[Bug tree-optimization/98544] [11 regression] Wrong code generated by tree vectorizer since r11-3917-g28290cb50c7dbf87

[martin--- via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98544

--- Comment #15 from Martin Reinecke <mar...@mpa-garching.mpg.de> ---
"Problem at length N" means that the FFT of length N is computed incorrectly.
Also, N==l1*ido*x.

For an FFT of length N, the computation is broken down into several passes.
Let's take N=15.
First the prome factors of N are computed, in this case 3 and 5.

The (simplified) procedure is then:

l1=1;

// first pass with x=3
x=3;
ido=N/(x*l1);
radb3(ido, l1, p1, p2, <twiddle factors>);
swap (p1,p2);

l1*=x;
x=5;
ido=N/(x*l1);
radb5(ido, l1, p1, p2, <twiddle factors>);
swap (p1,p2);