llvm dev - Jan 2015 - [LLVMdev] "Expected to drop a reference" assertion thrown

Hello,

A few days ago I upgraded to the LLVM/clang ToT (on Jan 19th or so)  and I just
encountered this assertion.

“Expected to drop a reference”  which is defined in llvm/lib/IR/Metadata.cpp
line 136.


I create DebugLoc objects using DebugLog:get(unsigned line, unsigned Col, MDNode
*Scope, MDNode *InlinedAt=nullptr)

The DebugLoc objects are wrapped in one of my classes and when instances get
destructed the assertion is thrown.

Would anyone have an idea what I’m doing wrong that causes this?

Best,

Christian Schafmeister
Professor
Chemistry Department
Temple University

> On 2015 Jan 23, at 22:31, Christian Schafmeister <chris.schaf at
verizon.net> wrote:
> 
> 
> Hello,
> 
> A few days ago I upgraded to the LLVM/clang ToT (on Jan 19th or so)  and I
just encountered this assertion.
> 
> “Expected to drop a reference”  which is defined in
llvm/lib/IR/Metadata.cpp line 136.
> 
> 
> I create DebugLoc objects using DebugLog:get(unsigned line, unsigned Col,
MDNode *Scope, MDNode *InlinedAt=nullptr)
> 
> The DebugLoc objects are wrapped in one of my classes and when instances
get destructed the assertion is thrown.
> 
> Would anyone have an idea what I’m doing wrong that causes this?
The first thing I'd try is recompiling with ASan support.  All the cases
I've seen of this assertion firing are use-after-frees.

Likely your `LLVMContext` has been destroyed already by the time this
`DebugLoc` is deleted.  Since `LLVMContext` teardown assumes that there
aren't any live tracking references to metadata, you need to `reset()` or
destroy the `DebugLoc`s before destroying the context.

Thank you - I’ll give ASan a try - it may alert/protect me from other problems
that come up down the road.
This is almost certainly a memory management problem on my end.   I was managing
the DebugLoc’s lifetime using my copying garbage collector.
I implemented a work around where I don’t expose the DebugLoc to my garbage
collector and the problem went away.



On Jan 25, 2015, at 8:55 PM, Duncan P. N. Exon Smith <dexonsmith at
apple.com> wrote:
> 
>> On 2015 Jan 23, at 22:31, Christian Schafmeister <chris.schaf at
verizon.net> wrote:
>> 
>> 
>> Hello,
>> 
>> A few days ago I upgraded to the LLVM/clang ToT (on Jan 19th or so) 
and I just encountered this assertion.
>> 
>> “Expected to drop a reference”  which is defined in
llvm/lib/IR/Metadata.cpp line 136.
>> 
>> 
>> I create DebugLoc objects using DebugLog:get(unsigned line, unsigned
Col, MDNode *Scope, MDNode *InlinedAt=nullptr)
>> 
>> The DebugLoc objects are wrapped in one of my classes and when
instances get destructed the assertion is thrown.
>> 
>> Would anyone have an idea what I’m doing wrong that causes this?
> 
> The first thing I'd try is recompiling with ASan support.  All the
cases
> I've seen of this assertion firing are use-after-frees.
> 
> Likely your `LLVMContext` has been destroyed already by the time this
> `DebugLoc` is deleted.  Since `LLVMContext` teardown assumes that there
> aren't any live tracking references to metadata, you need to `reset()`
or
> destroy the `DebugLoc`s before destroying the context.