[Bug tree-optimization/15524] [4.0 Regression] jump threading on trees is slow with switch statements with large # of cases

[Daniel Berlin]

On Sun, 31 Oct 2004, Jeffrey A Law wrote:

>
> More work to speed up 15524.  I was rather puzzled that "expand" was
> being charged with 30% of the compilation time.  I originally thought
> there was some lameness in the switch statement expanders.  However,
> it turned out I was totally wrong.
>
> The culprit is actually the code to record the number of iterations
> of each loop.  We never pushed a timevar for that pass and thus the
> time was charged to whatever was on the top of the timevar stack
> (which happens to be TV_EXPAND).
>
> The code to record the number of loop iterations was being rather
> dumb.  It basically did something like:
>
> foreach loop
>  foreach block
>     if block is in loop
>        code
>
>
> Clearly the problem is second loop -- it's walking *all* the blocks
> and testing for loop membership.  Ouch.

I actually had fixed this as part of my last patch, but was waiting till 
monday to commit it.