Fix for -wrap linker option and LTO, PR 33145

[dmikulin]

Summary

An attempt to fix PR33145, --wrap doesn't work with lto.
Wrapped symbols need to excluded from inter-procedural optimizations to prevent dropping symbols and allow the linker to process re-directs.
Current limitation: doesn't work with ThinLTO.

[davide]

There are no tests associated with this patch.

lld/ELF/SymbolTable.cpp

167–171

Can you explain why you need all three?

363–365

empty ifs are not really common in llvm.

lld/ELF/SymbolTable.h

140

A StringMap<bool> is a little silly.
If you really want a set container, you should use a proper one.

llvm/include/llvm/LTO/LTO.h

384

I'd say, LinkerRedefined.

llvm/lib/LTO/LTO.cpp

423

why do you need this being part of GlobalResolution?
If you do, please add a test showing that you fail switching the linkage earlier.

[ruiu]

Do you really need this much change? I expected something smaller than this. I mean, I thought what we need to do is to just pass a list of symbols which LTO should keep.

[dmikulin]

lld/ELF/SymbolTable.cpp

167–171

We need all three if we want to match non-LTO behavior.

• _wrap_bar is needed for the case where it is defined by the user but may be eliminated during LTO.
• _real_bar is needed for the test case in the PR: the direct call to _real_bar() from foo() will be bound and inlined by LTO to the user-defined _real_bar() instead of bar() if _real_bar() is not on the list.
• similarly, if bar() is not on the list, the call to bar() from foo() will be inlined instead of being routed to _wrap_bar().

363–365

I'll fix this. Negating the condition seamed counter-intuitive...

lld/ELF/SymbolTable.h

140

Is a set container like SmallSet good enough in this case? What's the worst case for the number of -wrap functions? I picked a map because we have to do a lookup in this container for every symbol.

llvm/include/llvm/LTO/LTO.h

384

I'll change this.

llvm/lib/LTO/LTO.cpp

423

You're right, I can do it sooner, in addRegularLTO(). I'll take out the GlobalRes stuff.

[davide]

lld/ELF/SymbolTable.cpp

167–171

Can you please add a test showing when 3) happens?

lld/ELF/SymbolTable.h

140

What I'm trying to say is that a map from something to bool is just a Set.
If you think your set will grow a lot (unlikely, but OK, I'd recommend DenseSet).

[dmikulin]

lld/ELF/SymbolTable.cpp

167–171

3. is also demonstrated by the test case in the PR. I'm working on regression tests for all known cases and will submit them with the new version of the patch.

lld/ELF/SymbolTable.h

140

OK, thanks. Will fix.

[pcc]

lld/ELF/SymbolTable.cpp

167–171

_wrap_bar is needed for the case where it is defined by the user but may be eliminated during LTO.

This one only requires VisibleToRegularObj, doesn't it?

363–365

Is the purpose of this code just to restore the binding of the original symbol? I think it would be simpler to have a post-LTO step that manually restores the binding for each renamed symbol rather than dealing with it here.

[pcc]

lld/ELF/SymbolTable.cpp

175

What about the rename target? If I pass --defsym=foo=bar, the linker should not allow IPO past foo, right?

[dmikulin]

lld/ELF/SymbolTable.cpp

167–171

Looks like it. Do we want to handle _wrap_* symbols specially and not set the weak binding for LTO?

175

Right. I'll add it to the list.

363–365

Yes, it was to restore the binding. I changed it to have a separate pass over renamed symbols after LTO.

[dmikulin]

Revised patch. Will add test cases tomorrow.

[pcc]

lld/ELF/SymbolTable.cpp

121

If you move the list of renamed symbols to Config you can avoid this lambda.

167–171

Up to you. Note that you can make LTO keep a symbol by calling addUndefined with the symbol name, so it wouldn't be *that* special.

187

You mean Alias here, right?

[davide]

I'll wait for your tests before another careful review. Some drive by comments.

lld/ELF/SymbolTable.cpp

131–133

Do you need an explicit iterator? Can't you use a range for ?

lld/ELF/SymbolTable.h

141

I'd be more explicit here. e.g. "this is a map from Symbol name to binding" or something like that.

[dmikulin]

lld/ELF/SymbolTable.cpp

121

OK, will do.

187

Of course.

lld/ELF/SymbolTable.h

141

OK

[dmikulin]

Addressed review comments. Added test cases. Tweaked saved pre-LTO bindings to more accurately reflect the state of the symbol table.

