Re: [PATCH] Fix middle-end/67133, part 1

[Marek Polacek <polacek@redhat.com>]

On Mon, Aug 17, 2015 at 11:31:57AM -0600, Jeff Law wrote:
> The funny thing here is we remove the statements after the trap to avoid
> this exact situation!
> 
> I think the problem with schemes that either change the order of block
> processing, or which ignore some blocks are going to run into issues. By
> walking blocks and statements in a backwards order, we address 99% of the
> problems, including uses in PHIs in a direct successor block.
> 
> What's not handled is a use in a PHI at the frontier of a subgraph that
> becomes unreachable.  We'd have to do the usual unreachable block analysis
> to catch and handle those properly.
> 
> I don't particularly like that idea....
> 
> But in walking through all that, I think I've stumbled on a simpler
> solution.  Specifically do as a little as possible and let the standard
> mechanisms clean things up :-)
> 
> 1. Delete the code that removes instructions after the trap.
> 
> 2. Split the block immediately after the trap and remove the edge
>    from the original block (with the trap) to the new block.
> 
> 
> THen let the standard mechanisms handle things when that pass is complete.
> 
> By setting cfg_altered, we'll  get unreachable code removal which will
> capture most of the intended effect.  DCE fires a couple more passes down in
> the pipeline to pick up the remaining tidbits.

Ok, thanks.
 
> Do you want to try and tackle this?

Sure.  I should have a patch tomorrow :-).

	Marek