[LLVMdev] Function inlining creates uninitialized stack roots

nicolas geoffray nicolas.geoffray at gmail.com
Sat Oct 2 14:44:39 PDT 2010


Sure. I think we can change the GC lowering pass to recognize all
llvm.gcroot (not only the ones in the first block), and move them to the
first block so that they are initialized by the pass later on.


On Sat, Oct 2, 2010 at 10:58 PM, Talin <viridia at gmail.com> wrote:

> On Sat, Oct 2, 2010 at 12:59 PM, nicolas geoffray <
> nicolas.geoffray at gmail.com> wrote:
>
>> Hi Talin,
>>
>> You are not doing something wrong, it is just that the LLVM optimizers
>> consider llvm.gcroot like a regular function call. The alloca is moved in
>> the first block most probably because the inliner anticipates another
>> optimization pass (the mem2reg).
>>
>
> OK, well, is there anything that can be done?
>
>>
>> Cheers,
>> Nicolas
>>
>> On Sat, Oct 2, 2010 at 8:28 PM, Talin <viridia at gmail.com> wrote:
>>
>>> I'm still putting the final touches on my stack crawler, and I've run
>>> into a problem having to do with function inlining and local stack roots.
>>>
>>> As you know, all local roots must be initialized before you can make any
>>> call to a function which might crawl the stack. My compiler ensures that all
>>> local variables of a function are allocated, declared as root, and
>>> initialized in the first block.
>>>
>>> However, the function inlining pass does not seem to preserve these
>>> constraints. For example, say I have a function F:
>>>
>>>    void F() {
>>>      f1();
>>>      f2();
>>>    }
>>>
>>> Let's say that f1() causes a stack crawl (perhaps by triggering a
>>> collection cycle), and that f2() declares a stack root 'a'. If the optimizer
>>> decides to inline f2, then by the time the GCPrinter is called the resulting
>>> code looks like this:
>>>
>>>    - a = alloca
>>>    - call f1
>>>    - call llvm.gcroot(a)
>>>    - store null, a
>>>
>>> The inliner moved the alloca instruction to the top of the function, but
>>> not the call to llvm.gcroot or the initialization of the variable. In other
>>> words, the initialization of the root occurs *after* the call to f1. This
>>> means that the stack crawler is seeing garbage and therefore crashes hard.
>>> (Also, I've observed that the call to llvm.gcroot and the initialization
>>> might not even be in the first block anymore.)
>>>
>>> As per usual, I'm not sure if this is a bug in LLVM or my doing something
>>> wrong...
>>>
>>> --
>>> -- Talin
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>
>>>
>>
>
>
> --
> -- Talin
>
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