[Bug target/109078] New: Missing optimization on aarch64 for types like `float32x4x2_t`

[dorazzsoft at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109078

            Bug ID: 109078
           Summary: Missing optimization on aarch64 for types like
                    `float32x4x2_t`
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dorazzsoft at gmail dot com
  Target Milestone: ---

Here is a simple code: https://godbolt.org/z/3qMTTfcfx

#include <arm_neon.h>
#include <stddef.h>
#include <stdbool.h>

void simple_gemm(
  float* restrict out,
  float const* restrict a,
  float const* restrict b,
  size_t k, bool zero_out
) {
  register float32x4x2_t o0;
  o0.val[0] = vdupq_n_f32(0.0f);
  o0.val[1] = vdupq_n_f32(0.0f);

  // begin dot
  {
    register float32x4_t a0;
    register float32x4x2_t b0;

    while (k >= 1) {
      b0 = vld1q_f32_x2(b);
      a0 = vdupq_n_f32(a[0]);

      o0.val[0] = vfmaq_f32(o0.val[0], a0, b0.val[0]);
      o0.val[1] = vfmaq_f32(o0.val[1], a0, b0.val[1]);

      b += 8;
      a += 1;
      k -= 1;
    }
  } // end dot

  // begin writeback
  {
    if (!zero_out) {
      register float32x4x2_t t0;
      t0 = vld1q_f32_x2(out);

      o0.val[0] = vaddq_f32(o0.val[0], t0.val[0]);
      o0.val[1] = vaddq_f32(o0.val[1], t0.val[1]);
    }

    // TODO: both clang and gcc generates redundant mov because of bad register
allocation.
    vst1q_f32_x2(out, o0);
  } // end writeback
}


The assembly generated:

simple_gemm:
        movi    v3.4s, 0
        and     w4, w4, 255
        mov     v4.16b, v3.16b
        cbz     x3, .L2
.L3:
        ld1     {v0.4s - v1.4s}, [x2], 32
        subs    x3, x3, #1
        ld1r    {v2.4s}, [x1], 4
        fmla    v3.4s, v2.4s, v0.4s
        fmla    v4.4s, v2.4s, v1.4s
        bne     .L3
.L2:
        cbnz    w4, .L4
        ld1     {v0.4s - v1.4s}, [x0]
        fadd    v3.4s, v3.4s, v0.4s
        fadd    v4.4s, v4.4s, v1.4s
.L4:
        mov     v0.16b, v3.16b
        mov     v1.16b, v4.16b
        st1     {v0.4s - v1.4s}, [x0]
        ret

The two values of float32x4x2_t o0 are assigned to v3 and v4. They should be
able to be used directly as operands of st1, so the mov at L4 is redundant.  

I also found that in some code, the register pair may not be neighboring, which
results in some redundant mov instructions.