[ELF] - Implemented support of default/non-default symbols versions

[grimar]

Summary

It is possible to create new version of symbol instead of depricated one
using combination of version script and asm commands. For example:

__asm__(".symver b_1,b@LIBSAMPLE_1.0");
int b_1() { return 10; }
__asm__(".symver b_2,b@@LIBSAMPLE_2.0");
int b_2() { return 20; }

This code makes b_2() to be default implementation for b().
b_1() is used for compatibility with binaries compiled against
library of older version LIBSAMPLE_1.0.

This patch implements support for above functionality in lld.

[ruiu]

This may be an expensive feature because it works for each symbol. Can you run benchmarks with this change?

[grimar]

In D21681#466350, @ruiu wrote:

This may be an expensive feature because it works for each symbol. Can you run benchmarks with this change?

Sure. But I think it should not be expensive. For symbols that does not have non-default versions almost nothing changed. (except single search of @ in name, not sure how much that is expensive).
I think there are no that many symbols that are affected by this change. Do you have suggestions for what to use for benchmark ?

[ruiu]

Maybe a clang and lld link times?

Specifically, I'm wondering whether Name.find("@") is expensive or not.

[grimar]

In D21681#466355, @ruiu wrote:

Maybe a clang and lld link times?

Specifically, I'm wondering whether Name.find("@") is expensive or not.

Yes, I just updated my comment about this. That is the only change for them.

[ruiu]

ELF/SymbolTable.cpp

167 ↗

(On Diff #61769)

Before running benchmarks, please change this to Name.find('@'). find(char) should be faster than find(StringRef).

[grimar]

ELF/SymbolTable.cpp

167 ↗

(On Diff #61769)

Ok. Btw we probably can avoid doing that search if not linking shared or have no Config->SymbolVersions.
I think we can assume that there is no versioning an that case.

[grimar]

Minor changes:

1. "@" -> '@'
2. Removed excessive empty lines in testcase.
3. Rebased.

[grimar]

So below is my benchmark results. In short - there is no any perfomance hit observed.
I tried to link clang-3.9 with and w/o this patch using next commandline. (Iook it from clang -v output).

