src/cpu/x86/vm/vm_version_x86.cpp

*** 863,880 ****
  
    // Use count trailing zeros instruction if available
    if (supports_bmi1()) {
      // tzcnt does not require VEX prefix
      if (FLAG_IS_DEFAULT(UseCountTrailingZerosInstruction)) {
        UseCountTrailingZerosInstruction = true;
      }
    } else if (UseCountTrailingZerosInstruction) {
      warning("tzcnt instruction is not available on this CPU");
      FLAG_SET_DEFAULT(UseCountTrailingZerosInstruction, false);
    }
  
!   // BMI instructions use an encoding with VEX prefix.
    // VEX prefix is generated only when AVX > 0.
    if (supports_bmi1() && supports_avx()) {
      if (FLAG_IS_DEFAULT(UseBMI1Instructions)) {
        UseBMI1Instructions = true;
      }
--- 863,885 ----
  
    // Use count trailing zeros instruction if available
    if (supports_bmi1()) {
      // tzcnt does not require VEX prefix
      if (FLAG_IS_DEFAULT(UseCountTrailingZerosInstruction)) {
+       if (!UseBMI1Instructions && !FLAG_IS_DEFAULT(UseBMI1Instructions)) {
+         // Don't use tzcnt if BMI1 is switched off on command line.
+         UseCountTrailingZerosInstruction = false;
+       } else {
          UseCountTrailingZerosInstruction = true;
        }
+     }
    } else if (UseCountTrailingZerosInstruction) {
      warning("tzcnt instruction is not available on this CPU");
      FLAG_SET_DEFAULT(UseCountTrailingZerosInstruction, false);
    }
  
!   // BMI instructions (except tzcnt) use an encoding with VEX prefix.
    // VEX prefix is generated only when AVX > 0.
    if (supports_bmi1() && supports_avx()) {
      if (FLAG_IS_DEFAULT(UseBMI1Instructions)) {
        UseBMI1Instructions = true;
      }