Re: How to extend SLP to support this case

["Kewen.Lin" <linkw@linux.ibm.com>]

Hi Richi,

on 2020/3/10 下午7:14, Richard Biener wrote:
> On Tue, Mar 10, 2020 at 12:12 PM Richard Biener
> <richard.guenther@gmail.com> wrote:
>>
>> On Tue, Mar 10, 2020 at 7:52 AM Kewen.Lin <linkw@linux.ibm.com> wrote:
>>>
>>> Hi all,
>>>
>>> I'm investigating whether GCC can vectorize the below case on ppc64le.
>>>
>>>   extern void test(unsigned int t[4][4]);
>>>
>>>   void foo(unsigned char *p1, int i1, unsigned char *p2, int i2)
>>>   {
>>>     unsigned int tmp[4][4];
>>>     unsigned int a0, a1, a2, a3;
>>>
>>>     for (int i = 0; i < 4; i++, p1 += i1, p2 += i2) {
>>>       a0 = (p1[0] - p2[0]) + ((p1[4] - p2[4]) << 16);
>>>       a1 = (p1[1] - p2[1]) + ((p1[5] - p2[5]) << 16);
>>>       a2 = (p1[2] - p2[2]) + ((p1[6] - p2[6]) << 16);
>>>       a3 = (p1[3] - p2[3]) + ((p1[7] - p2[7]) << 16);
>>>
>>>       int t0 = a0 + a1;
>>>       int t1 = a0 - a1;
>>>       int t2 = a2 + a3;
>>>       int t3 = a2 - a3;
>>>
>>>       tmp[i][0] = t0 + t2;
>>>       tmp[i][2] = t0 - t2;
>>>       tmp[i][1] = t1 + t3;
>>>       tmp[i][3] = t1 - t3;
>>>     }
>>>     test(tmp);
>>>   }
>>>
...
>>> From the above, the key thing is to group tmp[i][j] i=/0,1,2,3/ together, eg:
>>>   tmp[i][0] i=/0,1,2,3/ (one group)
>>>   tmp[i][1] i=/0,1,2,3/ (one group)
>>>   tmp[i][2] i=/0,1,2,3/ (one group)
>>>   tmp[i][3] i=/0,1,2,3/ (one group)
>>>
>>> which tmp[i][j] group have the same isomorphic computations.  But currently
>>> SLP is unable to divide group like this way. (call it as A-way for now)
>>>
>>> It's understandable since it has better adjacent store groups like,
>>>   tmp[0][i] i=/0,1,2,3/ (one group)
>>>   tmp[1][i] i=/0,1,2,3/ (one group)
>>>   tmp[2][i] i=/0,1,2,3/ (one group)
>>>   tmp[3][i] i=/0,1,2,3/ (one group)
> 
> Note this is how the non-SLP path will (try to) vectorize the loop.
> 

Oops, sorry for the confusion with poor writing, it's intended to show
how the current SLP group those 16 stores tmp[i][j] i,j=/0,1,2,3/
with completely unrolled.  I saw it split 16 stmts into 4 groups like
this way finally.

BR,
Kewen