sudo schedtool -F -p 99 -a 0x1 -e perf stat -r 50 /home/umb/LLVM/llvm-build/bin/ld.lld -z relro --hash-style=gnu --eh-frame-hdr -m elf_x86_64 -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o bin/clang-3.9 /usr/lib/gcc/x86_64-linux-gnu/5.3.1/../../../x86_64-linux-gnu/crt1.o /usr/lib/gcc/x86_64-linux-gnu/5.3.1/../../../x86_64-linux-gnu/crti.o /usr/lib/gcc/x86_64-linux-gnu/5.3.1/crtbegin.o -L/usr/lib/gcc/x86_64-linux-gnu/5.3.1 -L/usr/lib/gcc/x86_64-linux-gnu/5.3.1/../../../x86_64-linux-gnu -L/lib/x86_64-linux-gnu -L/lib/../lib64 -L/usr/lib/x86_64-linux-gnu -L/usr/lib/gcc/x86_64-linux-gnu/5.3.1/../../.. -L/home/umb/LLVM/llvm-build/bin/../lib -L/lib -L/usr/lib -allow-shlib-undefined --export-dynamic -O3 tools/clang/tools/driver/CMakeFiles/clang.dir/driver.cpp.o tools/clang/tools/driver/CMakeFiles/clang.dir/cc1_main.cpp.o tools/clang/tools/driver/CMakeFiles/clang.dir/cc1as_main.cpp.o lib/libLLVMAArch64CodeGen.a lib/libLLVMAArch64AsmPrinter.a lib/libLLVMAArch64AsmParser.a lib/libLLVMAArch64Desc.a lib/libLLVMAArch64Info.a lib/libLLVMAArch64Disassembler.a lib/libLLVMAMDGPUCodeGen.a lib/libLLVMAMDGPUAsmPrinter.a lib/libLLVMAMDGPUAsmParser.a lib/libLLVMAMDGPUDesc.a lib/libLLVMAMDGPUInfo.a lib/libLLVMAMDGPUDisassembler.a lib/libLLVMARMCodeGen.a lib/libLLVMARMAsmPrinter.a lib/libLLVMARMAsmParser.a lib/libLLVMARMDesc.a lib/libLLVMARMInfo.a lib/libLLVMARMDisassembler.a lib/libLLVMBPFCodeGen.a lib/libLLVMBPFAsmPrinter.a lib/libLLVMBPFDesc.a lib/libLLVMBPFInfo.a lib/libLLVMHexagonCodeGen.a lib/libLLVMHexagonAsmParser.a lib/libLLVMHexagonDesc.a lib/libLLVMHexagonInfo.a lib/libLLVMHexagonDisassembler.a lib/libLLVMMipsCodeGen.a lib/libLLVMMipsAsmPrinter.a lib/libLLVMMipsAsmParser.a lib/libLLVMMipsDesc.a lib/libLLVMMipsInfo.a lib/libLLVMMipsDisassembler.a lib/libLLVMMSP430CodeGen.a lib/libLLVMMSP430AsmPrinter.a lib/libLLVMMSP430Desc.a lib/libLLVMMSP430Info.a lib/libLLVMNVPTXCodeGen.a lib/libLLVMNVPTXAsmPrinter.a lib/libLLVMNVPTXDesc.a lib/libLLVMNVPTXInfo.a lib/libLLVMPowerPCCodeGen.a lib/libLLVMPowerPCAsmPrinter.a lib/libLLVMPowerPCAsmParser.a lib/libLLVMPowerPCDesc.a lib/libLLVMPowerPCInfo.a lib/libLLVMPowerPCDisassembler.a lib/libLLVMSparcCodeGen.a lib/libLLVMSparcAsmPrinter.a lib/libLLVMSparcAsmParser.a lib/libLLVMSparcDesc.a lib/libLLVMSparcInfo.a lib/libLLVMSparcDisassembler.a lib/libLLVMSystemZCodeGen.a lib/libLLVMSystemZAsmPrinter.a lib/libLLVMSystemZAsmParser.a lib/libLLVMSystemZDesc.a lib/libLLVMSystemZInfo.a lib/libLLVMSystemZDisassembler.a lib/libLLVMX86CodeGen.a lib/libLLVMX86AsmPrinter.a lib/libLLVMX86AsmParser.a lib/libLLVMX86Desc.a lib/libLLVMX86Info.a lib/libLLVMX86Disassembler.a lib/libLLVMXCoreCodeGen.a lib/libLLVMXCoreAsmPrinter.a lib/libLLVMXCoreDesc.a lib/libLLVMXCoreInfo.a lib/libLLVMXCoreDisassembler.a lib/libLLVMAnalysis.a lib/libLLVMCodeGen.a lib/libLLVMCore.a lib/libLLVMipo.a lib/libLLVMInstCombine.a lib/libLLVMInstrumentation.a lib/libLLVMMC.a lib/libLLVMMCParser.a lib/libLLVMObjCARCOpts.a lib/libLLVMOption.a lib/libLLVMScalarOpts.a lib/libLLVMSupport.a lib/libLLVMTransformUtils.a lib/libLLVMVectorize.a -lpthread lib/libclangBasic.a lib/libclangCodeGen.a lib/libclangDriver.a lib/libclangFrontend.a lib/libclangFrontendTool.a lib/libLLVMGlobalISel.a lib/libLLVMAArch64Desc.a lib/libLLVMAArch64AsmPrinter.a lib/libLLVMAArch64Info.a lib/libLLVMAArch64Utils.a lib/libLLVMAMDGPUDesc.a lib/libLLVMAMDGPUAsmPrinter.a lib/libLLVMAMDGPUInfo.a lib/libLLVMAMDGPUUtils.a lib/libLLVMARMDesc.a lib/libLLVMARMAsmPrinter.a lib/libLLVMARMInfo.a lib/libLLVMBPFAsmPrinter.a lib/libLLVMHexagonDesc.a lib/libLLVMHexagonInfo.a lib/libLLVMMipsAsmPrinter.a lib/libLLVMMipsInfo.a lib/libLLVMMSP430AsmPrinter.a lib/libLLVMNVPTXAsmPrinter.a lib/libLLVMPowerPCAsmPrinter.a lib/libLLVMPowerPCInfo.a lib/libLLVMSparcAsmPrinter.a lib/libLLVMSparcInfo.a lib/libLLVMSystemZDesc.a lib/libLLVMSystemZAsmPrinter.a lib/libLLVMSystemZInfo.a lib/libLLVMX86AsmPrinter.a lib/libLLVMX86Utils.a lib/libLLVMX86Info.a lib/libLLVMXCoreAsmPrinter.a lib/libLLVMAsmPrinter.a lib/libLLVMDebugInfoCodeView.a lib/libLLVMSelectionDAG.a lib/libLLVMCodeGen.a lib/libLLVMXCoreInfo.a lib/libLLVMMCDisassembler.a lib/libclangCodeGen.a lib/libLLVMipo.a lib/libLLVMVectorize.a lib/libLLVMInstrumentation.a lib/libLLVMObjCARCOpts.a lib/libLLVMScalarOpts.a lib/libLLVMInstCombine.a lib/libLLVMTarget.a lib/libLLVMBitWriter.a lib/libLLVMIRReader.a lib/libLLVMAsmParser.a lib/libLLVMLinker.a lib/libLLVMTransformUtils.a lib/libLLVMAnalysis.a lib/libLLVMCoverage.a lib/libLLVMObject.a lib/libclangRewriteFrontend.a lib/libclangARCMigrate.a lib/libclangStaticAnalyzerFrontend.a lib/libclangFrontend.a lib/libclangDriver.a lib/libLLVMOption.a lib/libLLVMProfileData.a lib/libclangParse.a lib/libLLVMMCParser.a lib/libclangSerialization.a lib/libLLVMBitReader.a lib/libclangSema.a lib/libclangEdit.a lib/libclangStaticAnalyzerCheckers.a lib/libclangStaticAnalyzerCore.a lib/libclangAnalysis.a lib/libclangAST.a lib/libclangRewrite.a lib/libclangLex.a lib/libclangBasic.a lib/libLLVMCore.a lib/libLLVMMC.a lib/libLLVMSupport.a -lrt -ldl -lpthread -lz -lm -rpath "\$ORIGIN/../lib" -lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc /usr/lib/gcc/x86_64-linux-gnu/5.3.1/crtend.o /usr/lib/gcc/x86_64-linux-gnu/5.3.1/../../../x86_64-linux-gnu/crtn.o

