Re: [Qemu-devel] [PATCH v1 2/5] s390x/tcg: Implement VECTOR FIND ELEMENT EQUAL

[David Hildenbrand <david@redhat.com>]

On 17.05.19 19:42, Richard Henderson wrote:
> On 5/17/19 9:47 AM, Richard Henderson wrote:
>>     first_equal = n;
>>     first_zero = n;
>>     for (i = n - 1; i >= 0; --i) {
>>         if (data1 == data2) {
>>             first_equal = i;
>>         }
>>         if (data1 == 0) {
>>             first_zero = i;
>>         }
>>     }
>>
>> // As an aside, there are bit tricks for the above,
>> // but let's stay simple(r) for now.
> 
> What the hell, it's not /that/ tricky.
> 
> 
> /*
>  * Returns a bit set in the MSB of each element that is zero,
>  * as defined by the mask M.
>  */
> static inline uint64_t zero_search(uint64_t a, uint64_t m)
> {
>     return ~(((a & m) + m) | a | m);
> }
> 
> /*
>  * Returns the byte offset for the first match, or 16 for no match.
>  */
> static inline int match_index(uint64_t c0, uint64_t c1)
> {
>     return (c0 ? clz64(c0) : clz64(c1) + 64) >> 3;
> }
> 
> Use
> 
>   dup_const(MO_8, 0x7f)
>   dup_const(MO_16, 0x7fff)
>   dup_const(MO_32, 0x7fffffff)
> 
> for the M parameter for the different element sizes.
> 
>     uint64_t a0, a1, b0, b1, e0, e1, z0, z1;
> 
>     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 = zero_search(a0 ^ b0, m);
>     e1 = zero_search(a1 ^ b1, m);
>     first_equal = match_index(e0, e1);
> 
>     if (zs) {
>         z0 = zero_search(a0, m);
>         z1 = zero_search(a1, m);
>         first_zero = match_index(z0, z1);
>     ...
> 
> 
> r~
> 


Crazy stuff, seems to work (not that I am surprised :D )

I now have:

+static int vfee(void *v1, const void *v2, const void *v3, bool zs,
uint8_t es)
+{
+    const uint64_t mask = dup_const(es, -1ull >> (65 - (1 << es) * 8));
+    uint64_t a0, a1, b0, b1, e0, e1, z0, z1;
+    uint64_t first_zero = 16;
+    uint64_t first_equal;
+
+    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 = zero_search(a0 ^ b0, mask);
+    e1 = zero_search(a1 ^ b1, mask);
+    first_equal = match_index(e0, e1);
+
+    if (zs) {
+        z0 = zero_search(a0, mask);
+        z1 = zero_search(a1, mask);
+        first_zero = match_index(z0, z1);
+    }
+
+    /* zero out the destination vector */
+    s390_vec_write_element64(v1, 0, 0);
+    s390_vec_write_element64(v1, 1, 0);
+
+    if (first_zero == 16 && first_equal == 16) {
+        s390_vec_write_element8(v1, 7, 16);
+        return 3; /* no match */
+    } else if (first_zero == 16) {
+        s390_vec_write_element8(v1, 7, first_equal);
+        return 1; /* matching elements, no match for zero */
+    } else if (first_equal < first_zero) {
+        s390_vec_write_element8(v1, 7, first_equal);
+        return 2; /* matching elements before match for zero */
+    }
+    s390_vec_write_element8(v1, 7, first_zero);
+    return 0; /* match for zero */
+}


-- 

Thanks,

David / dhildenb