[PDB] Fix hash function used to write /src/headerblock

lld-link used to write PDB files that DIA couldn't recover natvis
files from if:

- The global strings table was > 64kiB
- There were at least 3 natvis files

The cause was that the hash function for the /src/headerblock stream
was incorrect: It needs to be truncated to 16 bit.

If the global strings table was <= 64kiB, truncating to 16 bit is a
no-op, so this wasn't needed for small programs.

If there are only 1 or 2 natvis files, then the growth strategy in
HashTable::grow() would mean the hash table would have 2 buckets (for 1
natvis file) or 4 buckets (for 4 natvis files), and since the hash
function is used modulo number of buckets, and since 2 and 4 divide
0x10000, the missing `% 0x10000` is a no-op there too. For 3 natvis
files, the hash table grows to 6 buckets, which has a factor that's not
common with 0x10000 and the difference starts to matter.

Fixes PR41626.

Differential Revision: https://reviews.llvm.org/D61277

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@359515 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/DebugInfo/PDB/Native/HashTable.h

@@ -143,7 +143,7 @@ public:
       return EC;
     if (Present.intersects(Deleted))
       return make_error<RawError>(raw_error_code::corrupt_file,
-                                  "Present bit vector interesects deleted!");
+                                  "Present bit vector intersects deleted!");
 
     for (uint32_t P : Present) {
       if (auto EC = Stream.readInteger(Buckets[P].first))

lib/DebugInfo/PDB/Native/NamedStreamMap.cpp

@@ -34,6 +34,7 @@ uint16_t NamedStreamMapTraits::hashLookupKey(StringRef S) const {
   // Here, the type HASH is a typedef of unsigned short.
   // ** It is not a bug that we truncate the result of hashStringV1, in fact
   //    it is a bug if we do not! **
+  // See NMTNI::hash() in the reference implementation.
   return static_cast<uint16_t>(hashStringV1(S));
 }
 

lib/DebugInfo/PDB/Native/PDBStringTableBuilder.cpp

@@ -26,7 +26,13 @@ StringTableHashTraits::StringTableHashTraits(PDBStringTableBuilder &Table)
     : Table(&Table) {}
 
 uint32_t StringTableHashTraits::hashLookupKey(StringRef S) const {
-  return Table->getIdForString(S);
+  // The reference implementation doesn't include code for /src/headerblock
+  // handling, but it can only read natvis entries lld's PDB files if
+  // this hash function truncates the hash to 16 bit.
+  // PDB/include/misc.h in the reference implementation has a hashSz() function
+  // that returns an unsigned short, that seems what's being used for
+  // /src/headerblock.
+  return static_cast<uint16_t>(Table->getIdForString(S));
 }
 
 StringRef StringTableHashTraits::storageKeyToLookupKey(uint32_t Offset) const {

unittests/DebugInfo/PDB/StringTableBuilderTest.cpp

@@ -19,11 +19,7 @@ using namespace llvm;
 using namespace llvm::pdb;
 using namespace llvm::support;
 
-namespace {
-class StringTableBuilderTest : public ::testing::Test {};
-}
-
-TEST_F(StringTableBuilderTest, Simple) {
+TEST(StringTableBuilderTest, Simple) {
   // Create /names table contents.
   PDBStringTableBuilder Builder;
 
@@ -78,3 +74,21 @@ TEST_F(StringTableBuilderTest, Simple) {
   EXPECT_THAT_EXPECTED(Table.getIDForString("bazz"), HasValue(BazzID));
   EXPECT_THAT_EXPECTED(Table.getIDForString("barr"), HasValue(BarrID));
 }
+
+TEST(StringTableHashTraitsTest, Simple) {
+  PDBStringTableBuilder Builder;
+
+  // Create more than 64kiB of dummy entries.
+  for (int i = 0; i < 320; ++i) {
+    std::string aaaaa = std::string(220, 'a') + std::to_string(i);
+    Builder.insert(aaaaa);
+  }
+
+  std::string S = "foo.natvis";
+  uint32_t Pos = Builder.insert(S);
+
+  EXPECT_GT(Pos, 0xFFFFu);
+
+  StringTableHashTraits Traits(Builder);
+  EXPECT_LE(Traits.hashLookupKey(S), 0xFFFFu);
+}