Fix MSAN warning after r30777 (try 2)

src/sampler.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/sampler.h"
 
 #if V8_OS_POSIX && !V8_OS_CYGWIN
 
 #define USE_SIGNALS
 
@@ skipping 166 matching lines @@
 bool IsSamePage(byte* ptr1, byte* ptr2) {
   const uint32_t kPageSize = 4096;
   uintptr_t mask = ~static_cast<uintptr_t>(kPageSize - 1);
   return (reinterpret_cast<uintptr_t>(ptr1) & mask) ==
          (reinterpret_cast<uintptr_t>(ptr2) & mask);
 }
 
 
 // Check if the code at specified address could potentially be a
 // frame setup code.
-DISABLE_ASAN bool IsNoFrameRegion(Address address) {
+bool IsNoFrameRegion(Address address) {
   struct Pattern {
     int bytes_count;
     byte bytes[8];
     int offsets[4];
   };
   byte* pc = reinterpret_cast<byte*>(address);
   static Pattern patterns[] = {
 #if V8_HOST_ARCH_IA32
     // push %ebp
     // mov %esp,%ebp
@@ skipping 14 matching lines @@
     // popq %rbp
     // ret
     {2, {0x5d, 0xc3}, {0, 1, -1}},
 #endif
     {0, {}, {}}
   };
   for (Pattern* pattern = patterns; pattern->bytes_count; ++pattern) {
     for (int* offset_ptr = pattern->offsets; *offset_ptr != -1; ++offset_ptr) {
       int offset = *offset_ptr;
       if (!offset || IsSamePage(pc, pc - offset)) {
+        MSAN_MEMORY_IS_INITIALIZED(pc - offset, pattern->bytes_count);
         if (!memcmp(pc - offset, pattern->bytes, pattern->bytes_count))
           return true;
       } else {
         // It is not safe to examine bytes on another page as it might not be
         // allocated thus causing a SEGFAULT.
         // Check the pattern part that's on the same page and
         // pessimistically assume it could be the entire pattern match.
+        MSAN_MEMORY_IS_INITIALIZED(pc, pattern->bytes_count - offset);
         if (!memcmp(pc, pattern->bytes + offset, pattern->bytes_count - offset))
           return true;
       }
     }
   }
   return false;
 }
 
 }  // namespace
 
@@ skipping 595 matching lines @@
     SampleStack(state);
   }
   ResumeThread(profiled_thread);
 }
 
 #endif  // USE_SIGNALS
 
 
 }  // namespace internal
 }  // namespace v8