Re: [PATCH] Fix PR89463 (and dups?), wrong-debug with loop removal

[Richard Biener <rguenther at suse dot de>]

On Mon, 25 Mar 2019, Richard Biener wrote:

> 
> This PR and possibly quite some dups that have been accumulating lately
> run into an artifact of DCEs edge removal code when generating debug
> stmts.  Here DCE removes edges from the CFG at the same time it removes
> dead controlling stmts but before PHI nodes are removed.  This causes
> us to remove PHI arguments associated with removed edges and make
> those PHI appearing as degenerate, causing wrong debug stmts to be
> generated.  Consider for example i_1 = PHI <i_2, 0> where the edge
> leading to i_2 is removed - at the time we remove the PHI it looks
> like i_1 = PHI <0> and a i = 0 debug-stmt is generated.
> 
> The fix is to delay edge removal and move PHI removal back to the
> place it had originally been, doing that in reverse dominator order
> as intended for debug stmt creation (it jumped from where it'll be
> after the patch to all-before to all-after).
> 
> The testcase shows up as UNRESOLVED where it formerly was
> wrong-debug because we change from i = 0 to i = NULL.  I've seen
> no way to mark "optimized-out" as expected and at -O1 we
> retain the loop and the correct value of i.
> 
> Bootstrapped on x86_64-unknown-linux-gnu, testing in progress.
> 
> OK (well, I think it is, just double-checking).

I have applied this now, it doesn't immediately solve any of the
other issues.

Richard.