8285820: C2: LCM prioritizes locally dependent CreateEx nodes over projections after 8270090

src/hotspot/share/opto/lcm.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1998, 2018, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1998, 2022, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -489,9 +489,12 @@ void PhaseCFG::implicit_null_check(Block* block, Node *proj, Node *val, int allo
 
 
 //------------------------------select-----------------------------------------
-// Select a nice fellow from the worklist to schedule next. If there is only
-// one choice, then use it. Projections take top priority for correctness
-// reasons - if I see a projection, then it is next.  There are a number of
+// Select a nice fellow from the worklist to schedule next. If there is only one
+// choice, then use it. CreateEx nodes that are initially ready must start their
+// blocks and are given the highest priority, by being placed at the beginning
+// of the worklist. Next after initially-ready CreateEx nodes are projections,
+// which must follow their parents, and CreateEx nodes with local input
+// dependencies. Next are constants and CheckCastPP nodes. There are a number of
 // other special cases, for instructions that consume condition codes, et al.
 // These are chosen immediately. Some instructions are required to immediately
 // precede the last instruction in the block, and these are taken last. Of the
@@ -529,15 +532,28 @@ Node* PhaseCFG::select(
     Node *n = worklist[i];      // Get Node on worklist
 
     int iop = n->is_Mach() ? n->as_Mach()->ideal_Opcode() : 0;
-    if( n->is_Proj() ||         // Projections always win
-        n->Opcode()== Op_Con || // So does constant 'Top'
-        iop == Op_CreateEx ||   // Create-exception must start block
-        iop == Op_CheckCastPP
-        ) {
+    if (iop == Op_CreateEx || n->is_Proj()) {
+      // CreateEx nodes that are initially ready must start the block (after Phi
+      // and Parm nodes which are pre-scheduled) and get top priority. This is
+      // currently enforced by placing them at the beginning of the initial
+      // worklist and selecting them eagerly here. After these, projections and
+      // other CreateEx nodes are selected with equal priority.
       worklist.map(i,worklist.pop());
       return n;
     }
 
+    if (n->Opcode() == Op_Con || iop == Op_CheckCastPP) {
+      // Constants and CheckCastPP nodes have higher priority than the rest of
+      // the nodes tested below. Record as current winner, but keep looking for
+      // higher-priority nodes in the worklist.
+      choice  = 4;
+      // Latency and score are only used to break ties among low-priority nodes.
+      latency = 0;
+      score   = 0;
+      idx     = i;
+      continue;
+    }
+
     // Final call in a block must be adjacent to 'catch'
     Node *e = block->end();
     if( e->is_Catch() && e->in(0)->in(0) == n )
@@ -556,7 +572,7 @@ Node* PhaseCFG::select(
       }
     }
 
-    uint n_choice  = 2;
+    uint n_choice = 2;
 
     // See if this instruction is consumed by a branch. If so, then (as the
     // branch is the last instruction in the basic block) force it to the
@@ -1045,8 +1061,8 @@ bool PhaseCFG::schedule_local(Block* block, GrowableArray<int>& ready_cnt, Vecto
         // ties in scheduling by worklist order.
         delay.push(m);
       } else if (m->is_Mach() && m->as_Mach()->ideal_Opcode() == Op_CreateEx) {
-        // Force the CreateEx to the top of the list so it's processed
-        // first and ends up at the start of the block.
+        // Place CreateEx nodes that are initially ready at the beginning of the
+        // worklist so they are selected first and scheduled at the block start.
         worklist.insert(0, m);
       } else {
         worklist.push(m);         // Then on to worklist!