[llvm] Reland "[MC][NFC] Make ELFUniquingMap a StringMap (#95006)" (PR #95030)

[Fangrui Song via llvm-commits]

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@@ -548,16 +549,42 @@ MCSectionELF *MCContext::getELFSection(const Twine &Section, unsigned Type,
   if (GroupSym)
     Group = GroupSym->getName();
   assert(!(LinkedToSym && LinkedToSym->getName().empty()));
-  // Do the lookup, if we have a hit, return it.
-  auto IterBool = ELFUniquingMap.insert(std::make_pair(
-      ELFSectionKey{Section.str(), Group,
-                    LinkedToSym ? LinkedToSym->getName() : "", UniqueID},
-      nullptr));
-  auto &Entry = *IterBool.first;
-  if (!IterBool.second)
-    return Entry.second;
 
-  StringRef CachedName = Entry.first.SectionName;
+  // Sections are differentiated by the quadruple (section_name, group_name,
+  // unique_id, link_to_symbol_name). Sections sharing the same quadruple are
+  // combined into one section. As an optimization, non-unique sections without
+  // group or linked-to symbol have a shorter unique-ing key.
+  std::pair<StringMap<MCSectionELF *>::iterator, bool> EntryNewPair;
+  // Length of the section name, which are the first SectionLen bytes of the key
+  unsigned SectionLen;
+  if (GroupSym || LinkedToSym || UniqueID != MCSection::NonUniqueID) {
+    SmallString<128> Buffer;
+    Section.toVector(Buffer);
+    SectionLen = Buffer.size();
+    Buffer.push_back(0); // separator which cannot occur in the name
+    if (GroupSym)
+      Buffer.append(GroupSym->getName());
+    Buffer.push_back(0); // separator which cannot occur in the name
+    if (LinkedToSym)
+      Buffer.append(LinkedToSym->getName());
+    support::endian::write(Buffer, UniqueID, endianness::native);
+    StringRef UniqueMapKey = StringRef(Buffer);
+    EntryNewPair = ELFUniquingMap.insert(std::make_pair(UniqueMapKey, nullptr));
+  } else if (!Section.isSingleStringRef()) {
+    SmallString<128> Buffer;
+    StringRef UniqueMapKey = Section.toStringRef(Buffer);
+    SectionLen = UniqueMapKey.size();
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MaskRay wrote:

SectionLen is fixed now

https://github.com/llvm/llvm-project/pull/95030