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Re: GCC/JIT and precise garbage collection support?

On 07/10/2015 09:04 AM, Armin Rigo wrote:
Hi David,

On 10 July 2015 at 16:11, David Malcolm <> wrote:
AIUI, we have CALL_INSN instructions all the way through the RTL phase
of the backend, so we can identify which locations in the generated code
are calls; presumably we'd need at each CALL_INSN to determine somehow
which RTL expressions tagged as being GC-aware are live (perhaps a
mixture of registers and fp-offset expressions?)

So presumably we could use that information (maybe in the final pass) to
write out some metadata describing for each %pc callsite the relevant GC

Armin: does this sound like what you need?

Not quite.  I can understand that you're trying to find some solution
with automatic discovery of the live variables of a "GC pointer" type
and so on.  This is more than we need, and if
we had that, then we'd need to work harder to remove the extra stuff.
We only want the end result: attach to each CALL_INSN a list of
variables which should be stored in the stack map for that call, and
be ready to see these locations be modified from outside across the
call if a GC occurs.
I wonder how much overlap there is between this need and what we're going to need to do for resumable functions which are being discussed in the ISO C++ standards meetings.


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