[davide]

This is getting better.
Please rebase the patch because it didn't apply cleanly.

lld/ELF/SymbolTable.cpp

131

Convention says all Variable names should start with capital letter. Also, i is a little vague.

llvm/include/llvm/LTO/LTO.h

368–371

Is this formatted? Doesn't seem so.

llvm/lib/LTO/LTO.cpp

421

If I remove this line, all the tests still pass.

441–442

If I comment out this line, all the tests still pass.

547–548

This needs a comment.

[dmikulin]

lld/ELF/SymbolTable.cpp

131

Changed it to RSI (Renamed Symbol Iterator)

llvm/lib/LTO/LTO.cpp

421

This was meant for ThinLTO support, which hasn't been implemented yet. I can take it out from this patch.

441–442

Added a test point for this

[pcc]

lld/test/ELF/lto/defsym.ll

14–15

These are addresses of PLT entries, right? Can you avoid magic numbers here and make what you are testing for more explicit? I think you can do that by making the symbols hidden and checking that the calls correspond to the correct symbol addresses. Same for wrap-2.ll.

llvm/lib/LTO/LTO.cpp

442

You also need to add support for 'r' to llvm-lto2. Please also add an llvm-lto2 based test that verifies that the linkage has changed.

[dmikulin]

Addressed review comments.
Made changes and added a test to llvm-lto2.

[dmikulin]

Changed the mapping from string --> binding to Symbol* --> binding.

[pcc]

lld/ELF/SymbolTable.cpp

194–220

Can we now replace these functions with something that applies the rename to each symbol in Config->RenamedSymbols? I think it should be possible with this change to how we declare RenamedSymbols.

struct RenamedSymbol {
  uint8_t OrigBinding;
  Symbol *Target;
};
MapVector<Symbol*, RenamedSymbol> RenamedSymbols;

[dmikulin]

lld/ELF/SymbolTable.cpp

194–220

But __wrap_ symbols are different: they need to be on the list, but they are not being replaced.

[pcc]

lld/ELF/SymbolTable.cpp

194–220

They only need to be marked as IsUsedInRegularObj. And that's taken care of for you by addUndefined.

[dmikulin]

lld/ELF/SymbolTable.cpp

194–220

Map it to NULL?

[davide]

lld/ELF/Config.h

102

This is the only field in Config that has a comment. Not sure how I feel about it, I'll defer it to Rui.

lld/ELF/SymbolTable.cpp

194–197

Is this bit needed? I don't see a test for it.

198

Can this ever be null? If so, you should check for it.
If not, you may as well inline this in the single use.

lld/ELF/SymbolTable.h

42–43

Unsorted.

llvm/lib/LTO/LTO.cpp

407–410

This comment is now stale after your change.

[pcc]

lld/ELF/SymbolTable.cpp

194–220

I mean that you can just set IsUsedInRegularObj on the __wrap_ symbol before you assign it to Target. Then LTO will keep it alive. There's no need to add it (as a key) to RenamedSymbols then.
http://llvm-cs.pcc.me.uk/tools/lld/ELF/LTO.cpp#136

[dmikulin]

lld/ELF/SymbolTable.cpp

194–220

This works fine for -wrap, but not for -defsym.

For -wrap, we want to keep the __real_ and the original symbols on the list, so the mappings are:
Real --> Orig and Orig --> Wrap
After we're done with LTO, we copy targets of the mapping back to their keys, Orig --> Real and Wrap --> Orig.

For -defsym, we want to preserve the original symbol, so we would add Orig --> Alias mapping to communicate with LTO. But after we're done, we need to copy Orig --> Alias. I added a flag to the mapping structure to reverse the direction of the copy, so now in the -defsym test the call to bar3() gets inlined and the call to bar2() goes to its alias, bar3. This sounds reasonable to me, just want to make sure it's acceptable.

[dmikulin]

lld/ELF/Config.h

102

Removed the comment. Now it's next to the RemappedSymbol structure.

lld/ELF/SymbolTable.cpp

194–197

It was moved here from SymbolTable::alias, there is a test case for it already.

198

It can't be null, I just find it easier to read, that's all.

[dmikulin]

Changed the way -defsym and -wrap are handled.

