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From: Richard Henderson <richard.henderson@linaro.org>
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Date: Thu, 25 Jan 2018 23:29:07 -0800
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Subject: Re: [Qemu-devel] [PATCH v2 09/11] target/arm: Decode aa64 armv8.3
 fcmla
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: QEMU Developers <qemu-devel@nongnu.org>, qemu-arm <qemu-arm@nongnu.org>

On 01/15/2018 10:18 AM, Peter Maydell wrote:
>> +void HELPER(gvec_fcmlah)(void *vd, void *vn, void *vm,
>> +                         void *vfpst, uint32_t desc)
>> +{
>> +    uintptr_t opr_sz = simd_oprsz(desc);
>> +    float16 *d = vd;
>> +    float16 *n = vn;
>> +    float16 *m = vm;
>> +    float_status *fpst = vfpst;
>> +    intptr_t flip = extract32(desc, SIMD_DATA_SHIFT, 1);
>> +    uint32_t neg_imag = extract32(desc, SIMD_DATA_SHIFT + 1, 1);
>> +    uint32_t neg_real = flip ^ neg_imag;
>> +    uintptr_t i;
>> +
>> +    neg_real <<= 15;
>> +    neg_imag <<= 15;
>> +
>> +    for (i = 0; i < opr_sz / 2; i += 2) {
>> +        float16 e0 = n[H2(i + flip)];
>> +        float16 e1 = m[H2(i + flip)] ^ neg_real;
>> +        float16 e2 = e0;
>> +        float16 e3 = m[H2(i + 1 - flip)] ^ neg_imag;
> 
> This is again rather confusing to compare against the pseudocode.
> What order are your e0/e1/e2/e3 compared to the pseudocode's
> element1/element2/element3/element4 ?

The SVE pseudocode for the same operation is clearer than that in the main ARM
ARM, and is nearer to what I used:

  for e = 0 to elements-1
    if ElemP[mask, e, esize] == '1' then
        pair = e - (e MOD 2);  // index of first element in pair
        addend = Elem[result, e, esize];
        if IsEven(e) then  // real part
            // realD = realA [+-] flip ? (imagN * imagM) : (realN * realM)
            element1 = Elem[operand1, pair + flip, esize];
            element2 = Elem[operand2, pair + flip, esize];
            if neg_real then element2 = FPNeg(element2);
        else  // imaginary part
            // imagD = imagA [+-] flip ? (imagN * realM) : (realN * imagM)
            element1 = Elem[operand1, pair + flip, esize];
            element2 = Elem[operand2, pair + (1 - flip), esize];
            if neg_imag then element2 = FPNeg(element2);
        Elem[result, e, esize] = FPMulAdd(addend, element1, element2, FPCR);

In my version, e0/e1 are element1/element2 (real) and e2/e3 are
element1/element2 (imag).


r~