Re: fix left-over debug insns in DCE

[Alexandre Oliva <aoliva@redhat.com>]

On Jun  3, 2011, Eric Botcazou <ebotcazou@adacore.com> wrote:

> Does the same logic need to be replicated in all passes that do?  On
> the other hand, these passes call into DF when they remove insns, so
> DF is a central place here.

I went over (again?) a number of passes that call delete_insn directly
or indirectly.  The problem is that, at that point, it's not possible to
tell whether:

- equivalent DEFs are going to be re-emitted at the same place (say, a
split), so debug insns are to be left alone,

- whether they're being re-emitted elsewhere (say a move), so *some*
debug insns may have to be adjusted

- whether DEFs are really being removed, in which case debug temps may
have to be emitted; and the uses, adjusted; and other reaching defs also
followed by binding of the same debug temps

- whether many chained insns are being deleted (say an entire BB, or
part of a BB found to be shared between multiple BBs), in which case
debug insns may have to be left alone or adjusted depending on the
CFG adjustments to be made later

Given this analysis, I'm still of the opinion that passes that remove
insns that may be used by debug insns ought to decide on their own how
to make the adjustments (or not), as they currently do, but that we
should have better shared infrastructure to propagate
defs-to-be-removed, which is currently done in an ad hoc manner in
several passes.

-- 
Alexandre Oliva, freedom fighter    http://FSFLA.org/~lxoliva/
You must be the change you wish to see in the world. -- Gandhi
Be Free! -- http://FSFLA.org/   FSF Latin America board member
Free Software Evangelist      Red Hat Brazil Compiler Engineer