[LLVMdev] Patch to allow llvm.gcroot to work with non-pointer allocas.

[nicolas geoffray]

Hi Talin,

On Sat, Sep 25, 2010 at 4:18 AM, Talin <viridia at gmail.com> wrote:
>
>
> Many languages support the notion of a "value type". Value types are always
> passed by value, unlike reference types which are always passed by
> pointer. An example is the "struct" type in C#. Another example is a "tuple"
> type. A value type which is a local variable lives on the stack as an
> alloca, not on the heap. When a function is called with a value type as
> argument, the callee gets its own copy of the argument, rather than sharing
> a pointer with the caller.
>

Yes.


>
> Value types are represented in LLVM using structs, and may contain pointer
> fields which need to be traced.
>
>
Yes.


> The way that I handle non-pointer types is to generate an array of field
> offsets (containing the offset of each pointer field within the struct) as
> the metadata argument to llvm.gcroot. This meta argument is then processed
> in my GCStrategy, where I add the stack root offset to the offsets in the
> field offset array, which yields the stack offsets of the actual pointers in
> the call frame.
>
>
Did you think of the alternative of calling llvm.gcroot on pointers in this
struct? This requires to change the verifier to support non-alloca pointers
in llvm.gcroot, but it makes the solution more general and cleaner: pointers
given to llvm.gcroot only point to objects in the heap.

I think that, originally, the purpose of the second argument of llvm.gcroot
was to emit static type information.

Nicolas



> It's all pretty simple really.
>
>
>>
>> Nicolas
>>
>>  On Fri, Sep 24, 2010 at 7:00 PM, Chris Lattner <clattner at apple.com>wrote:
>>
>>>  On Sep 22, 2010, at 8:52 AM, Talin wrote:
>>> > I'm moving this thread to llvm-dev in the hopes of reaching a wider
>>> audience.
>>> >
>>> > This patch relaxes the restriction on llvm.gcroot so that it can work
>>> with non-pointer allocas. The only changes are to Verifier.cpp - it appears
>>> from my testing that llvm.gcroot always worked fine with non-pointer
>>> allocas, except that the verifier wouldn't allow it. I've used this patch to
>>> build an efficient stack crawler (an alternative to shadow-stack that uses
>>> only static constant data structures.)
>>> >
>>> > Here's a deal: If you accept this patch, I'll write up an extensive
>>> tutorial on how to write a stack crawler like mine. (Actually, it's already
>>> written, however without this patch the tutorial doesn't make any sense.)
>>>
>>> Hi Talin,
>>>
>>> I don't think anyone is really using the GC support, other than Nicolas
>>> in VMKit.  If he's ok with the change, I am too.  Please make sure the dox
>>> stay up to date though.
>>>
>>> -Chris
>>> _______________________________________________
>>> 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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