# HG changeset patch
# User kvn
# Date 1413585325 25200
# Node ID e48395e6a91b795a5a4ca58cff9cbd02433b99b0
# Parent  af6ff94a005ddb8192e13a1339c6bd36cb85942c
8059139: It should be possible to explicitly disable usage of TZCNT instr w/ -XX:-UseBMI1Instructions
Reviewed-by: iveresov

diff -r af6ff94a005d -r e48395e6a91b src/cpu/x86/vm/vm_version_x86.cpp
--- a/src/cpu/x86/vm/vm_version_x86.cpp	Mon Oct 20 16:37:02 2014 -0400
+++ b/src/cpu/x86/vm/vm_version_x86.cpp	Fri Oct 17 15:35:25 2014 -0700
@@ -873,14 +873,19 @@
   if (supports_bmi1()) {
     // tzcnt does not require VEX prefix
     if (FLAG_IS_DEFAULT(UseCountTrailingZerosInstruction)) {
-      UseCountTrailingZerosInstruction = true;
+      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 use an encoding with VEX prefix.
+  // 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)) {