[ctx_profile] Arena should zero-initialize its allocation area.

Author: mtrofin

compiler-rt/lib/ctx_profile/CtxInstrProfiling.cpp

@@ -132,6 +132,10 @@ __thread ContextNode **volatile __llvm_ctx_profile_callsite[2] = {0, 0};
 __thread ContextRoot *volatile __llvm_ctx_profile_current_context_root =
     nullptr;
 
+Arena::Arena(uint32_t Size) : Size(Size) {
+  __sanitizer::internal_memset(start(), 0, Size);
+}
+
 // FIXME(mtrofin): use malloc / mmap instead of sanitizer common APIs to reduce
 // the dependency on the latter.
 Arena *Arena::allocateNewArena(size_t Size, Arena *Prev) {

compiler-rt/lib/ctx_profile/CtxInstrProfiling.h

@@ -15,6 +15,9 @@
 
 using namespace llvm::ctx_profile;
 
+// Forward-declare for the one unittest checking Arena construction zeroes out
+// its allocatable space.
+class ArenaTest_ZeroInit_Test;
 namespace __ctx_profile {
 
 static constexpr size_t ExpectedAlignment = 8;
@@ -51,7 +54,8 @@ class Arena final {
   const char *pos() const { return start() + Pos; }
 
 private:
-  explicit Arena(uint32_t Size) : Size(Size) {}
+  friend class ::ArenaTest_ZeroInit_Test;
+  explicit Arena(uint32_t Size);
   ~Arena() = delete;
 
   char *start() { return reinterpret_cast<char *>(&this[1]); }

compiler-rt/lib/ctx_profile/tests/CtxInstrProfilingTest.cpp

@@ -12,6 +12,15 @@ class ContextTest : public ::testing::Test {
   ContextRoot Root;
 };
 
+TEST(ArenaTest, ZeroInit) {
+  char Buffer[1024];
+  memset(Buffer, 1, 1024);
+  Arena *A = new (Buffer) Arena(10);
+  for (auto I = 0U; I < A->size(); ++I)
+    EXPECT_EQ(A->pos()[I], 0);
+  EXPECT_EQ(A->size(), 10);
+}
+
 TEST(ArenaTest, Basic) {
   Arena *A = Arena::allocateNewArena(1024);
   EXPECT_EQ(A->size(), 1024U);