[Bug tree-optimization/98339] New: GCC could not vectorize loop with conditional reduced add and store

[wwwhhhyyy333 at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98339

            Bug ID: 98339
           Summary: GCC could not vectorize loop with conditional reduced
                    add and store
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wwwhhhyyy333 at gmail dot com
  Target Milestone: ---

For testcase

void foo(                
                int* restrict x,                      
                int n,
                int start,   
                int m,
                int* restrict ret
)   
{
    for (int i = 0; i < n; i++)
    {
        int pos = start + i;
        if ( pos <= m)
            ret[0] += x[i];    
    }
}

with -O3 -mavx2 it could not be vectorized because ret[0] += x[i] is zero step
MASK_STORE inside loop, and dr analysis failed for zero step store.

But with manually loop store motion

void foo2(                
                int* restrict x,                      
                int n,
                int start,   
                int m,
                int* restrict ret
)   
{
    int tmp = 0;

    for (int i = 0; i < n; i++)
    {
        int pos = start + i;
        if (pos <= m)
            tmp += x[i];    
    }

    ret[0] += tmp;
}

could be vectorized. 

godbolt: https://godbolt.org/z/Kcv8hP

There is no LIM between ifcvt and vect, and current LIM could not handle
MASK_STORE. Is there any possibility to vectorize foo, like by doing loop store
motion in ifcvt instead of creating MASK_STORE?