LoopVectorize fails to vectorize code with condition on reduction

Hello.
I'm not able to vectorize this simple C loop doing basically what could be called predicated sum-reduction:
#define NMAX 1000
int colOccupied[NMAX];
void Func(int N) {
int numSol = 0;
for (int c = 0; c < N; c++) {
if (colOccupied[c] == 0)
numSol++;
}
return numSol;
}

The compiler command I used was:
clang -O3 -emit-llvm -S -mllvm -debug -ffast-math A.cpp -fvectorize -mllvm -force-vector-width=16

Following is the output of LLVM's opt explaining why it fails to vectorize:
LV: Checking a loop in "_Z6Queensi" from A_DFS_last_level_wo_rec.cpp
LV: Loop hints: force=? width=16 unroll=0
LV: Found a loop: for.body
LV: PHI is not a poly recurrence.
LV: Found an unidentified PHI. %2 = phi i32 [ 0, %for.body.lr.ph ], [ %4, %for.inc ]
LV: Can't vectorize the instructions or CFG
LV: Not vectorizing: Cannot prove legality.

This was obtained with the latest LLVM - SVN rev 334367, from Jun 10 2018.

Could you please tell me why do you think LoopVectorize is not being able to vectorize the above C code.

BTW, I was hoping that LoopVectorize can change the code to be amenable to simple if-conversion like the following C code manually transformed:
void Func_manually_transformed(int N) {
int numSol = 0;
for (int c = 0; c < N; c++) {
if (colOccupied[c] == 0)
select[c] = 1;
else
select[c] = 0;
numSol += select[c];
}
return numSol;
}
Note that this manually transformed code is being vectorized by LLVM.

Thank you,
Alex

Hi,

Hi, Florian,
Yes, I've posted the right output - the small inadvertence is insignificant - the name of the function is really Func() and not Queens() - sorry for the "typo".

I'm really surprised LLVM is able to vectorize for you - are you maybe using a different loop vectorizer (maybe RV, which I understand it's able to vectorize such loops) instead of LLVM's LoopVectorize+VPlan? Can you maybe post the debug output of opt when it says it vectorizes the loop in the Func (not the Func_manually_transformed) function.

Thank you,
Alex