[PATCH] ADCE: Remove debug info intrinsics in dead scopes

[Mehdi Amini via llvm-commits]

> On Mar 30, 2016, at 9:18 AM, David Blaikie <dblaikie at gmail.com> wrote:
> 
> 
> 
> On Tue, Mar 29, 2016 at 10:59 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com <mailto:dexonsmith at apple.com>> wrote:
> +Mehdi
> 
> > On 2016-Mar-29, at 22:29, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
> >
> > Does it make sense to do this in ADCE and ADCE only? Lots of code can remove instructions & could be the place where a scope ceases to exist, so it seems strange to make this change only here, doesn't it?
> 
> I thought about the algorithm, and then looked for a pass that was
> already doing something similar.  ADCE seemed to be good fit since it
> also needed three passes through the function.
> 
> I had a look with Mehdi at the pass pipeline, and ADCE is run almost
> at the end.  It is run just before exit for -flto=thin, too.
> 
> > I'm assuming you want to do this to reduce IR size early on? (pre-(Thin)LTO merge?)
> 
> ^ ?

This is the motivation for this change (trying to emit less metadata in the end), but it is valuable in general: smaller basic blocks for all the optimization that will follow, less memory, etc.
This seems a good motivation to run ADCE in general as well.

-- 
Mehdi


>  
> Maybe as a separate pass?
> 
> > If it's just to reduce debug info output - maybe better to just delay until IR generation? (perhaps we already do this, I forget?)
> 
> Open to having a separate pass, but since ADCE was such a good fit
> (it similarly needed three passes through the IR, which I could just
> piggy-back on) and happened to run at the right times I decided not to
> think any further.
> 
> Hmm, rightio - I don't know too much about pass design (when we try to make them independent, when we don't). So I'll leave it to you folks & others to chime in if there are relevant guiding principles that should be applied differently here.
> 
> - Dave
>  
> 
> > On Tue, Mar 29, 2016 at 12:34 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com <mailto:dexonsmith at apple.com>> wrote:
> > During ADCE, track which debug info scopes still have live references
> > from the code, and delete debug info intrinsics for the dead ones.
> >
> > These intrinsics describe the locations of variables (in registers or
> > stack slots).  If there's no code left corresponding to a variable's
> > scope, then there's no way to reference the variable in the debugger and
> > it doesn't matter what its value is.
> >
> > I add a DEBUG printout when the described location in an SSA register,
> > in case it helps some trying to track down why locations get lost.
> > However, we still delete these; the root problem is that the scope
> > itself isn't attached to any real code.
> >
> >
> 
> 

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