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

Li Jia He helijia@linux.ibm.com
Thu Nov 21 09:22:00 GMT 2019

```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?

BR,
Lijia He

```