[pcc]

lld/ELF/Driver.cpp

972

This comment and the one below aren't really accurate any more because this code is used in the non-LTO case as well.

lld/ELF/LTO.cpp

139

I think this can just be Config->RenamedSymbols.count(Sym).

lld/ELF/SymbolTable.cpp

167–169

Move this comment to applySymbolRenames.

169

Is this function necessary? All callers pass a non-null SymbolBody, so they can just query the binding directly.

197

I'd minimise the diff here and rename this variable back.

[dmikulin]

Fixed comments.
Rebased the patch.

[davide]

lld/ELF/SymbolTable.cpp

202

You're setting IsUsedInRegularObj from what I can see, here, for all the symbols passed with --wrap.
Can you check this is really NFC? IIRC it could impact the way symbols are added (or not) to the symbol table.

220

Same here.

[• rafael]

This LGTM. I am not a big fan of keeping a copy or the original binding, but it sure saves a lot of plumbing.

Please just wait to see if Davide and Peter have something to add.

[pcc]

LGTM as well

lld/test/ELF/lto/wrap-1.ll

14

Please update this comment. I think you also want to test 'r' for __real_bar.

[davide]

I'm fine with this once Peter's comments are addressed. I'm not a fan of having to carry around the original binding as well, but I think it's tolerable.

[ruiu]

lld/ELF/Config.h

101

Please run clang-format-diff. It should be formatted as <Symbol *, RenamedSymbol>

lld/ELF/Driver.cpp

972

I prefer a more concrete comment. Handle -wrap option.

976

Ditto. Handle --defsym=sym=alias option.

lld/ELF/SymbolTable.cpp

201

Add a blank line before a comment.

219

Add a blank line before a comment.

225

It is not obvious why you need this function from the source code. You need to add a comment.

226

What is RSI? I found just KV (short for key-value) makes sense in most cases.

230

Ditto

[dmikulin]

lld/ELF/SymbolTable.cpp

226

RSI is supposed to be Renamed Symbol Iterator, but KV is ok with me too

lld/test/ELF/lto/wrap-1.ll

14

Resolutions file is driven by input file symbol tables. Since __real_bar is not defined in an object, it does not show up there. I added a definition and a test for the 'r' flag.

[davide]

There were some comments pending by Rui. You addressed them, but you still need to wait for an explicit LGTM before committing.
I don't think it's worth reverting this patch, but this is not okay and you should consider it next time.

lld/ELF/Config.h

struct Configuration {
   std::vector<VersionDefinition> VersionDefinitions;
   std::vector<llvm::StringRef> AuxiliaryList;
   std::vector<llvm::StringRef> SearchPaths;
   std::vector<llvm::StringRef> SymbolOrderingFile;
   std::vector<llvm::StringRef> Undefined;
   std::vector<SymbolVersion> VersionScriptGlobals;
   std::vector<SymbolVersion> VersionScriptLocals;
   std::vector<uint8_t> BuildIdVector;
+
+  // Mapping of renamed symbols to their original pre-LTO bindings.
+  llvm::StringMap<uint8_t> RenamedSymbols;
+
   bool AllowMultipleDefinition;
   bool AsNeeded = false;
   bool Bsymbolic;
   bool BsymbolicFunctions;
   bool ColorDiagnostics = false;
   bool CompressDebugSections;
   bool DefineCommon;
   bool Demangle = true;

lld/ELF/Driver.cpp

template <class ELFT> void LinkerDriver::link(opt::InputArgList &Args) {
   // Return if there were name resolution errors.
   if (ErrorCount)
     return;
 
   Symtab.scanUndefinedFlags();
   Symtab.scanShlibUndefined();
   Symtab.scanVersionScript();
 
+  // Pass wrapped symbols to LTO
+  for (auto *Arg : Args.filtered(OPT_wrap))
+    Symtab.addLTOSymbolWrap(Arg->getValue());
+
+  // Pass alias symbols to LTO
+  for (std::pair<StringRef, StringRef> &Def : getDefsym(Args))
+    Symtab.addLTOSymbolAlias(Def.first, Def.second);
+
   Symtab.addCombinedLTOObject();
   if (ErrorCount)
     return;
 
   // Some symbols (such as __ehdr_start) are defined lazily only when there
   // are undefined symbols for them, so we add these to trigger that logic.
   for (StringRef Sym : Script->Opt.ReferencedSymbols)
     Symtab.addUndefined(Sym);

lld/ELF/LTO.cpp

for (const lto::InputFile::Symbol &ObjSym : Obj.symbols()) {
     // Once IRObjectFile is fixed to report only one symbol this hack can
     // be removed.
     R.Prevailing = !ObjSym.isUndefined() && B->File == &F;
 
