llvm dev - Oct 2010 - [LLVMdev] Function inlining creates uninitialized stack roots

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
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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).

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
>
>
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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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