[LLD][COFF] Skip computation of the undefined symbols references that are not shown

[saudi]

Summary

The "undefined symbol" error message from lld-link displays up to 3 references to that symbol, and the number of extra references not shown.

This patch removes the computation of the strings for those extra references.

It fixes a freeze of lld-link we accidentally encountered, when we tried activating asan on one of our games without linking with the asan library.
In that case, __asan_report_load8 was referenced more than 2 million times, causing the computation of that many display strings, of which only 3 were used.

[mstorsjo]

LGTM (but maybe wait for a bit to see if someone else might have more opinions)

FWIW, I think this codepath exists in a fairly similar form in the ELF backend, that might benefit from a similar optimization.

[saudi]

Updated the test, adding an LTO obj file.

[mstorsjo]

Still looks good with the additional test update (thanks for that!)

[saudi]

In D83510#2142708, @mstorsjo wrote:

LGTM (but maybe wait for a bit to see if someone else might have more opinions)

FWIW, I think this codepath exists in a fairly similar form in the ELF backend, that might benefit from a similar optimization.

I double-checked, the corresponding code in ELF computes the strings in the same loop where they are displayed, instead of the 2 passes done for COFF.
So, no optimization seems necessary in the case of ELF.

[mstorsjo]

In D83510#2162794, @saudi wrote:

In D83510#2142708, @mstorsjo wrote:

LGTM (but maybe wait for a bit to see if someone else might have more opinions)

FWIW, I think this codepath exists in a fairly similar form in the ELF backend, that might benefit from a similar optimization.

I double-checked, the corresponding code in ELF computes the strings in the same loop where they are displayed, instead of the 2 passes done for COFF.
So, no optimization seems necessary in the case of ELF.

Oh, ok, that sounds good then. Please go ahead with this (and I think this could be worthwhile to backport to the release branch).

[MaskRay]

lld/test/COFF/Inputs/undefined-symbol-multi-lto.ll

20 ↗

(On Diff #279399)

llvm.ident should normally be removed.

lld/COFF/SymbolTable.cpp

if (!fileLine && config->mingw)
     fileLine = getFileLineDwarf(c, addr);
   return fileLine;
 }
 
 // Given a file and the index of a symbol in that file, returns a description
 // of all references to that symbol from that file. If no debug information is
 // available, returns just the name of the file, else one string per actual
 // reference as described in the debug info.
-std::vector<std::string> getSymbolLocations(ObjFile *file, uint32_t symIndex) {
+// Returns up to maxStrings string descriptions, along with the total number of
+// locations found.
+static std::pair<std::vector<std::string>, size_t>
+getSymbolLocations(ObjFile *file, uint32_t symIndex, size_t maxStrings) {
   struct Location {
     Symbol *sym;
     std::pair<StringRef, uint32_t> fileLine;
   };
   std::vector<Location> locations;
+  size_t numLocations = 0;
 
   for (Chunk *c : file->getChunks()) {
     auto *sc = dyn_cast<SectionChunk>(c);
     if (!sc)
       continue;
     for (const coff_relocation &r : sc->getRelocs()) {
       if (r.SymbolTableIndex != symIndex)
         continue;
+      numLocations++;
+      if (locations.size() >= maxStrings)
+        continue;
+
       Optional<std::pair<StringRef, uint32_t>> fileLine =
           getFileLine(sc, r.VirtualAddress);
       Symbol *sym = getSymbol(sc, r.VirtualAddress);
       if (fileLine)
         locations.push_back({sym, *fileLine});
       else if (sym)
         locations.push_back({sym, {"", 0}});
     }
   }
 
-  if (locations.empty())
-    return std::vector<std::string>({"\n>>> referenced by " + toString(file)});
+  if (maxStrings == 0)
+    return std::make_pair(std::vector<std::string>(), numLocations);
+
+  if (numLocations == 0)
+    return std::make_pair(
+        std::vector<std::string>{"\n>>> referenced by " + toString(file)}, 1);
 
   std::vector<std::string> symbolLocations(locations.size());
   size_t i = 0;
   for (Location loc : locations) {
     llvm::raw_string_ostream os(symbolLocations[i++]);
     os << "\n>>> referenced by ";
     if (!loc.fileLine.first.empty())
       os << loc.fileLine.first << ":" << loc.fileLine.second
          << "\n>>>               ";
     os << toString(file);
     if (loc.sym)
       os << ":(" << toString(*loc.sym) << ')';
   }
-  return symbolLocations;
+  return std::make_pair(symbolLocations, numLocations);
 }
 
-std::vector<std::string> getSymbolLocations(InputFile *file,
-                                            uint32_t symIndex) {
+std::vector<std::string> getSymbolLocations(ObjFile *file, uint32_t symIndex) {
+  return getSymbolLocations(file, symIndex, SIZE_MAX).first;
+}
+
+static std::pair<std::vector<std::string>, size_t>
+getSymbolLocations(InputFile *file, uint32_t symIndex, size_t maxStrings) {
   if (auto *o = dyn_cast<ObjFile>(file))
-    return getSymbolLocations(o, symIndex);
-  if (auto *b = dyn_cast<BitcodeFile>(file))
-    return getSymbolLocations(b);
+    return getSymbolLocations(o, symIndex, maxStrings);
+  if (auto *b = dyn_cast<BitcodeFile>(file)) {
+    std::vector<std::string> symbolLocations = getSymbolLocations(b);
+    size_t numLocations = symbolLocations.size();
+    if (symbolLocations.size() > maxStrings)
+      symbolLocations.resize(maxStrings);
+    return std::make_pair(symbolLocations, numLocations);
+  }
   llvm_unreachable("unsupported file type passed to getSymbolLocations");
-  return {};
+  return std::make_pair(std::vector<std::string>(), (size_t)0);
 }
 
 // For an undefined symbol, stores all files referencing it and the index of
 // the undefined symbol in each file.
 struct UndefinedDiag {
   Symbol *sym;
   struct File {
     InputFile *file;
     uint32_t symIndex;
   };
   std::vector<File> files;
 };
 
 static void reportUndefinedSymbol(const UndefinedDiag &undefDiag) {
   std::string out;
   llvm::raw_string_ostream os(out);
   os << "undefined symbol: " << toString(*undefDiag.sym);
 
   const size_t maxUndefReferences = 3;
-  size_t i = 0, numRefs = 0;
+  size_t numDisplayedRefs = 0, numRefs = 0;
   for (const UndefinedDiag::File &ref : undefDiag.files) {
-    std::vector<std::string> symbolLocations =
-        getSymbolLocations(ref.file, ref.symIndex);
-    numRefs += symbolLocations.size();
+    std::vector<std::string> symbolLocations;
+    size_t totalLocations = 0;
+    std::tie(symbolLocations, totalLocations) = getSymbolLocations(
+        ref.file, ref.symIndex, maxUndefReferences - numDisplayedRefs);
+
+    numRefs += totalLocations;
+    numDisplayedRefs += symbolLocations.size();
     for (const std::string &s : symbolLocations) {
-      if (i >= maxUndefReferences)
-        break;
       os << s;
-      i++;
     }
   }
-  if (i < numRefs)
-    os << "\n>>> referenced " << numRefs - i << " more times";
+  if (numDisplayedRefs < numRefs)
+    os << "\n>>> referenced " << numRefs - numDisplayedRefs << " more times";
   errorOrWarn(os.str());
 }
 
 void SymbolTable::loadMinGWAutomaticImports() {
   for (auto &i : symMap) {
     Symbol *sym = i.second;
     auto *undef = dyn_cast<Undefined>(sym);
     if (!undef)