Re: Sparse 0.5.1 RC5 released.

[Luc Van Oostenryck <luc.vanoostenryck@gmail.com>]

Another interesting thing is that even without removing the single-store
shortcut when using test-linearize we have an "instruction buffer overflow".
In other words, an instruction (a phi-node for sure) can't be displayed
in a 4096 bytes buffer  (because it as so much phi-sources).

This is something I have already seen before when trying to understand
the origin of some quadratic behaviour I had seen.
In this case it was clearly a problem related to SSA construction which
kinda wrongly accumulated phi-sources from parents.

In the current case we have a phi-node with:
- 241 sources
- 550 VOIDs (sources that have been removed during simplification)
- its BB has a single parent (thus should never have a phi-node).

With the shortcut removed, we then have hundreds and hundreds
of phi-nodes with hundreds of sources which fully exp^lain the
excessive time needed to process them. And it's very much
something possible with the removal of the shortcut since its
goal was to avoid to create phi-nodes when possible.

So this clearly points to the problem of the SSA construction
wich not only create misplaced phi-nodes but also, in some case,
create lots of them while there was no need for them.
The shortcut was very good at hiding this.

-- Luc