8063086: Math.pow yields different results upon repeated calls

C2 treats x^2 as a special case and computes x * x while the interpreter and c1 don't have special case code for X^2.

Reviewed-by: kvn

hotspot/src/cpu/x86/vm/macroAssembler_x86.cpp

@@ -3184,7 +3184,24 @@ void MacroAssembler::pow_or_exp(bool is_exp, int num_fpu_regs_in_use) {
     jmp(done);
   } else {
     // Stack: X Y
-    Label x_negative, y_odd;
+    Label x_negative, y_not_2;
+
+    static double two = 2.0;
+    ExternalAddress two_addr((address)&two);
+
+    // constant maybe too far on 64 bit
+    lea(tmp2, two_addr);
+    fld_d(Address(tmp2, 0));    // Stack: 2 X Y
+    fcmp(tmp, 2, true, false);  // Stack: X Y
+    jcc(Assembler::parity, y_not_2);
+    jcc(Assembler::notEqual, y_not_2);
+
+    fxch(); fpop();             // Stack: X
+    fmul(0);                    // Stack: X*X
+
+    jmp(done);
+
+    bind(y_not_2);
 
     fldz();                     // Stack: 0 X Y
     fcmp(tmp, 1, true, false);  // Stack: X Y

hotspot/test/compiler/floatingpoint/TestPow2.java

@@ -0,0 +1,73 @@
+/*
+ * Copyright (c) 2014, 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
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug 8063086
+ * @summary X^2 special case for C2 yields different result than interpreter
+ * @library /testlibrary /../../test/lib /compiler/whitebox
+ * @build TestPow2
+ * @run main ClassFileInstaller sun.hotspot.WhiteBox
+ * @run main/othervm -Xbootclasspath/a:. -XX:+UnlockDiagnosticVMOptions -XX:+WhiteBoxAPI
+ *                   -XX:-BackgroundCompilation -XX:-UseOnStackReplacement TestPow2
+ *
+ */
+
+import java.lang.reflect.*;
+import sun.hotspot.WhiteBox;
+
+public class TestPow2 {
+
+    private static final WhiteBox WHITE_BOX = WhiteBox.getWhiteBox();
+
+    private static final double base = 5350.456329377186;
+    private static final double exp = 2.0;
+
+    static double m() {
+        return Math.pow(base, exp);
+    }
+
+    static public void main(String[] args) throws NoSuchMethodException {
+        Method test_method = TestPow2.class.getDeclaredMethod("m");
+
+        double interpreter_result = m();
+
+        // Compile with C1 if possible
+        WHITE_BOX.enqueueMethodForCompilation(test_method, CompilerWhiteBoxTest.COMP_LEVEL_SIMPLE);
+
+        double c1_result = m();
+
+        WHITE_BOX.deoptimizeMethod(test_method);
+
+        // Compile it with C2 if possible
+        WHITE_BOX.enqueueMethodForCompilation(test_method, CompilerWhiteBoxTest.COMP_LEVEL_FULL_OPTIMIZATION);
+
+        double c2_result = m();
+
+        if (interpreter_result != c1_result || interpreter_result != c2_result ||
+            c1_result != c2_result) {
+            System.out.println("interpreter = " + interpreter_result + " c1 = " + c1_result + " c2 = " + c2_result);
+            throw new RuntimeException("Test Failed");
+        }
+    }
+}