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changeset 20555:e48395e6a91b

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8059139: It should be possible to explicitly disable usage of TZCNT instr w/ -XX:-UseBMI1Instructions Reviewed-by: iveresov
author kvn
date Fri, 17 Oct 2014 15:35:25 -0700
parents af6ff94a005d
children 03ca22ba9cf5
files src/cpu/x86/vm/vm_version_x86.cpp
diffstat 1 files changed, 7 insertions(+), 2 deletions(-) [+]
line wrap: on
line diff
--- 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)) {