[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

llvm-svn: 359515

2 files changed, 8 insertions, 1 deletions

diff --git a/llvm/lib/DebugInfo/PDB/Native/NamedStreamMap.cpp b/llvm/lib/DebugInfo/PDB/Native/NamedStreamMap.cpp
index a9965ff8d06..1c044e0c265 100644
--- a/llvm/lib/DebugInfo/PDB/Native/NamedStreamMap.cpp
+++ b/llvm/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));
 }
 
diff --git a/llvm/lib/DebugInfo/PDB/Native/PDBStringTableBuilder.cpp b/llvm/lib/DebugInfo/PDB/Native/PDBStringTableBuilder.cpp
index 4b69e09a83d..eabb9d44669 100644
--- a/llvm/lib/DebugInfo/PDB/Native/PDBStringTableBuilder.cpp
+++ b/llvm/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 {