[my_runtime] Ensure that the from space is zero-initialized.

Collect() maintains the invariant we set up in Init().

mycpp/gc_heap.cc

@@ -139,6 +139,8 @@ void Heap::Collect(bool must_grow) {
   // Swap spaces for next collection
   char* tmp = from_space_;
   from_space_ = to_space_;
+  // Maintain invariant of the space we will allocate from.
+  memset(from_space_, 0, space_size_);
   to_space_ = tmp;
 
 #if GC_DEBUG

mycpp/gc_heap.h

@@ -150,7 +150,9 @@ class Heap {
 
     roots_top_ = 0;
 
-    // slab scanning relies on 0 bytes (nullptr)
+    // Slab scanning relies on 0 bytes (nullptr).  e.g. for a List<Token*>*.
+    // Note: I noticed that memset() of say 400 MiB is pretty expensive.  Does
+    // it makes sense to zero the slabs instead?
     memset(from_space_, 0, num_bytes);
     memset(to_space_, 0, num_bytes);
 
@@ -527,8 +529,6 @@ class Str : public gc_heap::Obj {
   DISALLOW_COPY_AND_ASSIGN(Str)
 };
 
-constexpr int kStrHeaderSize = offsetof(Str, data_);
-
 template <int N>
 class GlobalStr {
   // A template type with the same layout as Str with length N-1 (which needs a
@@ -542,6 +542,10 @@ class GlobalStr {
   DISALLOW_COPY_AND_ASSIGN(GlobalStr)
 };
 
+// This is the same as offsetof(Str, data_), but doesn't give a warning,
+// because of the inheritance?
+constexpr int kStrHeaderSize = offsetof(GlobalStr<1>, data_);
+
 // This macro is a workaround for the fact that it's impossible to have a
 // a constexpr initializer for char[N].  The "String Literals as Non-Type
 // Template Parameters" feature of C++ 20 would have done it, but it's not
@@ -582,10 +586,7 @@ inline Str* NewStr(const char* data, int len) {
   // log("NewStr s->data_ %p len = %d", s->data_, len);
   // log("sizeof(Str) = %d", sizeof(Str));
   memcpy(s->data_, data, len);
-
-  // So we can pass it directly to C functions.  TODO: is this redundant
-  // because the  heap is zero'd?
-  s->data_[len] = '\0';
+  assert(s->data_[len] == '\0');  // should be true because Heap was zeroed
 
   s->SetCellLength(obj_len);  // So the GC can copy it
   return s;

mycpp/my_runtime.cc

@@ -49,7 +49,7 @@ Str* Str::replace(Str* old, Str* new_str) {
 
   // Second pass to copy into new 'result'
   Str* result = NewStr(length);
-  p_this = data_;  // back to beginning
+  p_this = data_;                  // back to beginning
   char* p_result = result->data_;  // advances through 'result'
 
   while (p_this < p_end) {
@@ -63,12 +63,7 @@ Str* Str::replace(Str* old, Str* new_str) {
       p_result++;
     }
   }
-  if (0) {
-    log("length = %d", length);
-    log("result->data_[length-1] = %c", result->data_[length-1]);
-    log("result->data_[length] = %c", result->data_[length]);
-    assert(result->data_[length] == '\0');  // buffer should have been zero'd
-  }
+  assert(result->data_[length] == '\0');  // buffer should have been zero'd
   return result;
 }
 

mycpp/my_runtime_test.cc

@@ -68,6 +68,9 @@ TEST collect_test() {
   auto s = NewStr("abcdefg");
   for (int i = 0; i < 40; ++i) {
     s = s->replace(b, bb);
+
+    Str* t = NewStr("NUL");
+
     // log("i = %d", i);
     // log("len(s) = %d", len(s));
   }