This is the GC_V4 specifics. It uses two bits of least significant
byte of the status word during GC, but it always return zeros to this
bits after GC.
Here is the atomic way to do the changes:
do {
unsigned char lsb = *P_HASH_CONTENTION(p_obj);
unsigned char new_lsb = lsb | hb;
unsigned char uptodate_lsb =
port_atomic_cas8(P_HASH_CONTENTION(p_obj), new_lsb, lsb);
while (lsb != uptodate_lsb);
BTW, atomic operations is only needed if GC wants to update those two
bits during mutator work. Otherwise, non-atomic operations is ok, as
the same value can be safely written to the same address by multiple
threads. This is one pros for moving this code to GC.
--
Ivan
On 8/28/06, Weldon Washburn <[EMAIL PROTECTED]> wrote:
While porting MMTk to harmony/drlvm, I hit an integration problem. It
could even be a bug. set_hash_bits() assumes the least significant
bit is zero. Assuming that the LSB can be "owned" by the garbage
collector for its purposes, set_hash_bits() will fail if the GC sets
this bit to one. Somehow I think the code should read the target
location, create the intended bit pattern before attempting to do the
atomic compare and swap. Currently the code assume the target CAS
location holds zero.
SInce I am working only in single thread right now, I hacked around
the problem with the below. Thoughts?
C:\t_harmony\drlvm\trunk\vm\vmcore\src\thread>svn diff mon_enter_exit.cpp
Index: mon_enter_exit.cpp
===================================================================
--- mon_enter_exit.cpp (revision 425482)
+++ mon_enter_exit.cpp (working copy)
@@ -368,7 +368,12 @@
hb = (23 & HASH_MASK); // NO hash = zero allowed, thus hard map hb = 0
to a fixed prime number
// don't care if the cmpxchg fails -- just means someone else already set t
he hash
- port_atomic_cas8(P_HASH_CONTENTION(p_obj),hb, 0);
+ //port_atomic_cas8(P_HASH_CONTENTION(p_obj),hb, 0);
+ unsigned char lsb = *P_HASH_CONTENTION(p_obj);
+ lsb = lsb & 0x01; //wjw need to keep the LSB, its used by MMTk Garbage Col
lector
+ hb = hb | lsb;
+ if ( (*P_HASH_CONTENTION(p_obj) & HASH_MASK) == 0 ) // wjw non-atomic hack
for now
+ *P_HASH_CONTENTION(p_obj) = hb;
}
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