Without this patch result was:

     277.083272      task-clock (msec)         #    0.999 CPUs utilized            ( +-  0.23% )
              0      context-switches          #    0.000 K/sec                    ( +- 69.99% )
              0      cpu-migrations            #    0.000 K/sec                  
         48,403      page-faults               #    0.175 M/sec                    ( +-  0.04% )
<not supported>      cycles                   
<not supported>      stalled-cycles-frontend  
<not supported>      stalled-cycles-backend   
<not supported>      instructions             
<not supported>      branches                 
<not supported>      branch-misses            

    0.277259064 seconds time elapsed                                          ( +-  0.23% )

With this patch result was:

     276.607353      task-clock (msec)         #    0.999 CPUs utilized            ( +-  0.23% )
              0      context-switches          #    0.000 K/sec                    ( +- 69.99% )
              0      cpu-migrations            #    0.000 K/sec                  
         48,383      page-faults               #    0.175 M/sec                    ( +-  0.06% )
<not supported>      cycles                   
<not supported>      stalled-cycles-frontend  
<not supported>      stalled-cycles-backend   
<not supported>      instructions             
<not supported>      branches                 
<not supported>      branch-misses            

    0.276840515 seconds time elapsed                                          ( +-  0.23% )

[ruiu]

ELF/SymbolTable.cpp

188 ↗

(On Diff #61783)

This is O(n)*O(m) where n is the number of symbols with "@" and m is the number of symbols in version scripts. This is probably too inefficient. Please use a hash table.

211 ↗

(On Diff #61783)

I don't see a reason to do this right here in insert(). You can add a new scan* path, no?

[grimar]

ELF/SymbolTable.cpp

188 ↗

(On Diff #61783)

I think you're mistaken. Point here that I am iterating over symbol versions, not over all globals for each version.
So it is sure O(n)*O(m), but m is number of versions, not the number of symbols in version script. That should be very fast.

I think there should be little amount of both versions and symbols with @.

211 ↗

(On Diff #61783)

scanVersionScript() iterates over globals of version script file, not iterates through all symtab.
We should set Sym->VersionedName flag somewhere. If I move this code to scanVersionScript(),
I`ll need to iterate over all symtab symbols to check the names, I do not think it is acceptable.

Also please look closer on a code from method above:

size_t I = 2;
  for (elf::Version &V : Config->SymbolVersions) {
    if (V.Name == Version) {

I am iterating over SymbolVersions here and check if versioned name has the same version.
Imagine that name of symbols are "foo@version1" and "foo@@version2".
Appropriate version script can be:

version0{  }; 
version1{  } version1; 
version2{ global: foo; } version2;

If we have only script, like we have in scanVersionScript(), we can imagine that
foo can also be foo@version0. I do not think we want to bruteforce name before
doing search in symtab, so again we return to the iterating over all symtab then.

So I think doing it here is just a faster and probably fastest way.

[ruiu]

ELF/SymbolTable.cpp

166 ↗

(On Diff #61783)

You can make this function side-effect-free.

static uint16_t getVersionId(Symbol *Sym, StringRef Name)

which returns a version ID. Then use it in insert() like this.

Sym->VersionId = getVersionId(Sym, Name);

[grimar]

ELF/SymbolTable.cpp

166 ↗

(On Diff #61783)

I can, but what about VersionedName flag ?
I need to set it too, I think the only way to do that without second search of '@' would be:

Sym->VersionId = getVersionId(Sym, Name); //search is inside
Sym->VersionedName = Sym->VersionId != VER_NDX_LOCAL && 
                                            Sym->VersionId != VER_NDX_GLOBAL;

Looks ok ?

