tcmalloc_unittest: generalize and fix ReallocTest

The test exercises the logic of "don't move allocation if change is
small" but relies too much on implementation knowledge. We now
explicitly exercise not moving only when old and new usable sizes are
the same, which is the correct expectation in general.

src/tests/tcmalloc_unittest.cc

diff --git a/src/tests/tcmalloc_unittest.cc b/src/tests/tcmalloc_unittest.cc
index 4c5da72..16d1852 100644
--- a/src/tests/tcmalloc_unittest.cc
+++ b/src/tests/tcmalloc_unittest.cc
@@ -716,22 +716,38 @@
   // turn off sampling
   tcmalloc::Cleanup cleanup = SetFlag(&TestingPortal::Get()->GetSampleParameter(), 0);
 
-  int start_sizes[] = { 100, 1000, 10000, 100000 };
-  int deltas[] = { 1, -2, 4, -8, 16, -32, 64, -128 };
+  std::vector<size_t> start_sizes = {100, 1000, 10000, 100000 };
 
-  for (int s = 0; s < sizeof(start_sizes)/sizeof(*start_sizes); ++s) {
-    void* p = noopt(malloc(start_sizes[s]));
+  for (size_t original_size : start_sizes) {
+    void* p = noopt(malloc(original_size));
     ASSERT_NE(p, nullptr);
-    // The larger the start-size, the larger the non-reallocing delta.
-    for (int d = 0; d < (s+1) * 2; ++d) {
-      void* new_p = noopt(realloc)(p, start_sizes[s] + deltas[d]);
-      ASSERT_EQ(p, new_p);  // realloc should not allocate new memory
+
+    size_t usable_size = nallocx(original_size, 0);
+    // Validate out expectation
+    ASSERT_EQ(MallocExtension::instance()->GetAllocatedSize(p), usable_size);
+
+    // Lets find range of request sizes that round up to the same
+    // usable size by using nallocx.
+    size_t minimal_size = original_size;
+    while (nallocx(minimal_size - 1, 0) == usable_size) {
+      minimal_size--;
+      ASSERT_NE(minimal_size, 0);
     }
-    // Test again, but this time reallocing smaller first.
-    for (int d = 0; d < s*2; ++d) {
-      void* new_p = noopt(realloc)(p, start_sizes[s] - deltas[d]);
-      ASSERT_EQ(p, new_p);  // realloc should not allocate new memory
-    }
+
+    void* new_p;
+
+    // Check growing up to usable size then shrinking
+    new_p = noopt(realloc)(p, usable_size);
+    ASSERT_EQ(new_p, p);
+    new_p = noopt(realloc)(p, minimal_size);
+    ASSERT_EQ(new_p, p);
+
+    // Checking shrinking then growing
+    new_p = noopt(realloc)(p, minimal_size);
+    ASSERT_EQ(new_p, p);
+    new_p = noopt(realloc)(p, usable_size);
+    ASSERT_EQ(new_p, p);
+
     free(p);
   }
 }