Turn fix off by default

Author: thurstond

llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp

@@ -280,7 +280,7 @@ static cl::opt<bool> ClPoisonUndefVectors(
              "considered fully initialized, which may lead to false "
              "negatives. Fully undefined constant vectors are "
              "unaffected by this flag (see -msan-poison-undef)."),
-    cl::Hidden, cl::init(true));
+    cl::Hidden, cl::init(false));
 
 static cl::opt<bool>
     ClHandleICmp("msan-handle-icmp",

llvm/test/Instrumentation/MemorySanitizer/partial-poison.ll

@@ -1,5 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
-; RUN: opt < %s -S -passes='msan' 2>&1 | FileCheck %s --check-prefixes=CHECK,CHECK-PRECISE
+; RUN: opt < %s -S -passes='msan' -msan-poison-undef-vectors=true  2>&1 | FileCheck %s --check-prefixes=CHECK,CHECK-PRECISE
 ; RUN: opt < %s -S -passes='msan' -msan-poison-undef-vectors=false 2>&1 | FileCheck %s --check-prefixes=CHECK,CHECK-IMPRECISE
 ;
 ; Regression test case for computing shadows of partially poisoned vectors.