Potential enhancement to Mask interface to help avoid FPEs

[jgfouca]

As I'm cleaning up FPEs in SHOC, I'm encountering this pattern over and over (this pattern is widespread in P3 too):

Spack pack1(data1), pack2(data2);
pack2.set(pack1 != 0, val / pack1);

... which needs to be changed in over to avoid the FPE (if they are on) to:

Spack pack1(data1), pack2(data2);
const auto mask = pack1 != 0;
if (mask.any()) {
  pack2.set(mask, val / pack1);
}

Because, even if pack2 is protected from getting infs by the mask, the divide-by-zero still occurs when val/pack1 gets evaluated. The code that needed to protect against this makes the code uglier and may impact performance since it introduces another branch.

I was thinking a macro like this would be nice:

#define FPE_SAFE_SET(pack, mask, val)
  #if defined(SCREAM_FPE)
    if (mask.any()) {
      pack.set(mask, val);
    }
  #else
    pack.set(mask, val);
  #endif

Which would be used like this:

Spack pack1(data1), pack2(data2);
FPE_SAFE_SET(pack2, pack1 != 0, val / pack1);

Thoughts?