     R.VisibleToRegularObj =
         Sym->IsUsedInRegularObj || (R.Prevailing && Sym->includeInDynsym());
     if (R.Prevailing)
       undefine(Sym);
+    R.LinkerRedefined = Config->RenamedSymbols.find(B->getName()) !=
+        Config->RenamedSymbols.end();
   }
   checkError(LTOObj->add(std::move(F.Obj), Resols));
 }
 
 // Merge all the bitcode files we have seen, codegen the result
 // and return the resulting ObjectFile(s).
 std::vector<InputFile *> BitcodeCompiler::compile() {
   std::vector<InputFile *> Ret;

lld/ELF/SymbolTable.h

 // add*() functions, which are called by input files as they are parsed. There
 // is one add* function per symbol type.
 template <class ELFT> class SymbolTable {
   typedef typename ELFT::Sym Elf_Sym;
 
 public:
   void addFile(InputFile *File);
   void addCombinedLTOObject();
+  void addLTOSymbolWrap(StringRef Name);
+  void addLTOSymbolAlias(StringRef Alias, StringRef Name);
 
   ArrayRef<Symbol *> getSymbols() const { return SymVector; }
   ArrayRef<ObjectFile<ELFT> *> getObjectFiles() const { return ObjectFiles; }
   ArrayRef<BinaryFile *> getBinaryFiles() const { return BinaryFiles; }
   ArrayRef<SharedFile<ELFT> *> getSharedFiles() const { return SharedFiles; }
 
   DefinedRegular *addAbsolute(StringRef Name,
                               uint8_t Visibility = llvm::ELF::STV_HIDDEN,
private:
   // A map from demangled symbol names to their symbol objects.
   // This mapping is 1:N because two symbols with different versions
   // can have the same name. We use this map to handle "extern C++ {}"
   // directive in version scripts.
   llvm::Optional<llvm::StringMap<std::vector<SymbolBody *>>> DemangledSyms;
 
   // For LTO.
   std::unique_ptr<BitcodeCompiler> LTO;
 };
 
 template <class ELFT> struct Symtab { static SymbolTable<ELFT> *X; };
 template <class ELFT> SymbolTable<ELFT> *Symtab<ELFT>::X;
 
 } // namespace elf
 } // namespace lld
 
 #endif

lld/ELF/SymbolTable.cpp

 // to the compiler at once, it can do whole-program optimization.
 template <class ELFT> void SymbolTable<ELFT>::addCombinedLTOObject() {
   if (BitcodeFiles.empty())
     return;
 
   // Compile bitcode files and replace bitcode symbols.
   LTO.reset(new BitcodeCompiler);
   for (BitcodeFile *F : BitcodeFiles)
     LTO->add(*F);
 
   for (InputFile *File : LTO->compile()) {
     ObjectFile<ELFT> *Obj = cast<ObjectFile<ELFT>>(File);
     DenseSet<CachedHashStringRef> DummyGroups;
     Obj->parse(DummyGroups);
     ObjectFiles.push_back(Obj);
   }
+
+  // Restore bindings of renamed symbols.
+  for (auto &RSI : Config->RenamedSymbols) {
+    SymbolBody *Body = find(RSI.first());
+    if (Body) {
+      Symbol *Sym = Body->symbol();
+      Sym->Binding = RSI.second;
+    }
+  }
 }
 
 template <class ELFT>
 DefinedRegular *SymbolTable<ELFT>::addAbsolute(StringRef Name,
                                                uint8_t Visibility,
                                                uint8_t Binding) {
   Symbol *Sym =
       addRegular(Name, Visibility, STT_NOTYPE, 0, 0, Binding, nullptr, nullptr);

 }
 
 // Set a flag for --trace-symbol so that we can print out a log message
 // if a new symbol with the same name is inserted into the symbol table.
 template <class ELFT> void SymbolTable<ELFT>::trace(StringRef Name) {
   Symtab.insert({CachedHashStringRef(Name), {-1, true}});
 }
 
