[Bug tree-optimization/97832] AoSoA complex caxpy-like loops: AVX2+FMA -Ofast 7 times slower than -O3

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97832

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
So for example we'd like to vectorize with SLP when reassociation is permitted
(thus with -Ofast for example):

double a[1024], b[1024], c[1024];

void foo()
{
  for (int i = 0; i < 256; ++i)
    {
      a[2*i] = 1. - a[2*i] + b[2*i];
      a[2*i+1] = a[2*i+1] + b[2*i+1] + 1.;
    }
}

it again works when written as follows and with -fno-tree-reassoc

double a[1024], b[1024], c[1024];

void foo()
{
  for (int i = 0; i < 256; ++i)
    {
      a[2*i] = 1. - a[2*i] + b[2*i];
      a[2*i+1] = 1 + a[2*i+1] + b[2*i+1];
    }
}