Merge switch statements in tree-cfgcleanup

[Bernd Schmidt]

On 07/19/2016 10:07 AM, Richard Biener wrote:
>
> This is not appropriate for CFG cleanup due to its complexity not
> being O(# bbs + # edges).
> I tried hard in the past to make it so (at least when no transform is done).

Why wouldn't it be, if no transform is done? Assuming we visit each bb 
once, we have at worst one walk over the successor edges of the 
predecessor (if we even find two switches on the same variable), and 
then we can decide whether or not to do the transformation.

When performing the transformation I could imagine one could construct a 
testcase where lots of these switches are nested inside each other, but 
I'm not convinved that's really a realistic worry.

> Please move this transform elsewhere.  I suggest the switch-conversion
> pass or if that
> is not a good fit then maybe if-combine (whose transforms are remotely related).

One problem is that this triggers rarely, but when it does, it occurs at 
various stages in the compilation after other optimizations have been 
done. Moving it to any given point is likely to limit the effectiveness.

> Not looking closer at the patch but missing some comments on how it deals with
> common cases (you see to handle fallthrus to the default label by
> ignoring them?)

If you are thinking of

switch (a)
  {
  case n:
  case m:
  default:
    switch (a) { .... }
  }

then the cases for n and m can simply be dropped when merging from the 
second switch into the first one. That's what happens, and there's a 
comment for it. So please elaborate what you mean.


Bernd