[LLVMdev] Interfacing llvm with a precise, relocating GC

[Sanjoy Das]

Hi Talin,

Thank you for your response!

Since you mention that you don't implement derived pointers, I'm a
little confused about how your approach solves the issue I brought up.
It seems to me that unless you plan on pinning some objects, support
for derived pointers isn't optional.  I'm not talking about derived
pointers that the front-end introduces (which can be tracked) but the
ones that are introduced by llvm during IR level optimizations or,
worse, during and after instruction selection.

To be sure that we're talking about the same thing, as an example,
consider the loop (in pseudo llvm IR):

for (int i = 0 to (length - 1)) {
 total += a[i];
 safepoint(); // This thread may be stopped here, and `a'
              // may be relocated.
}

llvm can transform this loop to

for (int *i = &a[0] to &a[length - 1]) {
 total += *i;
 safepoint(); // llvm has now introduced an additional
              // derived pointer, `i'.
}

>From llvm's point of view this is a valid transformation; but it ends
up creating a new pointer the GC has to be aware of, and needs to be
relocated in sync with a.

Thanks!
-- Sanjoy