs[RT+i] = count
- if test_CR_bit(CR[i+BA], BO):
- count += 1
-
-the variant of iotacr which is vidcr, this is not appropriate to have BA=0, plus, it is pointless to have it anyway. The integer version covers it, by not reading the int regfile at all.
-
-scalar variant which can be Vectorised to give iotacr:
-
- def crtaddi(RT, RA, BA, BO, D):
- if test_CR_bit(BA, BO):
- RT = RA + EXTS(D)
- else:
- RT = RA
-
-a Vector for-loop with zero-ing on dest will give the
-mask-out effect of resetting the count back to zero.
-However close examination shows that the above may actually
-be `sv.addi/mr/sm=EQ/dz r0.v, r0.v, 1`
-
-# Scalar
-
-These may all be viewed as suitable for fitting into a scalar bitmanip extension.
-
-## sbfm
+# sbfm
sbfm RT, RA, RB!=0
[[!inline quick="yes" raw="yes" pages="openpower/sv/sbf.py"]]
```
-## sifm
+# sifm
The vector mask set-including-first instruction is similar to set-before-first, except it also includes the element with a set bit.
[[!inline quick="yes" raw="yes" pages="openpower/sv/sif.py"]]
```
-## vmsof
+# vmsof
The vector mask set-only-first instruction is similar to set-before-first, except it only sets the first element with a bit set, if any.
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