[Bug tree-optimization/110449] New: Vect: use a small step to calculate the loop induction if the loop is unrolled during loop vectorization

["hliu at amperecomputing dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110449

            Bug ID: 110449
           Summary: Vect: use a small step to calculate the loop induction
                    if the loop is unrolled during loop vectorization
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hliu at amperecomputing dot com
  Target Milestone: ---

This is inspired by clang. Compile the follwing case with "-mcpu=neoverse-n2
-O3":

void foo(int *arr, int val, int step) {
  for (int i = 0; i < 1024; i++) {
    arr[i] = val;
    val += step;
  }
}

It will be unrolled by 2 during vectorization. GCC generates code:
        fmov    s29, w2                 # step
        shl     v27.2s, v29.2s, 3       # 8*step
        shl     v28.2s, v29.2s, 2       # 4*step
        ...
.L2:
        mov     v30.16b, v31.16b
        add     v31.4s, v31.4s, v27.4s  # += 8*step
        add     v29.4s, v30.4s, v28.4s  # += 4*step
        stp     q30, q29, [x0]
        add     x0, x0, 32
        cmp     x1, x0
        bne     .L2

The v27 (i.e. "8*step") is actually not necessary. We can use v29 + v28 (i.e.
"+ 4*step") and generate simpler code:
        fmov    s29, w2                 # step
        shl     v28.2s, v29.2s, 2       # 4*step
        ...
.L2:
        add     v29.4s, v30.4s, v28.4s  # += 4*step
        stp     q30, q29, [x0]
        add     x0, x0, 32
        add     v30.4s, v29.4s, v28.4s  # += 4*step
        cmp     x1, x0
        bne     .L2

This has two benefits:
(1) Save 1 vector register and one "mov" instructon
(2) For floating point, the result value of small step should be closer to the
original scalar result value than large step. I.e. "A + 4*step + ... + 4*step"
should be closer to "A + step + ... + step" than "A + 8*step + ... 8*step".

Do you think if this is reasonable? 

I have a simple patch to enhance the tree-vect-loop.cc
"vectorizable_induction()" to achieve this. Will send out the patch for code
review later.