Do we need to do a loop invariant motion after loop interchange ?

Richard Biener richard.guenther@gmail.com
Thu Nov 21 12:10:00 GMT 2019

```On Thu, Nov 21, 2019 at 10:22 AM Li Jia He <helijia@linux.ibm.com> wrote:
>
> Hi,
>
> I found for the follow code:
>
> #define N 256
> int a[N][N][N], b[N][N][N];
> int d[N][N], c[N][N];
> void __attribute__((noinline))
> double_reduc (int n)
> {
>    for (int k = 0; k < n; k++)
>    {
>      for (int l = 0; l < n; l++)
>       {
>         c[k][l] = 0;
>          for (int m = 0; m < n; m++)
>            c[k][l] += a[k][m][l] * d[k][m] + b[k][m][l] * d[k][m];
>       }
>    }
> }
>
> I dumped the file after loop interchange and got the following information:
>
> <bb 3> [local count: 118111600]:
>    # m_46 = PHI <0(7), m_45(11)>
>    # ivtmp_44 = PHI <_42(7), ivtmp_43(11)>
>    _39 = _49 + 1;
>
>    <bb 4> [local count: 955630224]:
>    # l_48 = PHI <0(3), l_47(12)>
>    # ivtmp_41 = PHI <_39(3), ivtmp_40(12)>
>    c_I_I_lsm.5_18 = c[k_28][l_48];
>    c_I_I_lsm.5_53 = m_46 != 0 ? c_I_I_lsm.5_18 : 0;
>    _2 = a[k_28][m_46][l_48];
>    _3 = d[k_28][m_46];
>    _4 = _2 * _3;
>    _5 = b[k_28][m_46][l_48];
>    _6 = _3 * _5;
>    _7 = _4 + _6;
>    _8 = _7 + c_I_I_lsm.5_53;
>    c[k_28][l_48] = _8;
>    l_47 = l_48 + 1;
>    ivtmp_40 = ivtmp_41 - 1;
>    if (ivtmp_40 != 0)
>      goto <bb 12>; [89.00%]
>    else
>      goto <bb 5>; [11.00%]
>
> we can see '_3 = d[k_28][m_46];'  is a loop invariant.
> Do we need to add a loop invariant motion pass after the loop interchange?

There is one at the end of the loop pipeline.

> BR,
> Lijia He
>

```