[ELF] Simplify GdbIndexSection. NFC

Author: MaskRay

lld/ELF/SyntheticSections.cpp

@@ -2805,21 +2805,16 @@ createSymbols(
     cuIdx += chunks[i].compilationUnits.size();
   }
 
-  // The number of symbols we will handle in this function is of the order
-  // of millions for very large executables, so we use multi-threading to
-  // speed it up.
+  // Collect the compilation unitss for each unique name. Speed it up using
+  // multi-threading as the number of symbols can be in the order of millions.
+  // Shard GdbSymbols by hash's high bits.
   constexpr size_t numShards = 32;
   const size_t concurrency =
       llvm::bit_floor(std::min<size_t>(config->threadCount, numShards));
-
-  // A sharded map to uniquify symbols by name.
+  const size_t shift = 32 - llvm::countr_zero(numShards);
   auto map =
       std::make_unique<DenseMap<CachedHashStringRef, size_t>[]>(numShards);
-  size_t shift = 32 - llvm::countr_zero(numShards);
-
-  // Instantiate GdbSymbols while uniqufying them by name.
   auto symbols = std::make_unique<SmallVector<GdbSymbol, 0>[]>(numShards);
-
   parallelFor(0, concurrency, [&](size_t threadId) {
     uint32_t i = 0;
     for (ArrayRef<NameAttrEntry> entries : nameAttrs) {
@@ -2829,14 +2824,12 @@ createSymbols(
           continue;
 
         uint32_t v = ent.cuIndexAndAttrs + cuIdxs[i];
-        size_t &idx = map[shardId][ent.name];
-        if (idx) {
-          symbols[shardId][idx - 1].cuVector.push_back(v);
-          continue;
-        }
-
-        idx = symbols[shardId].size() + 1;
-        symbols[shardId].push_back({ent.name, {v}, 0, 0});
+        auto [it, inserted] =
+            map[shardId].try_emplace(ent.name, symbols[shardId].size());
+        if (inserted)
+          symbols[shardId].push_back({ent.name, {v}, 0, 0});
+        else
+          symbols[shardId][it->second].cuVector.push_back(v);
       }
       ++i;
     }