[llvm][cas] Remove SubtrieHandle's NumUnusedBits

The "unused" bits were always zero, so remove the code around them to
make it easier to understand. The size of the header is not changed,
because we still want it to be 8 bytes (it is always followed by an
8-byte-aligned value anyway).

(cherry picked from commit cad7bdf)

Author: benlangmuir

llvm/lib/CAS/OnDiskHashMappedTrie.cpp

@@ -324,8 +324,7 @@ class HashMappedTrieHandle;
 /// Subtrie layout:
 /// - 2-bytes: StartBit
 /// - 1-bytes: NumBits=lg(num-slots)
-/// - 1-bytes: NumUnusedBits=lg(num-slots-unused)
-/// - 4-bytes: 0-pad
+/// - 5-bytes: 0-pad
 /// - <slots>
 class SubtrieHandle {
 public:
@@ -336,11 +335,8 @@ class SubtrieHandle {
     /// The number of bits this subtrie handles. It has 2^NumBits slots.
     uint8_t NumBits;
 
-    /// The number of extra bits this allocation *could* handle, due to
-    /// over-allocation. It has 2^NumUnusedBits unused slots.
-    uint8_t NumUnusedBits;
-
     /// 0-pad to 8B.
+    uint8_t ZeroPad1B;
     uint32_t ZeroPad4B;
   };
 
@@ -408,11 +404,9 @@ class SubtrieHandle {
   Header &getHeader() const { return *H; }
   uint32_t getStartBit() const { return H->StartBit; }
   uint32_t getNumBits() const { return H->NumBits; }
-  uint32_t getNumUnusedBits() const { return H->NumUnusedBits; }
 
   static Expected<SubtrieHandle> create(MappedFileRegionBumpPtr &Alloc,
-                                        uint32_t StartBit, uint32_t NumBits,
-                                        uint32_t NumUnusedBits = 0);
+                                        uint32_t StartBit, uint32_t NumBits);
 
   static SubtrieHandle getFromFileOffset(MappedFileRegion &Region,
                                          FileOffset Offset) {
@@ -558,19 +552,17 @@ struct OnDiskHashMappedTrie::ImplType {
 
 Expected<SubtrieHandle> SubtrieHandle::create(MappedFileRegionBumpPtr &Alloc,
                                               uint32_t StartBit,
-                                              uint32_t NumBits,
-                                              uint32_t NumUnusedBits) {
+                                              uint32_t NumBits) {
   assert(StartBit <= HashMappedTrieHandle::MaxNumHashBits);
   assert(NumBits <= UINT8_MAX);
-  assert(NumUnusedBits <= UINT8_MAX);
-  assert(NumBits + NumUnusedBits <= HashMappedTrieHandle::MaxNumRootBits);
+  assert(NumBits <= HashMappedTrieHandle::MaxNumRootBits);
 
-  auto Mem = Alloc.allocate(getSize(NumBits + NumUnusedBits));
+  auto Mem = Alloc.allocate(getSize(NumBits));
   if (LLVM_UNLIKELY(!Mem))
     return Mem.takeError();
   auto *H =
       new (*Mem) SubtrieHandle::Header{(uint16_t)StartBit, (uint8_t)NumBits,
-                                       (uint8_t)NumUnusedBits, /*ZeroPad4B=*/0};
+                                       /*ZeroPad1B=*/0, /*ZeroPad4B=*/0};
   SubtrieHandle S(Alloc.getRegion(), *H);
   for (auto I = S.Slots.begin(), E = S.Slots.end(); I != E; ++I)
     new (I) SlotT(0);