Re: [PATCH v3 30/51] target/arm: Implement FMOPA, FMOPS (non-widening)

[Richard Henderson]

On 6/24/22 05:31, Peter Maydell wrote:
> On Mon, 20 Jun 2022 at 19:07, Richard Henderson
> <richard.henderson@linaro.org> wrote:
> > Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
>
> > +void HELPER(sme_fmopa_s)(void *vza, void *vzn, void *vzm, void *vpn,
> > +                         void *vpm, void *vst, uint32_t desc)
> > +{
> > +    intptr_t row, col, oprsz = simd_maxsz(desc);
> > +    uint32_t neg = simd_data(desc) << 31;
> > +    uint16_t *pn = vpn, *pm = vpm;
> > +
> > +    bool save_dn = get_default_nan_mode(vst);
> > +    set_default_nan_mode(true, vst);
> > +
> > +    for (row = 0; row < oprsz; ) {
> > +        uint16_t pa = pn[H2(row >> 4)];
> > +        do {
> > +            if (pa & 1) {
> > +                void *vza_row = vza + row * sizeof(ARMVectorReg);
> > +                uint32_t n = *(uint32_t *)(vzn + row) ^ neg;
> > +
> > +                for (col = 0; col < oprsz; ) {
> > +                    uint16_t pb = pm[H2(col >> 4)];
> > +                    do {
> > +                        if (pb & 1) {
> > +                            uint32_t *a = vza_row + col;
> > +                            uint32_t *m = vzm + col;
> > +                            *a = float32_muladd(n, *m, *a, 0, vst);
> > +                        }
> > +                        col += 4;
> > +                        pb >>= 4;
> > +                    } while (col & 15);
> > +                }
> > +            }
> > +            row += 4;
> > +            pa >>= 4;
> > +        } while (row & 15);
> > +    }
>
> The code for the double version seems straightforward:
> row counts from 0 up to the number of rows, and we
> do something per row. Why is the single precision version
> doing something with an unrolled loop here? It's confusing
> that 'oprsz' in the two functions isn't the same thing --
> in the double version we divide by the element size, but
> here we don't.

It's all about the predicate addressing.  For doubles, the bits are spaced 8 bits apart, 
which makes it easy as you see.  For singles, the bits are spaced 4 bits apart, which is 
inconvenient.  Anyway, just as over in sve_helper.c, I load uint16_t at a time and shift 
to find each predicate bit.

So it's not unrolled, exactly.  There's second loop over predicates.  And since this is a 
matrix op, we get loops nested 4 deep.

> The pseudocode says that we ignore floating point exceptions
> (ie do not accumulate them in the FPSR) -- it passes fpexc == false
> to FPMulAdd(). Don't we need to do something special to arrange
> for that ?

Oops, somewhere I read that as "do not trap" not "do not accumulate".
But R_TGSKG is very clear on this as accumulate.


r~