[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

1 files changed, 19 insertions, 5 deletions

diff --git a/llvm/unittests/DebugInfo/PDB/StringTableBuilderTest.cpp b/llvm/unittests/DebugInfo/PDB/StringTableBuilderTest.cpp
index 719cb807a2b..2f7c0619440 100644
--- a/llvm/unittests/DebugInfo/PDB/StringTableBuilderTest.cpp
+++ b/llvm/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);
+}