[lld] r298124 - recommend using llvm-ar when finding undefined references and empty archives

[Bob Haarman via llvm-commits]

Author: inglorion
Date: Fri Mar 17 16:32:49 2017
New Revision: 298124

URL: http://llvm.org/viewvc/llvm-project?rev=298124&view=rev
Log:

recommend using llvm-ar when finding undefined references and empty archives

Summary:
When we perform LTO builds with a version of ar that does not
understand LLVM bitcode objects, we end up with undefined references,
because our archive files do not list the bitcode symbols in their
indices. The error messages do not make it clear what the real problem
is. This change adds a note that points out the likely problem and
solution. It is similar in spirit to r282633, but aims to avoid false
positives by only triggering when we see both undefined references and
archives without symbols in their indices.

Fixes PR32281.

Reviewers: davide, ruiu, tejohnson

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31011

Modified:
    lld/trunk/ELF/Config.h
    lld/trunk/ELF/InputFiles.cpp
    lld/trunk/ELF/Relocations.cpp

Modified: lld/trunk/ELF/Config.h
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Config.h?rev=298124&r1=298123&r2=298124&view=diff
==============================================================================
--- lld/trunk/ELF/Config.h (original)
+++ lld/trunk/ELF/Config.h Fri Mar 17 16:32:49 2017
@@ -98,6 +98,7 @@ struct Configuration {
   std::vector<SymbolVersion> VersionScriptLocals;
   std::vector<uint8_t> BuildIdVector;
   bool AllowMultipleDefinition;
+  bool ArchiveWithoutSymbolsSeen = false;
   bool AsNeeded = false;
   bool Bsymbolic;
   bool BsymbolicFunctions;

Modified: lld/trunk/ELF/InputFiles.cpp
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/InputFiles.cpp?rev=298124&r1=298123&r2=298124&view=diff
==============================================================================
--- lld/trunk/ELF/InputFiles.cpp (original)
+++ lld/trunk/ELF/InputFiles.cpp Fri Mar 17 16:32:49 2017
@@ -556,8 +556,12 @@ template <class ELFT> void ArchiveFile::
                MB.getBufferIdentifier() + ": failed to parse archive");
 
   // Read the symbol table to construct Lazy objects.
-  for (const Archive::Symbol &Sym : File->symbols())
+  for (const Archive::Symbol &Sym : File->symbols()) {
     Symtab<ELFT>::X->addLazyArchive(this, Sym);
+  }
+
+  if (File->symbols().begin() == File->symbols().end())
+    Config->ArchiveWithoutSymbolsSeen = true;
 }
 
 // Returns a buffer pointing to a member file containing a given symbol.

Modified: lld/trunk/ELF/Relocations.cpp
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Relocations.cpp?rev=298124&r1=298123&r2=298124&view=diff
==============================================================================
--- lld/trunk/ELF/Relocations.cpp (original)
+++ lld/trunk/ELF/Relocations.cpp Fri Mar 17 16:32:49 2017
@@ -610,10 +610,21 @@ static void reportUndefined(SymbolBody &
                     toString(Sym) + "'";
 
   if (Config->UnresolvedSymbols == UnresolvedPolicy::WarnAll ||
-      (Config->UnresolvedSymbols == UnresolvedPolicy::Warn && CanBeExternal))
+      (Config->UnresolvedSymbols == UnresolvedPolicy::Warn && CanBeExternal)) {
     warn(Msg);
-  else
+  } else {
     error(Msg);
+    if (Config->ArchiveWithoutSymbolsSeen) {
+      message("At least one archive listed no symbols in its index."
+              " This can happen when creating archives with a version"
+              " of ar that does not understand the object files in"
+              " the archive. For example, if you are using LLVM"
+              " bitcode objects (such as created by -flto), you may"
+              " need to use llvm-ar or GNU ar with a plugin.");
+      // Reset to false so that we print the message only once.
+      Config->ArchiveWithoutSymbolsSeen = false;
+    }
+  }
 }
 
 template <class RelTy>