# SV Vector Operations.
-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)
-
-Notes:
-
-* Some of these actually could be added to a scalar ISA as bitmanipulation instructions. These are separated out into their own section.
-* 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]].
-
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>
+* <https://bugs.libre-soc.org/show_bug.cgi?id=865> implementation in simulator
* <https://bugs.libre-soc.org/show_bug.cgi?id=213>
* <https://bugs.libre-soc.org/show_bug.cgi?id=142> specialist vector ops
- out of scope for this document
+ out of scope for this document [[openpower/sv/3d_vector_ops]]
* [[simple_v_extension/specification/bitmanip]] previous version,
contains pseudocode for sof, sif, sbf
+* <https://en.m.wikipedia.org/wiki/X86_Bit_manipulation_instruction_set#TBM_(Trailing_Bit_Manipulation)>
-# Vector
-
-## 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
-
-## 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.
+The core Power 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: details in [[discussion]]
- 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: 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
+Notes:
-These may all be viewed as suitable for fitting into a scalar bitmanip extension.
+* 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]].
-## sbfm
+# sbfm
sbfm RT, RA, RB!=0
The vmsbf.m instruction takes a mask register as input and writes results to a mask register. The instruction writes a 1 to all active mask elements before the first source element that is a 1, then writes a 0 to that element and all following active elements. If there is no set bit in the source vector, then all active elements in the destination are written with a 1.
-pseudocode:
+Executable pseudocode demo:
+
+```
+[[!inline quick="yes" raw="yes" pages="openpower/sv/sbf.py"]]
+```
- def sbf(rd, rs1, rs2):
- rd = 0
- # start setting if no predicate or if 1st predicate bit set
- setting_mode = rs2 == x0 or (regs[rs2] & 1)
- while i < XLEN:
- bit = 1<<i
- if rs2 != x0 and (regs[rs2] & bit):
- # reset searching
- setting_mode = False
- if setting_mode:
- if regs[rs1] & bit: # found a bit in rs1: stop setting rd
- setting_mode = False
- else:
- regs[rd] |= bit
- else if rs2 != x0: # searching mode
- if (regs[rs2] & bit):
- setting_mode = True # back into "setting" mode
- i += 1
-
-## 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.
vmsif.m v2, v3, v0.t
1 1 x x x x 1 1 v2 contents
-Pseudo-code:
-
- def sif(rd, rs1, rs2):
- rd = 0
- setting_mode = rs2 == x0 or (regs[rs2] & 1)
-
- while i < XLEN:
- bit = 1<<i
-
- # only reenable when predicate in use, and bit valid
- if !setting_mode && rs2 != x0:
- if (regs[rs2] & bit):
- # back into "setting" mode
- setting_mode = True
+Executable pseudocode demo:
- # skipping mode
- if !setting_mode:
- # skip any more 1s
- if regs[rs1] & bit == 1:
- i += 1
- continue
+```
+[[!inline quick="yes" raw="yes" pages="openpower/sv/sif.py"]]
+```
- # setting mode, search for 1
- regs[rd] |= bit # always set during search
- if regs[rs1] & bit: # found a bit in rs1:
- setting_mode = False
- # next loop starts skipping
-
- i += 1
-
-
-## 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.
vmsof.m v2, v3, v0.t
0 1 x x x x 0 0 v2 content
-Pseudo-code:
+Executable pseudocode demo:
- def sof(rd, rs1, rs2):
- rd = 0
- setting_mode = rs2 == x0 or (regs[rs2] & 1)
+```
+[[!inline quick="yes" raw="yes" pages="openpower/sv/sof.py"]]
+```
- while i < XLEN:
- bit = 1<<i
+# Carry-lookahead
- # only reenable when predicate in use, and bit valid
- if !setting_mode && rs2 != x0:
- if (regs[rs2] & bit):
- # back into "setting" mode
- setting_mode = True
+* cprop RT,RA,RB
+* cprop. RT,RA,RB
- # skipping mode
- if !setting_mode:
- # skip any more 1s
- if regs[rs1] & bit == 1:
- i += 1
- continue
+pseudocode:
- # setting mode, search for 1
- if regs[rs1] & bit: # found a bit in rs1:
- regs[rd] |= bit # only set when search succeeds
- setting_mode = False
- # next loop starts skipping
+ P = (RA)
+ G = (RB)
+ RT = ((P|G)+G)^P
- i += 1
+X-Form
-# Carry-lookahead
+| 0.5|6.10|11.15|16.20| 21..30 |31| name | Form |
+| -- | -- | --- | --- | --------- |--| ---- | ------- |
+| NN | RT | RA | RB | 0110001110 |Rc| cprop | X-Form |
used not just for carry lookahead, also a special type of predication mask operation.
`((P|G)+G)^P`
* <https://en.m.wikipedia.org/wiki/Carry-lookahead_adder>
+From QLSKY.png:
+
+```
+ x0 = nand(CIn, P0)
+ C0 = nand(x0, ~G0)
+
+ x1 = nand(CIn, P0, P1)
+ y1 = nand(G0, P1)
+ C1 = nand(x1, y1, ~G1)
+
+ x2 = nand(CIn, P0, P1, P2)
+ y2 = nand(G0, P1, P2)
+ z2 = nand(G1, P2)
+ C1 = nand(x2, y2, z2, ~G2)
+
+ # Gen*
+ x3 = nand(G0, P1, P2, P3)
+ y3 = nand(G1, P2, P3)
+ z3 = nand(G2, P3)
+ G* = nand(x3, y3, z3, ~G3)
+```
+
+```
P = (A | B) & Ci
G = (A & B)
+```
Stackoverflow algorithm `((P|G)+G)^P` works on the cumulated bits of P and G from associated vector units (P and G are integers here). The result of the algorithm is the new carry-in which already includes ripple, one bit of carry per element.
+```
At each id, compute C[id] = A[id]+B[id]+0
Get G[id] = C[id] > radix -1
Get P[id] = C[id] == radix-1
Join all P[id] together, likewise G[id]
- Compute newC = ((P|G)+G)^P << 1
+ Compute newC = ((P|G)+G)^P
result[id] = (C[id] + newC[id]) % radix
-
+```
two versions: scalar int version and CR based version.
projects / libreriscv.git / blobdiff