[Bug tree-optimization/98350] New: Reassociation breaks FMA chains

["ktkachov at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98350

            Bug ID: 98350
           Summary: Reassociation breaks FMA chains
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ktkachov at gcc dot gnu.org
  Target Milestone: ---

Consider the testcase:

#define N 1024
double a[N];
double b[N];
double c[N];
double d[N];
double e[N];
double f[N];
double g[N];
double h[N];
double j[N];
double k[N];
double l[N];
double m[N];
double o[N];
double p[N];


void
foo (void)
{
  for (int i = 0; i < N; i++)
  {
    a[i] += b[i]* c[i] + d[i] * e[i] + f[i] * g[i] + h[i] * j[i] + k[i] * l[i]
+ m[i]* o[i] + p[i];
  }
}

For -Ofast --param=tree-reassoc-width=1 GCC generates the loop:
.L2:
        ldr     q1, [x1, x0]
        ldr     q0, [x12, x0]
        ldr     q3, [x14, x0]
        fadd    v0.2d, v0.2d, v1.2d
        ldr     q1, [x13, x0]
        ldr     q2, [x11, x0]
        fmla    v0.2d, v3.2d, v1.2d
        ldr     q1, [x10, x0]
        ldr     q3, [x9, x0]
        fmla    v0.2d, v2.2d, v1.2d
        ldr     q1, [x8, x0]
        ldr     q2, [x7, x0]
        fmla    v0.2d, v3.2d, v1.2d
        ldr     q1, [x6, x0]
        ldr     q3, [x5, x0]
        fmla    v0.2d, v2.2d, v1.2d
        ldr     q1, [x4, x0]
        ldr     q2, [x3, x0]
        fmla    v0.2d, v3.2d, v1.2d
        ldr     q1, [x2, x0]
        fmla    v0.2d, v2.2d, v1.2d
        str     q0, [x1, x0]
        add     x0, x0, 16
        cmp     x0, 8192
        bne     .L2

with --param=tree-reassoc-width=4 it generates:
.L2:
        ldr     q5, [x11, x0]
        ldr     q4, [x7, x0]
        ldr     q0, [x3, x0]
        ldr     q3, [x12, x0]
        ldr     q1, [x8, x0]
        ldr     q2, [x4, x0]
        fmul    v3.2d, v3.2d, v5.2d
        fmul    v1.2d, v1.2d, v4.2d
        fmul    v2.2d, v2.2d, v0.2d
        ldr     q16, [x1, x0]
        ldr     q18, [x14, x0]
        ldr     q17, [x13, x0]
        ldr     q0, [x2, x0]
        ldr     q7, [x10, x0]
        ldr     q6, [x9, x0]
        ldr     q5, [x6, x0]
        ldr     q4, [x5, x0]
        fmla    v3.2d, v18.2d, v17.2d
        fadd    v0.2d, v0.2d, v16.2d
        fmla    v1.2d, v7.2d, v6.2d
        fmla    v2.2d, v5.2d, v4.2d
        fadd    v0.2d, v0.2d, v3.2d
        fadd    v1.2d, v1.2d, v2.2d
        fadd    v0.2d, v0.2d, v1.2d
        str     q0, [x1, x0]
        add     x0, x0, 16
        cmp     x0, 8192
        bne     .L2

The reassociation is evident. The problem here is that the fmla chains are
something we'd want to preserve.
Is there a way we can get the reassoc pass to handle FMAs more intelligently?