Safepoint timeout in scimark?

[Roland Westrelin]

> BTW, I tried -XX:+UseCountedLoopSafepoints, and it does indeed solve
> the problem, but ran into other problems in SPECjvm2008's compiler
> benchmarks:
>
> java -XX:+UseCountedLoopSafepoints -XX:+SafepointTimeout
> -XX:SafepointTimeoutDelay=2000 -jar SPECjvm2008.jar -ict -ikv compiler
>
> .. fails with:
>
> #
> # A fatal error has been detected by the Java Runtime Environment:
> #
> #  SIGSEGV (0xb) at pc=0x00007f7adff09814, pid=23268, tid=23296
> #
> # JRE version: OpenJDK Runtime Environment (9.0) (fastdebug build 9-
> internal+0-2016-07-11-184003.rkennke.jdk9)
> # Java VM: OpenJDK 64-Bit Server VM (fastdebug 9-internal+0-2016-07-11-
> 184003.rkennke.jdk9, mixed mode, tiered, compressed oops, g1 gc, linux-
> amd64)
> # Problematic frame:
> # V  [libjvm.so+0xfb0814]  PhaseIdealLoop::get_ctrl(Node*)+0x64

I took a look at the crash that Roman observe and I see that the failure
occurs when a loop has a safepoint on the backedge. In that case,
because in PhaseIdealLoop::is_counted_loop():

  // Allow funny placement of Safepoint
  if (back_control->Opcode() == Op_SafePoint)
    back_control = back_control->in(TypeFunc::Control);

the safepoint node is disconnected when the counted loop is created but
it's still in the list of safepts so IdealLoopTree::remove_safepoints()
removes it (because the loop has a call):

phase->lazy_replace(n, n->in(TypeFunc::Control));

with n->in(TypeFunc::Control) = NULL. The loop's tail is left to point
to the safepoint and when IdealLoopTree::tail() is called later on,
get_ctrl() is called with NULL. That problem doesn't occur when
safepoints are optimized out because. I suppose it's because:

  if (!UseCountedLoopSafepoints) {
    // Check for SafePoint on backedge and remove
    Node *sfpt = x->in(LoopNode::LoopBackControl);
    if (sfpt->Opcode() == Op_SafePoint && is_deleteable_safept(sfpt)) {
      lazy_replace( sfpt, iftrue );
      if (loop->_safepts != NULL) {
        loop->_safepts->yank(sfpt);
      }
      loop->_tail = iftrue;
    }
  }

in PhaseIdealLoop::is_counted_loop()

Anyway, I see 2 problems here: 1) the crash and 2) we drop a safepoint
that we would like to keep. I tried a fix with the patch below: I moved
the safepoint above the CountedLoopEnd. I duplicated the test of the
CountedLoopEnd and put the safepoint in one of the branch otherwise if
we deoptimize at the safepoint we would resume execution after a test
that's never executed.

Roland.


diff --git a/src/share/vm/opto/loopnode.cpp b/src/share/vm/opto/loopnode.cpp
--- a/src/share/vm/opto/loopnode.cpp
+++ b/src/share/vm/opto/loopnode.cpp
@@ -640,6 +640,32 @@
   // Replace the old IfNode with a new LoopEndNode
   Node *lex = _igvn.register_new_node_with_optimizer(new CountedLoopEndNode( iff->in(0), test, cl_prob, iff->as_If()->_fcnt ));
   IfNode *le = lex->as_If();
+
+  Node *sfpt = x->in(LoopNode::LoopBackControl);
+  if (sfpt->Opcode() == Op_SafePoint && UseCountedLoopSafepoints) {
+    Node* ctrl = le->in(0);
+    IfNode* iff = new IfNode(ctrl, test, cl_prob, le->_fcnt);
+    _igvn.register_new_node_with_optimizer(iff);
+    set_idom(iff, ctrl, dom_depth(ctrl)+1);
+    set_loop(iff, loop);
+    Node* iffalse = _igvn.register_new_node_with_optimizer(new IfFalseNode(iff));
+    Node* iftrue = _igvn.register_new_node_with_optimizer(new IfTrueNode(iff));
+    set_idom(iffalse, iff, dom_depth(iff)+1);
+    set_loop(iffalse, loop);
+    set_idom(iftrue, iff, dom_depth(iff)+1);
+    set_loop(iftrue, loop);
+    _igvn.replace_input_of(sfpt, 0, iftrue);
+    set_idom(sfpt, iftrue, dom_depth(iftrue)+1);
+    Node* region = new RegionNode(3);
+    region->init_req(1, sfpt);
+    region->init_req(2, iffalse);
+    _igvn.register_new_node_with_optimizer(region);
+    set_idom(region, iff, dom_depth(iff)+1);
+    set_loop(region, loop);
+    _igvn.replace_input_of(le, 0, region);
+    loop->_tail = iftrue;
+  }
+
   uint dd = dom_depth(iff);
   set_idom(le, le->in(0), dd); // Update dominance for loop exit
   set_loop(le, loop);