[Bug tree-optimization/114120] New: add reduction with promotion and then truncation poorly vectorized

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114120

            Bug ID: 114120
           Summary: add reduction with promotion and then truncation
                    poorly vectorized
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
            Blocks: 53947
  Target Milestone: ---
            Target: x86_64

Take:
```
unsigned char f(unsigned char *src)
{
        unsigned  sum = 0;
        for(int y = 0; y < 8; y++)
        {
                sum += src[y];
        }
        return sum;
}
```

On x86_64 we should vectorize to the same as what is done for:
```
unsigned char f0(unsigned char *src)
{
        unsigned char sum = 0;
        for(int y = 0; y < 8; y++)
        {
                sum += src[y];
        }
        return sum;
}
```

But GCC does not as GCC keeps sum in unsigned and the reduction is done in
`unsigned int`.

Note LLVM is able to vectorize this decently.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53947
[Bug 53947] [meta-bug] vectorizer missed-optimizations