[LLVMdev] getting closer!
gordonhenriksen at mac.com
Mon Apr 21 20:23:30 CDT 2008
On Apr 21, 2008, at 20:09, Terence Parr wrote:
> Ok, I *might* be getting this from the assembly code. ... From
> that, it will push/pop in functions? If so, that's easy enough. :)
Yup! Sounds like you've got it.
> What I was/am missing is the explicit link between types and
> variables in a GC.c file and the generated machine code. If I can
> get that last explicit link, I'm off to the races.
You mean, how do you know what sort of object you're tracing? You've
got 3 options here…
• If you have an type tree (as in Java or .NET), you can assume that
every root starts with a pointer to object metadata, which should
naturally include GC tracing information.
• If you have a type forest (as in C or C++) with optional vtables,
then no such assumption is possible, and you can include type layout
information in the %metadata parameter to @llvm.gcroot. The FrameMap
type includes this data.
• You can tag values, as in lisp or many functional languages. (e.g.,
integer values have the low bit set, pointers do not.) All fields in a
block must be of a uniform size, and you'll still need to know how
many words in a block.
This decision is completely agnostic to the decision to use the shadow
stack, or something more efficient.
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