Re: Do not account __builtin_unreachable guards in inliner

[Jan Hubicka <hubicka@ucw.cz>]

> 
> So you need to feed it with extra info on the optimized out stmts because
> as-is it will not remove __builtin_unreachable ().  That means you're

My plan was to add entry point to tree-ssa-dce that will take an
set of stmts declared dead by external force and will do the usual mark
stage bypassing mark_stmt_if_necessary if the stmt is in the set of
deads.

> doing the find_obviously_necessary_stmts manually, skipping the
> conditional and all stmts it controls to the __builtin_unreachable () path?
> 
> I also think you want something cheaper than non-cd-dce mark, you also don't
> want to bother with stores/loads?

You are probably right. cd-dce marking became bit of a monster and I do
not want to care about memory.
One can add extra flag to avoid processing of memory, but the code I
would re-use is quite small.

I can do my own mark&sweep  just considering phis, pre-identified
conditionals and basic gimple_assigns with no side effects as possibly
unnecesary stmts.  I can completely ignore debug stmts.

So it should be one pass through the statments to populate the worklist
& simple walk of the ssa graph to propagae it.

> 
> Also when you only do this conditional how do you plan to use the result?

Well, the analysis is a loop that walks all basic blocks and then all
stmts.  I can keep track if computation of live stmts was done and in
that case query the flag assume it is true otherwise.

Honza