C.9-10Vector Examples II: strcmp & Fractional LMUL

Two examples that show the vector ISA’s subtler features earning their keep: fault-only-first for unknown-length data, and fractional LMUL for register-pressure relief.

strcmp — the data-dependent loop

strcmp:
    li   t1, 0
loop:
    vsetvli t0, x0, e8, m2, ta, ma   # max-length byte strip
    add  a0, a0, t1
    vle8ff.v v8,  (a0)               # FOF: safe load past unknown end
    add  a1, a1, t1
    vle8ff.v v16, (a1)
    vmseq.vi v0, v8, 0               # NUL in src1?
    vmsne.vv v1, v8, v16             # bytes differ?
    vmor.mm  v0, v0, v1              # either = exit
    vfirst.m a2, v0                  # first exit position (-1 = none)
    csrr t1, vl                      # bytes actually fetched
    bltz a2, loop                    # no exit → next strip
    ...                              # compare the differing byte, return

The whole point is vle8ff.v: strcmp can’t know the string length, so a fixed-width load could fault reading past the end into an unmapped page. Fault-only-first loads as many bytes as are safely accessible, truncating vl at the fault boundary — then the mask logic (vmseq/vmsne/vmor + vfirst) finds the first NUL-or-mismatch across the whole strip in parallel.

Fractional LMUL — register pressure

The C.10 loop mixes one signed char array with a dozen long arrays. Holding SEW/LMUL constant, the byte data rides at a fractional LMUL (occupying a fraction of one register) while each long operand gets a full register. Without fractional LMUL, the narrowest type would force LMUL=1 and the wide types would each need multiple registers — exhausting the 32-register file. Fractional LMUL parks narrow data compactly so all the wide operands stay register-resident.