[11u] RFR: 8221092: UseAVX=3 has performance degredation on Skylake (X7) processors

[Roland Westrelin]

Hi,
 
Original bug:
  https://bugs.openjdk.java.net/browse/JDK-8221092
  https://hg.openjdk.java.net/jdk/jdk/rev/c6f1226cfb72
 
Original patch does not apply cleanly to 11u. See below for the rejected
hunk that I applied manually.
 
11u webrev:
  http://cr.openjdk.java.net/~roland/8221092.11u/webrev.00/
 
Testing: x86_64 build, tier1 (on non skylake system)
 
Roland.

--- macroAssembler_x86.cpp
+++ macroAssembler_x86.cpp
@@ -8104,12 +8103,13 @@
   shlq(length);
   xorq(result, result);
 
-  if ((UseAVX > 2) &&
+  if ((AVX3Threshold == 0) && (UseAVX > 2) &&
       VM_Version::supports_avx512vlbw()) {
     Label VECTOR64_LOOP, VECTOR64_NOT_EQUAL, VECTOR32_TAIL;
 
     cmpq(length, 64);
     jcc(Assembler::less, VECTOR32_TAIL);
+
     movq(tmp1, length);
     andq(tmp1, 0x3F);      // tail count
     andq(length, ~(0x3F)); //vector count