[Bug c/112612] New: [Missed Optimization] Holding on the loop variable rather than a derived value which can replace it

["eyalroz1 at gmx dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112612

            Bug ID: 112612
           Summary: [Missed Optimization] Holding on the loop variable
                    rather than a derived value which can replace it
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eyalroz1 at gmx dot com
  Target Milestone: ---

Consider the following function:

void foo(int* __restrict__ a) {
    int i, val;
    for (i = 0; i < 100; i++) {
        val = 2 * i;
        a[i] = val;
    }
}

When compiling it for x86_64 with -O3 -fno-unroll-loops -fno-tree-vectorize,
GCC 7.2 used to give:

foo:
        xor     eax, eax
.L2:
        mov     DWORD PTR [rdi], eax
        add     eax, 2
        add     rdi, 4
        cmp     eax, 200
        jne     .L2
        rep ret

which was rather wasteful, as eax and rdi - eax are linearly related. With GCC
13.2 or trunk on GodBolt as of today, this improves, but not really:

foo:
        xor     eax, eax
.L2:
        lea     edx, [rax+rax]
        mov     DWORD PTR [rdi+rax*4], edx
        add     rax, 1
        cmp     rax, 100
        jne     .L2
        ret

So, we don't increment two things; but - we do have an addition-via-lea in each
iteration. Is that really necessary? I mean, instead of keeping the i variable
(in rax), we could keep v = 2 * i, and that's good enough for both addressing
and condition checking. Indeed, clang 17 emits:

foo: # @foo
  xor eax, eax
.LBB0_1: # =>This Inner Loop Header: Depth=1
  mov dword ptr [rdi + 2*rax], eax
  add rax, 2
  cmp rax, 200
  jne .LBB0_1
  ret

which is almost the same, except that it holds v = 2 * i rather than i. (clang
has produced this code since v3.0.0 at least.)

GodBolt link: https://gcc.godbolt.org/z/MjzTbr831
Originally discussed in this SO question:
https://stackoverflow.com/q/48354636/1593077