[llvm] r226588 - Add an assertion and prefer a crash over an infinite loop.

[Duncan P. N. Exon Smith]

> On 2015 Jan 20, at 11:10, David Blaikie <dblaikie at gmail.com> wrote:
> 
> 
> 
> On Tue, Jan 20, 2015 at 10:58 AM, Adrian Prantl <aprantl at apple.com> wrote:
> 
> 
> > Begin forwarded message:
> >
> > Subject: Re: [llvm] r226588 - Add an assertion and prefer a crash over an infinite loop.
> > From: "Duncan P. N. Exon Smith" <dexonsmith at apple.com>
> > Date: January 20, 2015 at 10:53:38 AM PST
> > Cc: llvm-commits at cs.uiuc.edu
> > To: Adrian Prantl <aprantl at apple.com>
> >
> >
> >> On 2015 Jan 20, at 10:03, Adrian Prantl <aprantl at apple.com> wrote:
> >>
> >> Author: adrian
> >> Date: Tue Jan 20 12:03:37 2015
> >> New Revision: 226588
> >>
> >> URL: http://llvm.org/viewvc/llvm-project?rev=226588&view=rev
> >> Log:
> >> Add an assertion and prefer a crash over an infinite loop.
> >>
> >> Modified:
> >>   llvm/trunk/lib/IR/DebugInfo.cpp
> >>
> >> Modified: llvm/trunk/lib/IR/DebugInfo.cpp
> >> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/IR/DebugInfo.cpp?rev=226588&r1=226587&r2=226588&view=diff
> >> ==============================================================================
> >> --- llvm/trunk/lib/IR/DebugInfo.cpp (original)
> >> +++ llvm/trunk/lib/IR/DebugInfo.cpp Tue Jan 20 12:03:37 2015
> >> @@ -527,12 +527,15 @@ bool DISubprogram::Verify() const {
> >>        while ((IA = DL.getInlinedAt()))
> >>          DL = DebugLoc::getFromDILocation(IA);
> >>        DL.getScopeAndInlinedAt(Scope, IA);
> >> +        assert(Scope && "debug location has no scope");
> >>        assert(!IA);
> >>        while (!DIDescriptor(Scope).isSubprogram()) {
> >>          DILexicalBlockFile D(Scope);
> >>          Scope = D.isLexicalBlockFile()
> >>                      ? D.getScope()
> >>                      : DebugLoc::getFromDILexicalBlock(Scope).getScope();
> >> +          if (!Scope)
> >> +            llvm_unreachable("lexical block file has no scope");
> >
> > It's strange that this isn't simply an assertion:
> >
> >    assert(Scope && "lexical block file has no scope");
> >
> > But I guess that's so that it crashes even when NDEBUG?  (Is that
> > reliable?)
> 
> Yes, that’s the intention (and llvm_unreachable is meant to be used that way).
> 
> I don't believe that's the intention of the LLVM style guide - generally we try to remove conditionals like this and replace the conditional with the assertion so the dead code isn't exercised in release builds.

I don't mind an exception here myself (as long as there's a comment).  In
release builds this code is only exercised in the verifier (I think?), and
I don't think it makes sense to infinite loop there.

Although another option is:

    if (!Scope)
      return false;

(That's certainly a valid answer.)

>  
> >
> > If that's the reason, you should probably add a comment to that
> > effect since otherwise I'm likely to switch it to an assert next
> > time I look at it ;).
> >
> > (Also, do you know when this situation arises?)
> 
> Happened to me when I compiled an old .ll file without the MDLocation upgrade.
> 
> -- adrian
> 
> >
> >>        }
> >>        if (!DISubprogram(Scope).describes(F))
> >>          return false;
> >>
> >>
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> >
> 
> 
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