+// Get pre-LTO symbol binding for -wrap and -defsym symbols.
+// If it's in the symbol table, read it from the symbol.
+// Otherwise an undef for it will be created with STB_GLOBAL.
+static uint8_t getSymbolBinding(SymbolBody *SB) {
+  uint8_t Binding = STB_GLOBAL;
+  if (SB) {
+    Symbol *S = SB->symbol();
+    Binding = S->Binding;
+  }
+  return Binding;
+}
+
+template <class ELFT> void SymbolTable<ELFT>::addLTOSymbolWrap(StringRef Name) {
+  std::string WrappedName = "__wrap_" + Name.str();
+  std::string RealName = "__real_" + Name.str();
+  Config->RenamedSymbols[WrappedName] = getSymbolBinding(find(WrappedName));
+  Config->RenamedSymbols[RealName] = getSymbolBinding(find(RealName));
+  Config->RenamedSymbols[Name] = getSymbolBinding(find(Name));
+}
+
+template <class ELFT> void SymbolTable<ELFT>::addLTOSymbolAlias(StringRef Alias,
+                                                                StringRef Name) {
+  Config->RenamedSymbols[Alias] = getSymbolBinding(find(Alias));
+  Config->RenamedSymbols[Name] = getSymbolBinding(find(Name));
+}
+
 // Rename SYM as __wrap_SYM. The original symbol is preserved as __real_SYM.
 // Used to implement --wrap.
 template <class ELFT> void SymbolTable<ELFT>::wrap(StringRef Name) {
   SymbolBody *B = find(Name);
   if (!B)
     return;
   Symbol *Sym = B->symbol();
   Symbol *Real = addUndefined(Saver.save("__real_" + Name));
   Symbol *Wrap = addUndefined(Saver.save("__wrap_" + Name));
 
   // We rename symbols by replacing the old symbol's SymbolBody with the new
   // symbol's SymbolBody. This causes all SymbolBody pointers referring to the
   // old symbol to instead refer to the new symbol.
   memcpy(Real->Body.buffer, Sym->Body.buffer, sizeof(Sym->Body));
   memcpy(Sym->Body.buffer, Wrap->Body.buffer, sizeof(Wrap->Body));
 }
 
 // Creates alias for symbol. Used to implement --defsym=ALIAS=SYM.
 template <class ELFT>
 void SymbolTable<ELFT>::alias(StringRef Alias, StringRef Name) {
   SymbolBody *B = find(Name);
   if (!B) {
     error("-defsym: undefined symbol: " + Name);
     return;
   }
   Symbol *Sym = B->symbol();
   Symbol *AliasSym = addUndefined(Alias);
   memcpy(AliasSym->Body.buffer, Sym->Body.buffer, sizeof(AliasSym->Body));
 }
 
 static uint8_t getMinVisibility(uint8_t VA, uint8_t VB) {
   if (VA == STV_DEFAULT)
     return VB;
   if (VB == STV_DEFAULT)
     return VA;
   return std::min(VA, VB);
 }
 
 // Find an existing symbol or create and insert a new one.
 template <class ELFT>
 std::pair<Symbol *, bool> SymbolTable<ELFT>::insert(StringRef Name) {
   auto P = Symtab.insert(
       {CachedHashStringRef(Name), SymIndex((int)SymVector.size(), false)});
   SymIndex &V = P.first->second;
   bool IsNew = P.second;
 
   if (V.Idx == -1) {
static int compareDefinedNonCommon(Symbol *S, bool WasInserted, uint8_t Binding,
                                    bool IsAbsolute, typename ELFT::uint Value) {
   if (int Cmp = compareDefined(S, WasInserted, Binding)) {
     if (Cmp > 0)
       S->Binding = Binding;
     return Cmp;
   }
   SymbolBody *B = S->body();
   if (isa<DefinedCommon>(B)) {
     // Non-common symbols take precedence over common symbols.
     if (Config->WarnCommon)
       warn("common " + S->body()->getName() + " is overridden");
     return 1;
   } else if (auto *R = dyn_cast<DefinedRegular>(B)) {
     if (R->Section == nullptr && Binding == STB_GLOBAL && IsAbsolute &&
         R->Value == Value)
       return -1;
   }
   return 0;
 }

lld/test/ELF/lto/Inputs/defsym-bar.ll

+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define hidden void @bar1() {
+  ret void
+}
+
+define hidden void @bar2() {
+  ret void
+}
+
+define hidden void @bar3() {
+  ret void
+}

lld/test/ELF/lto/Inputs/wrap-bar.ll

+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define hidden void @bar() {
+  ret void
+}
+
+define hidden void @__real_bar() {
+  ret void
+}
+
+define hidden void @__wrap_bar() {
+  ret void
+}

