Thank you, Bertrand and John
I added this conversation to the bug report.
Thanks,
Vladimir
On 1/15/15 3:13 AM, Bertrand Delsart wrote:
On 14/01/2015 20:12, John Rose wrote:
On Jan 14, 2015, at 6:42 AM, Bertrand Delsart
<bertrand.dels...@oracle.com <mailto:bertrand.dels...@oracle.com>> wrote:
I would not prevent the JITs from using RBP as long as the changeset
is not sufficient to guarantee the profiling will work... and IMHO
solving the JSR292 issue will be much more intrusive (impacting
HotSpot stack walking code).
Here are some thoughts on that.
SPARC uses L7 (L7_mh_SP_save) for the same purpose of method handle
support as x86 uses RBP (rbp_mh_SP_save). So there's not a hard
requirement for x86 to take over RBP.
(Deep background: This purpose, in method handle support, is to allow
an adapter to make changes to the caller's SP. The adapter is the
initial callee from the caller, but may change argument shape, and
tail-calls the ultimate callee. Because it is a tail-call, the original
caller must have a spot where his original SP can be preserved. The
preservation works because the original caller knows he is calling a
MH.invoke method, which requires the extra argument preservation. The
repertoire of argument shape changes is quite small, actually; it is not
a very general mechanism since the LF machinery was put in. Perhaps the
whole thing could be removed somehow, by finding alternative techniques
for the few remaining changes. OTOH, this SP-restoring mechanism may be
helpful in doing more a general tail-call mechanism, and perhaps in
managing int/comp mode changes more cleanly, so I'd like us to keep it.
And document it better.)
Any register or stack slot will do for this purpose, as long as (i) its
value can be recovered after the MH.invoke call returns to the caller,
and (ii) its value can be dug up somehow during stack walking. There
are only a couple of places where stack walking code needs to sample the
value, so they should be adjustable.
Both x86 and SPARC use registers which are callee-save (or "non-volatile
across calls") which satisfy properties (i) and (ii). A standard stack
slot (addressed based on caller's RBP) would probably also satisfy those
properties.
A variably-positioned stack slot would also work, which would require
registering the position in each CodeBlob. That's unpleasant extra
detail, but it would align somewhat with the current logic which allows
each CodeBlob (nmethod, actually) to advertise which call sites need the
special processing (see the function is_method_handle_return(caller_pc)).
I recommend reserving a dead word of space in every stack frame that
makes MH.invoke calls, at a fixed position relative to that frame's RBP.
— John
I perfectly agree that it is doable (and with your proposed approach).
I just wanted to be sure people were aware that the RFE is more complex
than what the current changeset may suggest. We are not just taking
about reviewing and integrating a complete changeset contributed by the
community. There is more work needed, either by the community or by
Oracle. This will require changes at least in C1 and C2 call sequences,
in the stack walking, in the creation and sizing of compiled frames...
Regards,
Bertrand.
