[PDB] Don't ignore bucket 0 when writing the PDB string table.

The hash table is a list of buckets, and the *value* stored in
the bucket cannot be 0 since that is reserved.  However, the code
here was incorrectly skipping over the 0'th bucket entirely.
The 0'th bucket is perfectly fine, just none of these buckets
can contain the value 0.

As a result, whenever there was a string where hash(S) % Size
was equal to 0, we would write the value in the next bucket
instead.  We never caught this in our tests due to *another*
bug, which is that we would iterate the entire list of buckets
looking for the value, only using the hash value as a starting
point.  However, the real algorithm stops when it finds 0 in
a bucket since it takes that to mean "the item is not in the
hash table".

The unit test is updated to carefully construct a set of hash
values that will cause one item to hash to 0 mod bucket count,
and the reader is also updated to return an error indicating that
the item is not found when it encounters a 0 bucket.

llvm-svn: 328162

2 files changed, 3 insertions, 2 deletions

diff --git a/llvm/lib/DebugInfo/PDB/Native/PDBStringTable.cpp b/llvm/lib/DebugInfo/PDB/Native/PDBStringTable.cpp
index f1c10357132..afeea32043d 100644
--- a/llvm/lib/DebugInfo/PDB/Native/PDBStringTable.cpp
+++ b/llvm/lib/DebugInfo/PDB/Native/PDBStringTable.cpp
@@ -122,7 +122,10 @@ Expected<uint32_t> PDBStringTable::getIDForString(StringRef Str) const {
     // we iterate the entire array.
     uint32_t Index = (Start + I) % Count;
 
+    // If we find 0, it means the item isn't in the hash table.
     uint32_t ID = IDs[Index];
+    if (ID == 0)
+      return make_error<RawError>(raw_error_code::no_entry);
     auto ExpectedStr = getStringForID(ID);
     if (!ExpectedStr)
       return ExpectedStr.takeError();
diff --git a/llvm/lib/DebugInfo/PDB/Native/PDBStringTableBuilder.cpp b/llvm/lib/DebugInfo/PDB/Native/PDBStringTableBuilder.cpp
index ece3e00b1a8..cdd23f8df52 100644
--- a/llvm/lib/DebugInfo/PDB/Native/PDBStringTableBuilder.cpp
+++ b/llvm/lib/DebugInfo/PDB/Native/PDBStringTableBuilder.cpp
@@ -89,8 +89,6 @@ Error PDBStringTableBuilder::writeHashTable(BinaryStreamWriter &Writer) const {
 
     for (uint32_t I = 0; I != BucketCount; ++I) {
       uint32_t Slot = (Hash + I) % BucketCount;
-      if (Slot == 0)
-        continue; // Skip reserved slot
       if (Buckets[Slot] != 0)
         continue;
       Buckets[Slot] = Offset;