lld/test/ELF/lto/defsym.ll

+; REQUIRES: x86
+; RUN: llvm-as %s -o %t.o
+; RUN: llvm-as %S/Inputs/defsym-bar.ll -o %t1.o
+; RUN: ld.lld %t.o %t1.o -shared -o %t.so -defsym=bar2=bar3
+; RUN: llvm-objdump -d %t.so | FileCheck %s
+
+; Call to bar2() should not be inlined and should be routed to bar3()
+; Symbol bar3 should not be eliminated
+
+; CHECK:      foo:
+; CHECK-NEXT: pushq %rax
+; CHECK-NEXT: callq{{.*}}<bar3>
+; CHECK-NEXT: callq{{.*}}<bar3>
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+declare void @bar1()
+declare void @bar2()
+declare void @bar3()
+
+define void @foo() {
+  call void @bar1()
+  call void @bar2()
+  call void @bar3()
+  ret void
+}

lld/test/ELF/lto/wrap-1.ll

+; REQUIRES: x86
+; RUN: llvm-as %s -o %t.o
+; RUN: ld.lld %t.o -o %t.out -wrap=bar -save-temps
+; RUN: llvm-readobj -t %t.out | FileCheck %s
+; RUN: cat %t.out.resolution.txt | FileCheck -check-prefix=RESOLS %s
+
+; CHECK:      Name: __wrap_bar
+; CHECK-NEXT: Value:
+; CHECK-NEXT: Size:
+; CHECK-NEXT: Binding: Global
+; CHECK-NEXT: Type: Function
+
+; Make sure that the 'r' (linker redefined) bit is set for bar and __wrap_bar
+; in the resolutions file.
+; RESOLS: ,bar,r
+; RESOLS: ,__wrap_bar,pr
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+declare void @bar()
+
+define void @_start() {
+  call void @bar()
+  ret void
+}
+
+define void @__wrap_bar() {
+  ret void
+}

lld/test/ELF/lto/wrap-2.ll

+; REQUIRES: x86
+; RUN: llvm-as %s -o %t.o
+; RUN: llvm-as %S/Inputs/wrap-bar.ll -o %t1.o
+; RUN: ld.lld %t.o %t1.o -shared -o %t.so -wrap=bar
+; RUN: llvm-objdump -d %t.so | FileCheck %s
+
+; Make sure that calls in foo() are not eliminated and that bar is
+; routed to __wrap_bar and __real_bar is routed to bar.
+
+; CHECK:      foo:
+; CHECK-NEXT: pushq %rax
+; CHECK-NEXT: callq{{.*}}<__wrap_bar>
+; CHECK-NEXT: callq{{.*}}<bar>
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+declare void @bar()
+declare void @__real_bar()
+
+define void @foo() {
+  call void @bar()
+  call void @__real_bar()
+  ret void
+}

llvm/include/llvm/LTO/LTO.h

Error runThinLTO(AddStreamFn AddStream, NativeObjectCache Cache,
                    bool HasRegularLTO);
 
   mutable bool CalledGetMaxTasks = false;
 };
 
 /// The resolution for a symbol. The linker must provide a SymbolResolution for
 /// each global symbol based on its internal resolution of that symbol.
 struct SymbolResolution {
   SymbolResolution()
-      : Prevailing(0), FinalDefinitionInLinkageUnit(0), VisibleToRegularObj(0) {
-  }
+      : Prevailing(0), FinalDefinitionInLinkageUnit(0), VisibleToRegularObj(0),
+        LinkerRedefined(0) {}
+
   /// The linker has chosen this definition of the symbol.
   unsigned Prevailing : 1;
 
   /// The definition of this symbol is unpreemptable at runtime and is known to
   /// be in this linkage unit.
   unsigned FinalDefinitionInLinkageUnit : 1;
 
   /// The definition of this symbol is visible outside of the LTO unit.
   unsigned VisibleToRegularObj : 1;
+
+  /// Linker redefined version of the symbol which appeared in -wrap or -defsym
+  /// linker option.
+  unsigned LinkerRedefined : 1;
 };
 
