8279676: Dubious YMM register clearing in x86_64 arraycopy stubs

Reviewed-by: kvn, vlivanov, neliasso

Author: shipilev

src/hotspot/cpu/x86/stubGenerator_x86_64.cpp

@@ -1221,11 +1221,6 @@ class StubGenerator: public StubCodeGenerator {
       }
       __ addptr(qword_count, 4);
       __ BIND(L_end);
-      if (UseAVX >= 2) {
-        // clean upper bits of YMM registers
-        __ vpxor(xmm0, xmm0);
-        __ vpxor(xmm1, xmm1);
-      }
     } else {
       // Copy 32-bytes per iteration
       __ BIND(L_loop);
@@ -1299,11 +1294,6 @@ class StubGenerator: public StubCodeGenerator {
       }
       __ subptr(qword_count, 4);
       __ BIND(L_end);
-      if (UseAVX >= 2) {
-        // clean upper bits of YMM registers
-        __ vpxor(xmm0, xmm0);
-        __ vpxor(xmm1, xmm1);
-      }
     } else {
       // Copy 32-bytes per iteration
       __ BIND(L_loop);