From 4856ddc9b06d4baefc2a43e86c05e9b276eaefe6 Mon Sep 17 00:00:00 2001
From: kvn <unknown>
Date: Fri, 17 Oct 2014 15:35:25 -0700
Subject: [PATCH] 8059139: It should be possible to explicitly disable usage of
 TZCNT instr w/ -XX:-UseBMI1Instructions Reviewed-by: iveresov

---
 src/cpu/x86/vm/vm_version_x86.cpp | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/src/cpu/x86/vm/vm_version_x86.cpp b/src/cpu/x86/vm/vm_version_x86.cpp
index 62cb56d3a..adfdfd67e 100644
--- a/src/cpu/x86/vm/vm_version_x86.cpp
+++ b/src/cpu/x86/vm/vm_version_x86.cpp
@@ -873,14 +873,19 @@ void VM_Version::get_processor_features() {
   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)) {
-- 
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