 } // namespace lto
 } // namespace llvm
 
 #endif

llvm/lib/LTO/LTO.cpp

 // Add the given symbol to the GlobalResolutions map, and resolve its partition.
 void LTO::addSymbolToGlobalRes(const InputFile::Symbol &Sym,
                                SymbolResolution Res, unsigned Partition) {
   auto &GlobalRes = GlobalResolutions[Sym.getName()];
   GlobalRes.UnnamedAddr &= Sym.isUnnamedAddr();
   if (Res.Prevailing)
     GlobalRes.IRName = Sym.getIRName();
 
   // Set the partition to external if we know it is used elsewhere, e.g.
   // it is visible to a regular object, is referenced from llvm.compiler_used,
   // or was already recorded as being referenced from a different partition.
-  if (Res.VisibleToRegularObj || Sym.isUsed() ||
+  if (Res.LinkerRedefined || Res.VisibleToRegularObj || Sym.isUsed() ||
       (GlobalRes.Partition != GlobalResolution::Unknown &&
        GlobalRes.Partition != Partition)) {
     GlobalRes.Partition = GlobalResolution::External;
   } else
     // First recorded reference, save the current partition.
     GlobalRes.Partition = Partition;
 
   // Flag as visible outside of ThinLTO if visible from a regular object or
   // if this is a reference in the regular LTO partition.
   GlobalRes.VisibleOutsideThinLTO |=
       (Res.VisibleToRegularObj || Sym.isUsed() ||
        Partition == GlobalResolution::RegularLTO);
 }
 
 static void writeToResolutionFile(raw_ostream &OS, InputFile *Input,
                                   ArrayRef<SymbolResolution> Res) {
   StringRef Path = Input->getName();
   OS << Path << '\n';
   auto ResI = Res.begin();
   for (const InputFile::Symbol &Sym : Input->symbols()) {
     assert(ResI != Res.end());
     SymbolResolution Res = *ResI++;
 
     OS << "-r=" << Path << ',' << Sym.getName() << ',';
     if (Res.Prevailing)
       OS << 'p';
     if (Res.FinalDefinitionInLinkageUnit)
       OS << 'l';
     if (Res.VisibleToRegularObj)
       OS << 'x';
+    if (Res.LinkerRedefined)
+      OS << 'r';
     OS << '\n';
   }
   OS.flush();
   assert(ResI == Res.end());
 }
 
 Error LTO::add(std::unique_ptr<InputFile> Input,
                ArrayRef<SymbolResolution> Res) {
for (const InputFile::Symbol &Sym : Syms) {
     ModuleSymbolTable::Symbol Msym = *MsymI++;
     Skip();
 
     if (GlobalValue *GV = Msym.dyn_cast<GlobalValue *>()) {
       if (Res.Prevailing) {
         if (Sym.isUndefined())
           continue;
         Keep.push_back(GV);
+        // For symbols re-defined with linker -wrap and -defsym options,
+        // set the linkage to weak to inhibit IPO. The linkage will be
+        // restored by the linker.
+        if (Res.LinkerRedefined)
+          GV->setLinkage(GlobalValue::WeakAnyLinkage);
         switch (GV->getLinkage()) {
         default:
           break;
         case GlobalValue::LinkOnceAnyLinkage:
           GV->setLinkage(GlobalValue::WeakAnyLinkage);
           break;
         case GlobalValue::LinkOnceODRLinkage:
           GV->setLinkage(GlobalValue::WeakODRLinkage);

llvm/test/LTO/Resolution/X86/linker-redef.ll

+; RUN: llvm-as %s -o %t.o
+; RUN: llvm-lto2 run -o %t1.o %t.o -r %t.o,bar,pr
+; RUN: llvm-readobj -t %t1.o.0 | FileCheck %s
+
+; CHECK: Name: bar
+; CHECK-NEXT: Value:
+; CHECK-NEXT: Size:
+; CHECK-NEXT: Binding: Weak
+; CHECK-NEXT: Type: Function
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @bar() {
+  ret void
+}

llvm/tools/llvm-lto2/llvm-lto2.cpp

for (std::string R : SymbolResolutions) {
     SymbolResolution Res;
     for (char C : Rest) {
       if (C == 'p')
         Res.Prevailing = true;
       else if (C == 'l')
         Res.FinalDefinitionInLinkageUnit = true;
       else if (C == 'x')
         Res.VisibleToRegularObj = true;
+      else if (C == 'r')
+        Res.LinkerRedefined = true;
       else {
         llvm::errs() << "invalid character " << C << " in resolution: " << R
                      << '\n';
         return 1;
       }
     }
     CommandLineResolutions[{FileName, SymbolName}].push_back(Res);
   }