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

[Talin]

On Fri, Sep 24, 2010 at 10:44 AM, nicolas geoffray <
nicolas.geoffray at gmail.com> wrote:

> Thanks for the heads up Chris.
>
> Talin, how is your GC dealing with non-pointers (be it allocas or not)?
> What is the use-case (either in C or LLVM)?
>

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.

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

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.

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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20100924/f1726127/attachment.html>