[LLVMdev] weird issue with mem2reg, should have guessed

Chris Lattner
Wed, 23 Jun 2004 16:14:03 -0000

On Wed, 23 Jun 2004, Patrick Meredith wrote:

> What's different about code that's been mem2reg'd from straight
front end
> code, or anything that mem2reg hasn't been run on?  PHINODES!

Yup, front-ends generally don't produce SSA form. :)

> It appears to be crashing when I try to cast a Value* that's really a
> BB* (from the PHInode operands) to a User*, insteresting since I am
> dyn_casting.  I just caught this on cerr though (printing out what the
> Value* was each time).

Yeah, you shouldn't do that. :)  Also, you should use the 'cast'
template
instead of the dyn_cast template unless you are prepared to handle the
null return value.  Use 'cast' when you _know_ that something is a
particular type.  This will give you a nice assertion failure instead of a
segfault when you deref the null pointer :)

> Let me check bugpoint.

Cool, post the testcase it produces.  Hopefully it will be small :)

-Chris

> ----- Original Message -----
> From: "Misha Brukman" <brukman at uiuc.edu>
> To: <llvmdev at cs.uiuc.edu>
> Sent: Wednesday, June 23, 2004 3:56 PM
> Subject: Re: [LLVMdev] weird issue with mem2reg
>
>
> > On Wed, Jun 23, 2004 at 03:50:09PM -0500, Patrick Meredith wrote:
> > > MetaSplit is an anlysis I just finished writing.  It doesn't
alter
> > > anything, all it does is build a set of "program
instructions".  For
> > > some reason even though if I run it with any other combination of
> > > passes I've found, anytime I run it with mem2reg I get a seg
fault in
> > > dyn_cast!  Here's output:
> >
> > Since the crash is after your code is entered, the problem is probably
> > not in mem2reg, but in your pass.  To see if that's the case, you
can do
> > this:
> >
> > % opt -mem2reg < orig.bc > m2r.bc
> > % opt -load=... -metasplit < m2r.bc > output.bc
> >
> > And see if it crashes in mem2reg or in your pass.  To narrow your
> > testcase down further, we recommend the use of bugpoint, the automatic
> > test case reducer:
> >
> >   http://llvm.cs.uiuc.edu/docs/HowToSubmitABug.html
> >
> > In this case, this should work:
> >
> > % bugpoint -load=... -mem2reg -metasplit orig.bc
> >
> > --
> > Misha Brukman :: http://misha.brukman.net :: http://llvm.cs.uiuc.edu
> >
> > _______________________________________________
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> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://mail.cs.uiuc.edu/mailman/listinfo/llvmdev
>
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-Chris

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