1 # Parallelism using Bitmaps
3 If you think about it this way you can combine setvl, and predication,
4 and indeed vector length, by always working with bitmaps.
6 So: you have 32 WARL CSRs , called X0, ... X31 (or perhaps 2 banks of
7 32 CSR's and have a set of additional CSR's FX0,... FX31)
9 Each contains a bitmap of length 32 (assuming we only have the standard
12 By default, X0 contains 1<<0, X1 contains 1<<1, X2 contains 1 << 2, ...
14 now an instruction like
18 is reinterpreted as referring to the CSR's rather than individual
19 registers. i.e. under simple V it means
23 and it has the following semantics:
25 let rds = registers in bitmap X1
26 let rs1s = registers in bitmap X2 repeated periodically in order of register number to the length of X1
27 let rs2s = registers in bitmap X3 repeated periodically in order of register number to the length of X1
30 parallelfor (rd, rs1, rs2) in (rds[i],rs1s[i], rs2s[i]) where i = 0 to length(rds) - 1
41 rd1s = [x1, x2, x3, x4, x5]
42 rs1s = [x0, x2, x3, x0, x2]
43 rs2s = [x3, x3, x3, x3, x3]
55 add x4, x0, x3 # x2 and x3 have their original values!
56 add x5, x2, x3 # x2 and x3 have their original values!
59 This means that the analogue of setvl is simply the "write any" of
60 setting the bitmap, and the analogue of the return value of setvl,
61 is the "read legal" of the CSR. Moreover popc would tell you how many
62 operations are scheduled in parallel so you know how often you have to
63 repeat a sequential loop.