8270090: C2: LCM may prioritize CheckCastPP nodes over projections

Backport-of: 8ebea443f333ecf79d6b0fc725ededb231e83ed5

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 +489,10 @@ 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 must start their blocks and are selected
+// eagerly. After them, projections take top priority for correctness. Next
+// after projections 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 +530,34 @@ 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) {
+      // CreateEx must start the block (after Phi and Parm nodes which are
+      // pre-scheduled): select it right away.
       worklist.map(i,worklist.pop());
       return n;
     }
 
+    uint n_choice = 2;
+    if (n->is_Proj()) {
+      // Projections should follow their parents.
+      n_choice = 5;
+    } else if (n->Opcode() == Op_Con || iop == Op_CheckCastPP) {
+      // Constants and CheckCastPP nodes have higher priority than the rest of
+      // the nodes tested below.
+      n_choice = 4;
+    }
+
+    if (n_choice >= 4 && choice < n_choice) {
+      // n is a constant, a projection, or a CheckCastPP node: record as current
+      // winner, but keep looking for higher-priority nodes in the worklist.
+      choice  = n_choice;
+      // 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 )
@@ -556,8 +576,6 @@ Node* PhaseCFG::select(
       }
     }
 
-    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
     // end of the basic block
@@ -1044,10 +1062,6 @@ bool PhaseCFG::schedule_local(Block* block, GrowableArray<int>& ready_cnt, Vecto
         // of the phi to be scheduled first. The select() method breaks
         // 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.
-        worklist.insert(0, m);
       } else {
         worklist.push(m);         // Then on to worklist!
       }