Why can GCC only do loop interchange optimization when the int size is a compile-time constant?

When I compile this snippet (with -Ofast -fnest-loop-optimize) gcc generates assembly which traverses the array in source order.

However if I uncomment the line // n = 32767 and assign any number to n, it interchanges the index order to x[i * n + j]. Traversing memory in contiguous row-major order is much more cache-friendly than striding down columns.

float matrix_sum_column_major(float* x, int n) {    // n = 32767;    float sum = 0;    for (int i = 0; i < n; i++)        for (int j = 0; j < n; j++)            sum += x[j * n + i];    return sum;}

On godbolt

Why can't GCC or clang do loop interchange with a runtime-variable int size? Real-world code won't usually have the size declared explicitly.

PD: I've tried this with different versions of gcc and clang-9 and it seems to happen in both.
PD2: Even if I make x be a local variable malloced inside the function it still happens.