[Bug tree-optimization/109088] GCC does not always vectorize conditional reduction

[juzhe.zhong at rivai dot ai via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109088

--- Comment #17 from JuzheZhong <juzhe.zhong at rivai dot ai> ---
Sorry for confusing and not enough information.

I am trying to transform:

+          reduc_1 = PHI <..., reduc_2>
+          ...
+          if (...)
+          tmp1 = reduc_1 + rhs1;
+          tmp2 = tmp1 + rhs2;
+          tmp3 = tmp2 + rhs3;
+          ...
+          reduc_3 = tmpN-1 + rhsN-1;
+
+          reduc_2 = PHI <reduc_1, reduc_3>

First transform the first statement:

tmp1 = reduc_1 + rhs1; into  tmp1 = rhs1 + 0;

Then it will become bogus data move assignment: tmp1 = rhs1.
The later PASS will eliminate it.

Then, transform the reduction PHI: 

reduc_1 = PHI <..., reduc_2>

into if-convert statement:

reduc_1 = PHI <_ifc__35(8), 0(18)>

Thid, transform 

+          reduc_3 = tmpN-1 + rhsN-1;
+
+          reduc_2 = PHI <reduc_1, reduc_3>

into :

reduc_3 = tmpN-1 + rhsN-1;
_ifc__35 = .COND_ADD (condition, reduc_1, reduc_3, reduc_1);


So finally:

result_1 = PHI <_ifc__35(8), 0(18)>
...
tmp1 = rhs1;
tmp2 = tmp1 + rhs2;
tmp3 = tmp2 + rhs3;
...
reduc_3 = tmpN-1 + rhsN-1;
_ifc__35 = .COND_ADD (condition, reduc_1, reduc_3, reduc_1);