Note of RISC-V Vector ISA Spec v0.6

Calendar December 15, 2018

Vector regfile#

  • 32 of them, v0 to v31
  • Each is VLEN bits
  • Each can be divided into several elements
    • The max element width is ELEN
    • CSR vsew maps to SEW (standard element width) controls their width dynamically
    • CSR vl controls the number of elements to operate on for vector instructions
  • Packing of shorter vector
    • when SEW is smaller than ELEN, multiple SEW will be packed into one ELEN unit
      • Following little-endian rule
    • ELEN units are packed into VLEN register also
      • Following little-endiam rule
  • Storage of longer vector
    • If operand longer than SEW is needed, then
      • Even-numbered vector register holds the even-numbered elements
      • Odd-numbered vector register holds the odd-numberred elements
      • WHY? the author said it’s designed to simplify data path alignment
  • Register grouping via vlmul field
    • Any operation applies on one vector register n will apply on all the other vector registers in the same group
    • Depending on the vlmul, 00 = no group, 01 = v[n+16], 02 = v[n+8], 03 = v[n+4]
      • VLMAX will change accordingly, by double, quarod, 8 times
  • Reuse for floating-point regfile
    • Floating-point registers reside at the LSB FLEN of the vector registers
    • Lower precision floating-point types are NaN-boxed (MSB bits are set to 'b1)
    • Loading floating-point data doesn’t change the upper bits where VLEN > FLEN

Vector operation#

  • Masking
    • Masked off elements are skipped, and will not generate exceptions
    • Use the LSB of each SEW in v0 as the masking bit
    • 4 types in vm[1:0]: true, false, scalar, no-masking
  • Vector load/store
    • Unit-stride
      • Starting address = base address in RS1 + imm offset
      • Fault-first version
        • WHY?
    • Constant-stride
      • Base in RS1, stride in RS2, by an unit of byte
    • Indexed (scatter-gather)
      • Base in RS1, indices in VRS2 (signed integers)
        • If SEW is larger than XLEN, LSB bits are taken
      • For store, there is an extra unordered-indexed load, in contrast to ordered-indexed load
        • JW: we can make unordered version to be higher performance
  • Vector arithmetic
    • Widening operation with w suffix puts 2xSEW wide destination into an even-odd vector register pair
    • Merge: use the mask field to merge 2 source operands
    • Narrowing: convert multi-width vector into single-width vector
    • Reduction: scalar <= vector op scalar
    • Matrix multiplication support
      • Fused-multiply-add + reduction
    • Mask
      • Mask population count
      • Find-first-set mask bit
      • viota: count the number of elements in v0 that’s masked or un-masked
        • Can be combined with scatter/gather instructions to perform vector compress/expand instructions
        • They are still discussing if this should be able to take any registers, not only v0
      • Set-before-frst mask bit
    • Permutation
      • insert/extract
      • slide
      • gather

Exception#

  • CSR progress logs the element index that caused the trap
    • With this, failed operations can resume from where exactly it traps exception
  • Precise vs imprecise traps
    • Precise trap: slow, but support debug
    • Imprecise trap: fast, but possibly obscure error conditions
    • Implementations can choose