[BOLT][NFCI] Use FileSymbols for local symbol disambiguation (#89088)

Remove SymbolToFileName mapping from every local symbol to its
containing FILE symbol name, and reuse FileSymbols to disambiguate
local symbols instead.

Also removes the check for `ld-temp.o` file symbol which was added to
prevent LTO build mode from affecting the disambiguated name. This may
cause incompatibility when using the profile collected on a binary built
in a different mode than the input binary.

Addresses #90661.

Speeds up discover file objects by 5-10% for large binaries:
- binary with ~1.2M symbols: 12.6422s -> 12.0297s
- binary with ~4.5M symbols: 48.8851s -> 43.7315s

Author: aaupov

bolt/lib/Rewrite/RewriteInstance.cpp

@@ -780,14 +780,6 @@ void RewriteInstance::discoverFileObjects() {
 
   // For local symbols we want to keep track of associated FILE symbol name for
   // disambiguation by combined name.
-  StringRef FileSymbolName;
-  bool SeenFileName = false;
-  struct SymbolRefHash {
-    size_t operator()(SymbolRef const &S) const {
-      return std::hash<decltype(DataRefImpl::p)>{}(S.getRawDataRefImpl().p);
-    }
-  };
-  std::unordered_map<SymbolRef, StringRef, SymbolRefHash> SymbolToFileName;
   for (const ELFSymbolRef &Symbol : InputFile->symbols()) {
     Expected<StringRef> NameOrError = Symbol.getName();
     if (NameOrError && NameOrError->starts_with("__asan_init")) {
@@ -806,21 +798,8 @@ void RewriteInstance::discoverFileObjects() {
     if (cantFail(Symbol.getFlags()) & SymbolRef::SF_Undefined)
       continue;
 
-    if (cantFail(Symbol.getType()) == SymbolRef::ST_File) {
+    if (cantFail(Symbol.getType()) == SymbolRef::ST_File)
       FileSymbols.emplace_back(Symbol);
-      StringRef Name =
-          cantFail(std::move(NameOrError), "cannot get symbol name for file");
-      // Ignore Clang LTO artificial FILE symbol as it is not always generated,
-      // and this uncertainty is causing havoc in function name matching.
-      if (Name == "ld-temp.o")
-        continue;
-      FileSymbolName = Name;
-      SeenFileName = true;
-      continue;
-    }
-    if (!FileSymbolName.empty() &&
-        !(cantFail(Symbol.getFlags()) & SymbolRef::SF_Global))
-      SymbolToFileName[Symbol] = FileSymbolName;
   }
 
   // Sort symbols in the file by value. Ignore symbols from non-allocatable
@@ -1028,14 +1007,14 @@ void RewriteInstance::discoverFileObjects() {
       // The <id> field is used for disambiguation of local symbols since there
       // could be identical function names coming from identical file names
       // (e.g. from different directories).
-      std::string AltPrefix;
-      auto SFI = SymbolToFileName.find(Symbol);
-      if (SymbolType == SymbolRef::ST_Function && SFI != SymbolToFileName.end())
-        AltPrefix = Name + "/" + std::string(SFI->second);
+      auto SFI = llvm::upper_bound(FileSymbols, ELFSymbolRef(Symbol));
+      if (SymbolType == SymbolRef::ST_Function && SFI != FileSymbols.begin()) {
+        StringRef FileSymbolName = cantFail(SFI[-1].getName());
+        if (!FileSymbolName.empty())
+          AlternativeName = NR.uniquify(Name + "/" + FileSymbolName.str());
+      }
 
       UniqueName = NR.uniquify(Name);
-      if (!AltPrefix.empty())
-        AlternativeName = NR.uniquify(AltPrefix);
     }
 
     uint64_t SymbolSize = ELFSymbolRef(Symbol).getSize();
@@ -1294,7 +1273,7 @@ void RewriteInstance::discoverFileObjects() {
                              FDE->getAddressRange());
   }
 
-  BC->setHasSymbolsWithFileName(SeenFileName);
+  BC->setHasSymbolsWithFileName(FileSymbols.size());
 
   // Now that all the functions were created - adjust their boundaries.
   adjustFunctionBoundaries();