[llvm-dev] Deleting function IR after codegen

[Hal Finkel via llvm-dev]

----- Original Message -----

> From: "Chandler Carruth" <chandlerc at gmail.com>
> To: "Quentin Colombet" <qcolombet at apple.com>, "Eric Christopher"
> <echristo at gmail.com>
> Cc: "Pete Cooper" <peter_cooper at apple.com>, "Duncan P. N. Exon Smith"
> <dexonsmith at apple.com>, "Hal Finkel" <hfinkel at anl.gov>, "Philip
> Reames" <listmail at philipreames.com>, "Mehdi Amini"
> <mehdi.amini at apple.com>, "Rafael Espíndola"
> <rafael.espindola at gmail.com>, "llvm-dev" <llvm-dev at lists.llvm.org>
> Sent: Thursday, May 12, 2016 6:11:34 PM
> Subject: Re: [llvm-dev] Deleting function IR after codegen

> FWIW, +1 from me as well.

> But I don't think you need to make this a module pass or anything
> else. I think you should leave the husks of the functions around and
> just nuke the IR out from under them. That way the module surface
> remains essentially identical. You can also probably nuke all the
> instructions from BBs with their addresses taken for jump tables,
> etc.
I agree. We need to be careful about invalidating inter-procedural IR-level analysis results that we make use of using CodeGen (like AA). Consider, for example, the following situation: 

1. We CodeGen a function foo(), and then remove its IR. During this process, we never used an AA query that reached CFL-AA 
2. We CodeGen a function bar(), and bar() calls foo(). 
3. During (2), we make an AA query on instructions in bar(), which have MMOs with IR Values in bar() 
4. CFL-AA is reached and does not have a cached graph for bar(), so it builds one 
5. While building the graph for bar(), it reaches the call to foo(), and calls tryInterproceduralAnalysis 
6. CFL-AA does not have a cached graph for foo(), so tryInterproceduralAnalysis triggers one to be constructed 
7. But foo() is now empty, and so has trivial aliasing properties 
8. We return an incorrect AA result when compiling bar() 

-Hal 

> -Chandler

> On Tue, Mar 8, 2016 at 1:16 PM Quentin Colombet < qcolombet at apple.com
> > wrote:

> > > On Mar 8, 2016, at 11:50 AM, Eric Christopher <
> > > echristo at gmail.com
> > > >
> > > wrote:
> > 
> 

> > > > I could attach a patch, but first i’d really like to know if
> > > > anyone
> > > > is fundamentally opposed to this.
> > > 
> > 
> 

> > > Not necessarily. I think that any information that isn't being
> > > serialized in MI right now for a function could be as well.
> > > Definitely something for GlobalISel to keep in mind.
> > 
> 
> > +1.
> 
> > That’s basically where I would like to go with
> > MachineModule/MachineModulePass.
> 
> > http://lists.llvm.org/pipermail/llvm-dev/2016-January/094426.html
> 

> > Cheers,
> 
> > -Quentin
> 

> > > > I should note, a couple of issues have come up in the
> > > > prototype.
> > > 
> > 
> 
> > > > - llvm::getDISubprogram was walking the function body to find
> > > > the
> > > > subprogram. This is trivial to fix as functions now have !dbg
> > > > on
> > > > them.
> > > 
> > 
> 

> > > This is definitely worth it, please go ahead and do this.
> > 
> 

> > > > - The AsmPrinter is calling canBeOmittedFromSymbolTable on
> > > > GlobalValue’s which then walks all their uses. I think this
> > > > should
> > > > be done earlier in codegen as an analysis whose results are
> > > > available to the AsmPrinter.
> > > 
> > 
> 

> > > I think this makes sense, but I worry about late added
> > > GlobalValues
> > > during code gen? How would we cope with that? Example: Let's say
> > > we
> > > start lowering a target specific construct as an MI pass etc and
> > > it
> > > constructs a global value, when do we run the analysis to make
> > > sure
> > > that all such things happen? Late as possible I'd assume. Maybe
> > > this
> > > isn't an issue, but thought I'd bring it up. At any rate, could
> > > you
> > > provide a bit more detail here?
> > 
> 

> > > > - BB’s whose addresses are taken, i.e. jump tables, can’t be
> > > > deleted.
> > > > Those functions will just keep their IR around so no changes
> > > > there.
> > > 
> > 
> 

> > > Oh well. Conveniently there aren't a lot of these.
> > 
> 

> > > -eric
> > 
> 

-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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