[LLVMdev] Am I missing something in this Value debug output code?

[Daniel Berlin]

I think this just a bug.
I actually had the same bug in some other code.
Nowadays, i think this should just be "for (I : users())"

The main use of the use_begin/use_end idiom nowadays is to be able to
modify the use list itself (IE people removing uses) without iterator
invalidation.



On Tue, Mar 10, 2015 at 12:23 PM, Kaylor, Andrew <andrew.kaylor at intel.com>
wrote:

>  In the course of adding some new code I came across a debug output
> message before an assert that was extremely unhelpful.  If I incorrectly
> try to delete a Value object that still has uses in a debug build, an debug
> message is printed just before an assertion that says it is telling me
> where the value is still being used but actually just tries to print the
> value itself multiple times, which in my case was crashing because the
> Value was already partially destructed.
>
>
>
> It looked like a pretty simple and obvious fix and I was just going to go
> ahead and commit it, but when I looked at the revision history I saw that
> the current behavior has been in place since 2002.  So I thought maybe it
> would be a good idea to get a reality check and make sure I wasn’t being an
> idiot in some way.
>
>
>
> Can someone give me a second opinion?
>
>
>
> Here’s the change I am proposing:
>
>
>
> Index: lib/IR/Value.cpp
>
> ===================================================================
>
> --- lib/IR/Value.cpp         (revision 231388)
>
> +++ lib/IR/Value.cpp      (working copy)
>
> @@ -69,15 +69,15 @@
>
> #ifndef NDEBUG      // Only in -g mode...
>
>    // Check to make sure that there are no uses of this value that are
> still
>
>    // around when the value is destroyed.  If there are, then we have a
> dangling
>
> -  // reference and something is wrong.  This code is here to print out
> what is
>
> -  // still being referenced.  The value in question should be printed as
>
> -  // a <badref>
>
> +  // reference and something is wrong.  This code is here to print out
> where
>
> +  // the value is still being referenced.  The value will be printed as a
>
> +  // <badref>.
>
>    //
>
>    if (!use_empty()) {
>
>      dbgs() << "While deleting: " << *VTy << " %" << getName() << "\n";
>
>      for (use_iterator I = use_begin(), E = use_end(); I != E; ++I)
>
>        dbgs() << "Use still stuck around after Def is destroyed:"
>
> -           << **I << "\n";
>
> +           << *(I->getUser()) << "\n";
>
>    }
>
> #endif
>
>    assert(use_empty() && "Uses remain when a value is destroyed!");
>
>
>
> =======================================================================
>
>
>
> This code appears in the Value::~Value().  I’m pretty sure that “**I” here
> is just going to be a reference to the Value being destructed, whereas
> *(I->getUser()) will provide useful information.
>
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