[BOLT][DWARF] Deduplicate Foreign TU list (llvm#97629)

There could be multiple TUs with the same hash in various DWO files. In
bigger binaries this could be in the thousands. Although they could be
structurally different and we need to output Entries for all of them,
for the purposes of figuring out a TU hash we only need one entry in
Foreign TU list.

Author: ayermolo

bolt/include/bolt/Core/DebugNames.h

@@ -91,6 +91,10 @@ class DWARF5AcceleratorTable {
   uint64_t CurrentUnitOffset = 0;
   const DWARFUnit *CurrentUnit = nullptr;
   std::unordered_map<uint32_t, uint32_t> AbbrevTagToIndexMap;
+  /// Contains a map of TU hashes to a Foreign TU indecies.
+  /// This is used to reduce the size of Foreign TU list since there could be
+  /// multiple TUs with the same hash.
+  DenseMap<uint64_t, uint32_t> TUHashToIndexMap;
 
   /// Represents a group of entries with identical name (and hence, hash value).
   struct HashData {

bolt/lib/Core/DebugNames.cpp

@@ -90,7 +90,11 @@ void DWARF5AcceleratorTable::addUnit(DWARFUnit &Unit,
       auto Iter = CUOffsetsToPatch.insert({*DWOID, CUList.size()});
       if (Iter.second)
         CUList.push_back(BADCUOFFSET);
-      ForeignTUList.push_back(cast<DWARFTypeUnit>(&Unit)->getTypeHash());
+      const uint64_t TUHash = cast<DWARFTypeUnit>(&Unit)->getTypeHash();
+      if (!TUHashToIndexMap.count(TUHash)) {
+        TUHashToIndexMap.insert({TUHash, ForeignTUList.size()});
+        ForeignTUList.push_back(TUHash);
+      }
     } else {
       LocalTUList.push_back(CurrentUnitOffset);
     }
@@ -231,8 +235,13 @@ DWARF5AcceleratorTable::addAccelTableEntry(
     IsTU = Unit.isTypeUnit();
     DieTag = Die.getTag();
     if (IsTU) {
-      if (DWOID)
-        return ForeignTUList.size() - 1;
+      if (DWOID) {
+        const uint64_t TUHash = cast<DWARFTypeUnit>(&Unit)->getTypeHash();
+        auto Iter = TUHashToIndexMap.find(TUHash);
+        assert(Iter != TUHashToIndexMap.end() &&
+               "Could not find TU hash in map");
+        return Iter->second;
+      }
       return LocalTUList.size() - 1;
     }
     return CUList.size() - 1;

bolt/test/X86/dwarf5-df-types-debug-names.test

@@ -18,19 +18,19 @@
 ; BOLT: type_signature = [[TYPE1:0x[0-9a-f]*]]
 ; BOLT: Compile Unit
 ; BOLT: type_signature = [[TYPE2:0x[0-9a-f]*]]
-; BOLT: type_signature = [[TYPE3:0x[0-9a-f]*]]
+; BOLT: type_signature = [[TYPE1]]
 ; BOLT: Compile Unit
 ; BOLT: [[OFFSET:0x[0-9a-f]*]]: Compile Unit
 ; BOLT: [[OFFSET1:0x[0-9a-f]*]]: Compile Unit
 
 ; BOLT:       Name Index @ 0x0 {
 ; BOLT-NEXT:   Header {
-; BOLT-NEXT:     Length: 0x17E
+; BOLT-NEXT:     Length: 0x176
 ; BOLT-NEXT:     Format: DWARF32
 ; BOLT-NEXT:     Version: 5
 ; BOLT-NEXT:     CU count: 2
 ; BOLT-NEXT:     Local TU count: 0
-; BOLT-NEXT:     Foreign TU count: 4
+; BOLT-NEXT:     Foreign TU count: 3
 ; BOLT-NEXT:     Bucket count: 9
 ; BOLT-NEXT:     Name count: 9
 ; BOLT-NEXT:     Abbreviations table size: 0x37
@@ -44,7 +44,6 @@
 ; BOLT-NEXT:     ForeignTU[0]: [[TYPE]]
 ; BOLT-NEXT:     ForeignTU[1]: [[TYPE1]]
 ; BOLT-NEXT:     ForeignTU[2]: [[TYPE2]]
-; BOLT-NEXT:     ForeignTU[3]: [[TYPE3]]
 ; BOLT-NEXT:   ]
 ; BOLT-NEXT: Abbreviations [
 ; BOLT-NEXT:     Abbreviation [[ABBREV:0x[0-9a-f]*]] {
@@ -173,7 +172,7 @@
 ; BOLT-NEXT:       Entry @ {{.+}} {
 ; BOLT-NEXT:         Abbrev: [[ABBREV]]
 ; BOLT-NEXT:         Tag: DW_TAG_structure_type
-; BOLT-NEXT:         DW_IDX_type_unit: 0x03
+; BOLT-NEXT:         DW_IDX_type_unit: 0x01
 ; BOLT-NEXT:         DW_IDX_compile_unit: 0x01
 ; BOLT-NEXT:         DW_IDX_die_offset: 0x00000021
 ; BOLT-NEXT:         DW_IDX_parent: <parent not indexed>
@@ -237,7 +236,7 @@
 ; BOLT-NEXT:       Entry @ {{.+}} {
 ; BOLT-NEXT:         Abbrev: 0x5
 ; BOLT-NEXT:         Tag: DW_TAG_base_type
-; BOLT-NEXT:         DW_IDX_type_unit: 0x03
+; BOLT-NEXT:         DW_IDX_type_unit: 0x01
 ; BOLT-NEXT:         DW_IDX_compile_unit: 0x01
 ; BOLT-NEXT:         DW_IDX_die_offset: 0x00000048
 ; BOLT-NEXT:         DW_IDX_parent: <parent not indexed>