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.guent...@gmail.com> wrote:
>>
>> On Tue, Mar 10, 2020 at 7:52 AM Kewen.Lin <li...@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
