Apply trapping nullcheck to the uncompressed target for compressed pointer.

Trapping nullcheck might generate two uncompress instructions for the
same compressed oop in Graal. One is inserted by the backend when it
emits nullcheck. If the pointer is a compressed object, it should be
uncompressed before the nullcheck is emitted. And another one is
generated by the normal uncompressing operation. These two instructions
are duplicated with each other.
The generated codes on AArch64 like:

  ldr   w0, [x0,oracle#112]
  lsl   x2, x0, oracle#3      ; uncompressing (first)
  ldr   xzr, [x2]       ; implicit exception: deoptimizes
  ......                ; fixed operations
  lsl   x0, x0, oracle#3      ; uncompressing (second)
  str   w1, [x0,oracle#12]

A simple way to avoid this is to apply the nullcheck to the uncompressed
result if it exists instead of to the compressed pointer when generating
the trapping nullcheck.
With the modification, the codes above could be optimized to:

  ldr   w0, [x0,oracle#112]
  lsl   x0, x0, oracle#3      ; uncompressing
  ldr   xzr, [x0]       ; implicit exception: deoptimizes
  ......                ; fixed operations
  str   w1, [x0,oracle#12]

Change-Id: Iabfe47bbf984ed11c42555f84bdd0ccf2a5bdddb

Author: Xiaohong Gong

compiler/src/org.graalvm.compiler.phases.common/src/org/graalvm/compiler/phases/common/UseTrappingNullChecksPhase.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2014, 2019, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2014, 2020, 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
@@ -264,8 +264,13 @@ private static void replaceWithTrappingNullCheck(AbstractDeoptimizeNode deopt, I
         }
 
         if (trappingNullCheck == null) {
-            // Need to add a null check node.
-            trappingNullCheck = deopt.graph().add(new NullCheckNode(value));
+            // Need to add a null check node. Move the null check forward to the uncompressed object
+            // if it exists.
+            NullCheckNode nullCheck = tryUseUncompressedNullCheck(value, isNullNode);
+            if (nullCheck == null) {
+                nullCheck = new NullCheckNode(value);
+            }
+            trappingNullCheck = deopt.graph().add(nullCheck);
             deopt.graph().replaceSplit(ifNode, trappingNullCheck, nonTrappingContinuation);
             deopt.getDebug().log("Inserted NullCheckNode %s", trappingNullCheck);
         }
@@ -287,4 +292,22 @@ private static void replaceWithTrappingNullCheck(AbstractDeoptimizeNode deopt, I
         GraphUtil.killCFG(trappingContinuation);
         GraphUtil.tryKillUnused(isNullNode);
     }
+
+    private static NullCheckNode tryUseUncompressedNullCheck(ValueNode value, IsNullNode isNullNode) {
+        assert isNullNode.getValue().equals(value);
+        CompressionNode uncompressed = null;
+        for (Node usage : value.usages()) {
+            if (usage instanceof CompressionNode) {
+                if (((CompressionNode) usage).getOp() == CompressionNode.CompressionOp.Uncompress) {
+                    assert ((CompressionNode) usage).getValue().equals(value);
+                    uncompressed = (CompressionNode) usage;
+                    break;
+                }
+            }
+        }
+        if (uncompressed != null) {
+            return new NullCheckNode(uncompressed);
+        }
+        return null;
+    }
 }