bitmap_parallelism_extension.mdwn

   1 # Parallelism using Bitmaps
   2
   3 If you think about it this way you can combine setvl, and predication,
   4 and indeed vector length, by always working with bitmaps.
   5
   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)
   8
   9 Each contains a bitmap of length 32 (assuming we only have the standard
  10 registers)
  11
  12 By default, X0 contains 1<<0, X1 contains 1<<1, X2 contains  1 << 2, ...
  13
  14 now an instruction like
  15
  16     add x1 x2 x3
  17
  18 is reinterpreted as referring to the CSR's rather than individual
  19 registers.  i.e. under simple V it means
  20
  21     add X1, X2, X3
  22
  23 and it has the following semantics:
  24
  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
  28
  29
  30     parallelfor (rd, rs1, rs2)  in  (rds[i],rs1s[i], rs2s[i]) where i = 0 to length(rds) - 1
  31           add rd rs1 rs2
  32
  33
  34 example:
  35
  36     X1 <- 0b011111
  37     X2 <- 0b1011
  38     X3 <-  0b00010
  39
  40 then
  41     rd1s = [x1, x2, x3, x4, x5]
  42     rs1s = [x0, x2, x3, x0, x2]
  43     rs2s = [x3, x3, x3, x3, x3]
  44
  45 and
  46
  47     add X1, X2, X3
  48
  49 is interpreted as
  50
  51     parallel{
  52          add x1, x0, x3
  53          add x2, x2, x3
  54          add 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!
  57     }
  58
  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.
  64
  65 Notes:
  66
  67 > > Thinking about it more, a bitset for X0 seems a bad idea, or equivalently X0
  68 > > should be
  69 > > the immutable  bitset {x0}. That suggests FX0, ... FX31 _is_ a good idea.
  70
  71 >  what would it mean, to do ops with x0?  it would mean "always add 0"
  72 > and so on.  it sounds kinda useful.  like MV being add r1, r2, x0.
  73 > it would completely pointless to *have* anything other than "all 1s"
  74 > in it though i think :)
  75
  76 # pseudocode for decoding ops
  77
  78     uint32 XB[32];   // global, assume RV32 for now: CSRs for bitmapping
  79     uint32 regs[32]; // global, actual (integer) register file
  80
  81     // gets current ACTUAL register to be used
  82     // XB had better not be empty...
  83     int regdecode(int rn, int *offs)
  84     {
  85         int bmap = XB[rn];
  86         int _offs = *offs;
  87         while (1)
  88         {
  89             int _newoffs = (_offs + 1) & 0x1f; // 32 regs, modulo
  90             if (bmap & (1<<_offs))
  91             {
  92                 *offs = _newoffs;
  93                 return _offs;
  94             }
  95             _offs = _newoffs;
  96         }
  97     }
  98
  99 example usage (pseudo-implementation of add):
 100
 101     op_add(int rd, int rs1, int rs2)
 102     {
 103         int id=0, irs1=0, irs2=0;
 104         int VL = pcnt(XB[rd];
 105         for (int i = 0; i < VL; i++)
 106         {
 107             int actualrd  = regdecode(rd , &id);
 108             int actualrs1 = regdecode(rs1, &irs1);
 109             int actualrs2 = regdecode(rs2, &irs2);
 110             regs[actualrd] = regs[actualrs1] + regs[actualrs2];
 111         }
 112     }
 113