X64: Use roundsd for DoMathFloor.

src/x64/disasm-x64.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1003 matching lines @@
     int mod, regop, rm;
     if (opcode == 0x3A) {
       byte third_byte = *current;
       current = data + 3;
       if (third_byte == 0x17) {
         get_modrm(*current, &mod, &regop, &rm);
         AppendToBuffer("extractps ");  // reg/m32, xmm, imm8
         current += PrintRightOperand(current);
         AppendToBuffer(", %s, %d", NameOfCPURegister(regop), (*current) & 3);
         current += 1;
+      } else if (third_byte == 0x0b) {
+        get_modrm(*current, &mod, &regop, &rm);
+         // roundsd xmm, xmm/m64, imm8
+        AppendToBuffer("roundsd %s, ", NameOfCPURegister(regop));
+        current += PrintRightOperand(current);
+        AppendToBuffer(", %d", (*current) & 3);
+        current += 1;
       } else {
         UnimplementedInstruction();
       }
     } else {
       get_modrm(*current, &mod, &regop, &rm);
       if (opcode == 0x6E) {
         AppendToBuffer("mov%c %s,",
                        rex_w() ? 'q' : 'd',
                        NameOfXMMRegister(regop));
         current += PrintRightOperand(current);
@@ skipping 743 matching lines @@
     for (int i = 6 - static_cast<int>(pc - prev_pc); i >= 0; i--) {
       fprintf(f, "  ");
     }
     fprintf(f, "  %s\n", buffer.start());
   }
 }
 
 }  // namespace disasm
 
 #endif  // V8_TARGET_ARCH_X64