[Jikesrvm-core] sysCallSigWait and nonvolatile registers

[David P G. <gr...@us...>]

Hi,

      Could someone explain how sysCallSigWait is supposed to be dealing
with nonvolatile registers?

      As I understand it, the point of sysCallSigWait is to allow a sysCall
transition into C while enabling a GC to be taken while the thread that did
the sysCallSigWait is in C.  On both PPC and IA32, it appears that the
sysCallSigWait implementation is not saving the nonvolatile registers in
the VM_Registers object.  Therefore, I don't see how it is possible to
actually correctly take a GC from this point.  The nonvolatile registers
may in fact contain references, and they have been passed off into C-land
in the hardware registers and potentially saved at some arbitrary point in
the C portion of the stack frame.  The only way this might possibly work is
if every frame on the stack above the call to sysCallSigWait is a baseline
compiled frame, and even this seems pretty fragile.

--dave