possible CMS problem with C2

Tom Rodriguez tom.rodriguez at oracle.com
Thu Mar 31 12:41:14 PDT 2011

While looking at the code we generate for a simple program I noticed that we were eliminating card marks with CMS in a illegal fashion.  For this simple program:

public class test {
    static Object a1;
    static Object a2;
    static Object a3;
    public static void main(String[] args) {
        a1 = args;
        a2 = args;
        a3 = args;

we generate this:

000   B1: #     N1 <- BLOCK HEAD IS JUNK   Freq: 1
000     SAVE   R_SP,-72,R_SP
004     SET    precise klass test: 0x00843a50:Constant:exact *,R_L0     !ptr
00c +   SET    0xf92d4000,R_L2  !ptr
010 +   STW    R_I0,[R_L0 + #352]       ! ptr ! Field test.a3
014 +   STW    R_I0,[R_L0 + #348]       ! ptr ! Field test.a2
018 +   SRL    R_L0,#9,R_L1     ! Cast ptr R_L0 to int and shift
01c     STW    R_I0,[R_L0 + #344]       ! ptr ! Field test.a1
020 +   STB    #0,[R_L2 + R_L1] ! CMS card-mark byte 0
024     SETHI  #PollAddr,L0     ! Load Polling address
        LDUW   [L0],G0  !Poll for Safepointing
034 +   ! return

which happens to be ok because the STB happens last but we don't actually have enough dependences to ensure that we get this schedule.  We emit 3 separate StoreCMs for each field references and each one has a dependence on the store that it covers.  What's going wrong is that we're allowing StoreCM to participate in the the store elimination in StoreNode::Ideal so we end up with this:

 45     StoreP  ===  5  7  44  10  [[ 16  50 ]]  @precise klass test: 0x00843a50:Constant:exact+352 *, name=a3, idx=6;  Memory: @precise klass test: 0
x00843a50:Constant:exact+352 *, name=a3, idx=6; !jvms: test::main @ bci:9
 36     StoreP  ===  5  7  35  10  [[ 16 ]]  @precise klass test: 0x00843a50:Constant:exact+348 *, name=a2, idx=5;  Memory: @precise klass test: 0x008
43a50:Constant:exact+348 *, name=a2, idx=5; !jvms: test::main @ bci:5
 25     StoreP  ===  5  7  24  10  [[ 16 ]]  @precise klass test: 0x00843a50:Constant:exact+344 *, name=a1, idx=4;  Memory: @precise klass test: 0x008
43a50:Constant:exact+344 *, name=a1, idx=4; !jvms: test::main @ bci:1
 50     StoreCM ===  5  7  31  23  45  [[ 16 ]]  @rawptr:BotPTR, idx=Raw;  Memory: @rawptr:BotPTR, idx=Raw; !jvms: test::main @ bci:9

The store to a3 has a StoreCM and it has killed the StoreCMs for a1 and a2 but it doesn't have a dependence on it.  Since the slices are independent the a3 operations could be scheduled before the store to a1 and a2.  It may be that this is rare in practice and/or the local schedule tends to put the StoreCM last but it's clearly wrong.  G1 is safe from this because the StoreCMs are never close enough to be eliminated.

It's easy to fix but it might hurt CMS performance a bit.  It could also be done safely if the StoreCM could have dependences on multiple stores but since it's not using normal precedence edges I'm not sure how this would be implemented.

Does this sound like any issues that have been seen in the past?


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