enable some production CHECK in F14 code

Summary:
Production-enabled assertions have shown value in detecting
a variety of issues that should be impossible but are observed to
occur. This diff adds some checks to F14 that may fail under some types
of memory corruption, tag missearch, or hash miscomputation.

Reviewed By: yfeldblum

Differential Revision: D17739592

fbshipit-source-id: 92c9839ed5dab030350408d704a01c97573e3123

folly/container/detail/F14Table.h

@@ -437,10 +437,12 @@ struct alignas(kRequiredVectorAlignment) F14Chunk {
   void setTag(std::size_t index, std::size_t tag) {
     FOLLY_SAFE_DCHECK(
         this != emptyInstance() && tag >= 0x80 && tag <= 0xff, "");
+    FOLLY_SAFE_CHECK(tags_[index] == 0, "");
     tags_[index] = static_cast<uint8_t>(tag);
   }
 
   void clearTag(std::size_t index) {
+    FOLLY_SAFE_CHECK((tags_[index] & 0x80) != 0, "");
     tags_[index] = 0;
   }
 
@@ -1696,7 +1698,7 @@ class F14Table : public Policy {
           auto&& srcArg = std::forward<T>(src).buildArgForItem(srcItem);
           auto const& srcKey = src.keyForValue(srcArg);
           auto hp = splitHash(this->computeKeyHash(srcKey));
-          FOLLY_SAFE_DCHECK(hp.second == srcChunk->tag(i), "");
+          FOLLY_SAFE_CHECK(hp.second == srcChunk->tag(i), "");
           insertAtBlank(
               allocateTag(fullness, hp),
               hp,
@@ -1928,7 +1930,7 @@ class F14Table : public Policy {
           Item& srcItem = srcChunk->item(srcI);
           auto hp = splitHash(
               this->computeItemHash(const_cast<Item const&>(srcItem)));
-          FOLLY_SAFE_DCHECK(hp.second == srcChunk->tag(srcI), "");
+          FOLLY_SAFE_CHECK(hp.second == srcChunk->tag(srcI), "");
 
           auto dstIter = allocateTag(fullness, hp);
           this->moveItemDuringRehash(dstIter.itemAddr(), srcItem);