Re: [Qemu-devel] [PATCH v1 1/5] s390x/tcg: Implement VECTOR FIND ANY ELEMENT EQUAL

[David Hildenbrand]

On 23.05.19 14:27, Richard Henderson wrote:
> On 5/23/19 3:50 AM, David Hildenbrand wrote:
>> /*
>>  * Returns the number of bits composing one element.
>>  */
>> static uint8_t get_element_bits(uint8_t es)
>> {
>>     return (1 << es) * BITS_PER_BYTE;
>> }
>>
>> /*
>>  * Returns the bitmask for a single element.
>>  */
>> static uint64_t get_single_element_mask(uint8_t es)
>> {
>>     return -1ull >> (64 - get_element_bits(es));
>> }
>>
>> /*
>>  * Returns the bitmask for a single element (excluding the MSB).
>>  */
>> static uint64_t get_single_element_lsbs_mask(uint8_t es)
>> {
>>     return -1ull >> (65 - get_element_bits(es));
>> }
>>
>> /*
>>  * Returns the bitmasks for multiple elements (excluding the MSBs).
>>  */
>> static uint64_t get_element_lsbs_mask(uint8_t es)
>> {
>>     return dup_const(es, get_single_element_lsbs_mask(es));
>> }
>>
>> static int vfae(void *v1, const void *v2, const void *v3, bool in,
>>                 bool rt, bool zs, uint8_t es)
>> {
>>     const uint64_t mask = get_element_lsbs_mask(es);
>>     const int bits = get_element_bits(es);
>>     uint64_t a0, a1, b0, b1, e0, e1, t0, t1, z0, z1;
>>     uint64_t first_zero = 16;
>>     uint64_t first_equal;
>>     int i;
>>
>>     a0 = s390_vec_read_element64(v2, 0);
>>     a1 = s390_vec_read_element64(v2, 1);
>>     b0 = s390_vec_read_element64(v3, 0);
>>     b1 = s390_vec_read_element64(v3, 1);
>>     e0 = 0;
>>     e1 = 0;
>>     /* compare against equality with every other element */
>>     for (i = 0; i < 64; i += bits) {
>>         t0 = i ? rol64(b0, i) : b0;
>>         t1 = i ? rol64(b1, i) : b1;
>>         e0 |= zero_search(a0 ^ t0, mask);
>>         e0 |= zero_search(a0 ^ t1, mask);
>>         e1 |= zero_search(a1 ^ t0, mask);
>>         e1 |= zero_search(a1 ^ t1, mask);
>>     }
> 
> I don't see that this is doing what you want.  You're shifting one element of 
> B
> down, but not broadcasting it so that it is compared against every element of 
> A.
> 
> I'd expect something like
> 
>       t0 = dup_const(es, b0 >> i);
>       t1 = dup_const(es, b1 >> i);
> 
> (I also don't see what rol is getting you that shift doesn't.)

Let's assume

a0 = [0, 1, 2, 3]
a1 = [4, 5, 6, 7]

b0 = [8, 8, 8, 4]
b1 = [8, 8, 8, 8]

What I would check is

First iteration

a0 == [8, 8, 8, 4] -> no match
a0 == [8, 8, 8, 8] -> no match
a1 == [8, 8, 8, 4] -> no match
a1 == [8, 8, 8, 8] -> no match

Second iteration

a0 == [8, 8, 4, 8] -> no match
a0 == [8, 8, 8, 8] -> no match
a1 == [8, 8, 4, 8]
a1 == [8, 8, 8, 8] -> no match

...

Last iteration

a0 == [4, 8, 8, 8] -> no match
a0 == [8, 8, 8, 8] -> no match
a1 == [4, 8, 8, 8] -> match in first element
a1 == [8, 8, 8, 8] -> no match

What am i missing?


-- 

Thanks,

David / dhildenb