--- old/src/cpu/x86/vm/assembler_x86.cpp	2014-03-24 17:32:13.000000000 -0700
+++ new/src/cpu/x86/vm/assembler_x86.cpp	2014-03-24 17:32:13.000000000 -0700
@@ -1112,8 +1112,12 @@
 }
 
 void Assembler::bsrl(Register dst, Register src) {
-  assert(!VM_Version::supports_lzcnt(), "encoding is treated as LZCNT");
+#ifdef ASSERT
+  address prefix = pc();
+#endif
   int encode = prefix_and_encode(dst->encoding(), src->encoding());
+  // x86 oddity: bsr with a rep prefix (f3) is lzcnt when CPU supports it
+  assert(!VM_Version::supports_lzcnt() || pc() == prefix || *prefix != REP, "encoding is treated as LZCNT with REP prefix");
   emit_int8(0x0F);
   emit_int8((unsigned char)0xBD);
   emit_int8((unsigned char)(0xC0 | encode));
@@ -4977,8 +4981,12 @@
 }
 
 void Assembler::bsrq(Register dst, Register src) {
-  assert(!VM_Version::supports_lzcnt(), "encoding is treated as LZCNT");
+#ifdef ASSERT
+  address prefix = pc();
+#endif
   int encode = prefixq_and_encode(dst->encoding(), src->encoding());
+  // x86 oddity: bsr with a rep prefix (f3) is lzcnt when CPU supports it
+  assert(!VM_Version::supports_lzcnt() || pc() == prefix || *prefix != REP, "encoding is treated as LZCNT with REP prefix");
   emit_int8(0x0F);
   emit_int8((unsigned char)0xBD);
   emit_int8((unsigned char)(0xC0 | encode));