alt_rvp.mdwn

   1 This RVP proposal is an alternative that uses only the integer register file.  It provides both SIMD groups equal to XLEN by partitioning the standard ALU, and provides SIMD groups larger than XLEN by effectively expanding the register file with additional lanes using a scheme inspired by the Hwacha microarchitecture.
   2
   3 (All examples on this page assume RV32.)
   4
   5 ----
   6
   7 # Partitioning
   8
   9 Each bit set in the "part" CSR inhibits carry-in to that position and defines an element boundary.  Only the packed-data instructions honor the part CSR.
  10
  11 ADD
  12
  13 |    | reg | 31..0 |
  14 | -- | --- | ----- |
  15 |    | rs1 |    x1 |
  16 | +  | rs2 |    y1 |
  17 | -> | rd  | x1+y1 |
  18
  19 PADD ("packed ADD") (with bits 5, 11, 16, 21, and 27 set in the "part" CSR for pairs of RGB565 data)
  20
  21 |    | reg | 31..27 | 26..21 | 20..16 | 15..11 | 10..5 | 4..0 |
  22 | -- | --- | ------ | ------ | ------ | ------ | ----- | ---- |
  23 |    | rs1 |   x2r  |   x2g  |   x2b  |   x1r  |  x1g  |  x1b |
  24 | +  | rs2 |   y2r  |   y2g  |   y2b  |   y1r  |  y1g  |  y1b |
  25 | -> | rd  | x2r+y2r| x2g+y2g| x2b+y2b| x1r+y1r|x1g+y1g| x1b+y1b|
  26
  27 ----
  28
  29 # Lanes
  30
  31 Each bit set in the "plane" CSR activates that lane.  Only bits corresponding to implemented lanes are writable.  Writing zero to "plane" disables all lanes, zeroes all registers except for lane 0, clears the status bits that indicate that other lanes need to be saved/restored, and stores "1" to "plane" to leave lane 0 active.
  32
  33 ADD (with "plane" CSR == 0x0000000F)
  34
  35 |    | reg | Lane 0 | Lane 1 | Lane 2 | Lane 3 |
  36 | -- | --- | ------ | ------ | ------ | ------ |
  37 |    | reg | 31..0  | 31..0  | 31..0  | 31..0  |
  38 |    | rs1 |   x1   |   x2   |   x3   |   x4   |
  39 | +  | rs2 |   y1   |   y2   |   y3   |   y4   |
  40 | -> | rd  | x1+y1  | x2+y2  | x3+y3  | x4+y4  |
  41
  42 Example parallel add:
  43
  44     /* XLEN and N are "baked-in" to the hardware */
  45     parameter XLEN;
  46     parameter N;
  47     /* note that N cannot be greater than XLEN */
  48
  49     register plane[XLEN];
  50     register x[N][32][XLEN];
  51
  52     function op_add(rd, rs1, rs2) {
  53        /* note that this is ADD, not PADD */
  54        int i;
  55        for (i = 0; i<N; i++)
  56           if (plane[i])
  57              x[i][rd] <= x[i][rs1] + x[i][rs2];
  58     }
  59     /* note that "<=" is the Verilog non-blocking assignment operator */
  60
  61 The above pseudocode works equally well for packed-ADD, by simply replacing the "+" operator with a packed-ADD.  All lanes use the shared part CSR for packed element boundaries.
  62
  63
  64 The reuse of the baseline operations makes trap-and-emulate for RVP lanes infeasible, but this seems to be less of a problem than it appears to be at first glance:  the entire purpose of RVP lanes is increased performance and lanes *can* be emulated by using software emulation until the plane CSR is written with zero.