Re: [Qemu-devel] [Qemu-arm] [PATCH v2 31/67] target/arm: Implement SVE conditionally broadcast/extract element

[Richard Henderson]

On 02/23/2018 07:44 AM, Peter Maydell wrote:
>> +/* Similar to the ARM LastActiveElement pseudocode function, except the
>> +   result is multiplied by the element size.  This includes the not found
>> +   indication; e.g. not found for esz=3 is -8.  */
>> +int32_t HELPER(sve_last_active_element)(void *vg, uint32_t pred_desc)
>> +{
>> +    intptr_t oprsz = extract32(pred_desc, 0, SIMD_OPRSZ_BITS) + 2;
>> +    intptr_t esz = extract32(pred_desc, SIMD_DATA_SHIFT, 2);
> 
> pred_desc is obviously an encoding of some stuff, so the comment would
> be a good place to mention what it is.

Yeah, and I've also just noticed I'm not totally consistent about it.
I probably want to re-think how some of this is done.


>> +/* Compute CLAST for a scalar.  */
>> +static void do_clast_scalar(DisasContext *s, int esz, int pg, int rm,
>> +                            bool before, TCGv_i64 reg_val)
>> +{
>> +    TCGv_i32 last = tcg_temp_new_i32();
>> +    TCGv_i64 ele, cmp, zero;
>> +
>> +    find_last_active(s, last, esz, pg);
>> +
>> +    /* Extend the original value of last prior to incrementing.  */
>> +    cmp = tcg_temp_new_i64();
>> +    tcg_gen_ext_i32_i64(cmp, last);
>> +
>> +    if (!before) {
>> +        incr_last_active(s, last, esz);
>> +    }
>> +
>> +    /* The conceit here is that while last < 0 indicates not found, after
>> +       adjusting for cpu_env->vfp.zregs[rm], it is still a valid address
>> +       from which we can load garbage.  We then discard the garbage with
>> +       a conditional move.  */
> 
> That's a bit ugly. Can we at least do a compile time assert that the
> worst case (which I guess is offset of zregs[0] minus largest-element-size)
> is still positive ? That way if for some reason we reshuffle fields
> in CPUARMState we'll notice if it's going to fall off the beginning
> of the struct.

I suppose so.  Though as commented above find_last_active, the minimal value is
-8.  I feel fairly confident that zregs[0] will never be shuffled to the
absolute start of the structure.


r~