Re: [RFC] Don't move cold code out of loop by checking bb count

[Ulrich Drepper <drepper.fsp@gmail.com>]

On Tue, Aug 10, 2021 at 4:03 AM Xionghu Luo via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
> For this case, theorotically I think the master GCC will optimize it to:
>
>   invariant;
>   for (;;)
>     if (unlikely_cond)
>       for (;;)
>          ;
>
> 'invariant' is moved out of outer loop, but with the patch, it will get:
>
>   for (;;)
>     if (unlikely_cond)
>       {
>         invariant;
>         for (;;)
>            ;
>       }
>
> 'invariant' is *cold* for outer loop, but it is still *hot* for inner loop,
> so hoist it out of inner loop, this is exactly what we want, right?

Is relying on absolute numbers really what you want?  If the
'unlikely_cond' condition depends on the iteration count of the outer
loop the probability of it being true in each individual iteration can
be low (at least that's how I use unlikely) but the overall
probability of needing the code is higher 1 - (1 - p)^n  if 'p' is the
probability of 'unlikely_cond' and 'n' is the number of iterations.
Assuming complete independence of the loop iterations, otherwise it's
rather an upper limit.

At the very least I'd generate code like this:

  first = true;
  for (;;)
    if (unlikely_cond)
      {
        if (first)
          {
            invariant;
            first = false;
          }
        for (;;)
           ;
      }

If it's worth hoisting the code the the extra test and flag should be
small in cost in comparison.

If 'unlikely_cond' does not in any way depend on the loop iteration
then I think your code generation is fine.