[ruiu]

ELF/SymbolTable.cpp

166 ↗

(On Diff #61783)

Well, if you can compute VersionedName from VersionId, you don't need to store it to a symbol. Instead, you can define bool isVersioned().

[grimar]

• Addressed review comments.

ELF/SymbolTable.cpp

166 ↗

(On Diff #61783)

No, generally I can't compute it.
Only in this place it is possible to set this flag, because getVersionId() can return version > than VER_NDX_GLOBAL only if
version was extracted from symbol name, so it is versioned name. Later, scanVersionScript() will assign
versions to regular symbols and it will not be possible to compute if name is versioned or not.
So I need this flag I think.

[ruiu]

ELF/SymbolTable.cpp

166 ↗

(On Diff #61939)

This function needs a function comment as it is not obvious what this is for.

189 ↗

(On Diff #61939)

I wouldn't strip '@' and following characters because it is what users want to know.

191 ↗

(On Diff #61939)

Since this is a dummy value, return something meaningless such as just 0.

[ruiu]

ELF/SymbolTable.cpp

172 ↗

(On Diff #61939)

Remove Version variable and inline Config->VersionScriptGlobalByDefault ? ... here.

[grimar]

• Addressed review comments.
• Reformatted comment.

[grimar]

ELF/SymbolTable.cpp

166 ↗

(On Diff #61939)

Done.

172 ↗

(On Diff #61939)

Done.

189 ↗

(On Diff #61939)

Done.

191 ↗

(On Diff #61939)

Done.

[ruiu]

ELF/SymbolTable.cpp

166 ↗

(On Diff #61944)

// A symbol version may be included in a symbol name as a prefix after '@'.
// This function parses that part and returns a version ID number.

ELF/Symbols.cpp

106 ↗

(On Diff #61944)

N is usually a number, so name this S.

109 ↗

(On Diff #61944)

"@" -> '@'

[grimar]

• Addressed review comments.

[ruiu]

LGTM

[ruiu]

Unfortunately this change have slowed down the linker. Without this change, the linker was able to link LLD in 6.3426 seconds. With this, it takes 6.6993 seconds. It's not negligible. I'll investigate how to speed this up.

[grimar]

In D21681#486066, @ruiu wrote:

Unfortunately this change have slowed down the linker. Without this change, the linker was able to link LLD in 6.3426 seconds. With this, it takes 6.6993 seconds. It's not negligible. I'll investigate how to speed this up.

Forgot to reply here for history. So my comments are on page of your change (D22433).

lld/trunk/ELF/SymbolTable.cpp

 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);
 }
 
+// A symbol version may be included in a symbol name as a prefix after '@'.
+// This function parses that part and returns a version ID number.
+static uint16_t getVersionId(Symbol *Sym, StringRef Name) {
+  size_t VersionBegin = Name.find('@');
+  if (VersionBegin == StringRef::npos)
+    return Config->VersionScriptGlobalByDefault ? VER_NDX_GLOBAL
+                                                : VER_NDX_LOCAL;
+
+  // If symbol name contains '@' or '@@' we can assign its version id right
+  // here. '@@' means version by default. It is usually the most recent one.
+  // VERSYM_HIDDEN flag should be set for all non-default versions.
+  StringRef Version = Name.drop_front(VersionBegin + 1);
+  bool Default = Version.startswith("@");
+  if (Default)
+    Version = Version.drop_front();
+
+  size_t I = 2;
+  for (elf::Version &V : Config->SymbolVersions) {
+    if (V.Name == Version)
+      return Default ? I : (I | VERSYM_HIDDEN);
+    ++I;
+  }
+  error("symbol " + Name + " has undefined version " + Version);
+  return 0;
+}
+
 // Find an existing symbol or create and insert a new one.
 template <class ELFT>
 std::pair<Symbol *, bool> SymbolTable<ELFT>::insert(StringRef Name) {
   unsigned NumSyms = SymVector.size();
   auto P = Symtab.insert(std::make_pair(Name, NumSyms));
   Symbol *Sym;
   if (P.second) {
     Sym = new (Alloc) Symbol;
     Sym->Binding = STB_WEAK;
     Sym->Visibility = STV_DEFAULT;
     Sym->IsUsedInRegularObj = false;
     Sym->ExportDynamic = false;
-    if (Config->VersionScriptGlobalByDefault)
-      Sym->VersionId = VER_NDX_GLOBAL;
-    else
-      Sym->VersionId = VER_NDX_LOCAL;
+    Sym->VersionId = getVersionId(Sym, Name);
+    Sym->VersionedName =
+        Sym->VersionId != VER_NDX_LOCAL && Sym->VersionId != VER_NDX_GLOBAL;
     SymVector.push_back(Sym);
   } else {
     Sym = SymVector[P.first->second];
   }
   return {Sym, P.second};
 }
 
 // Find an existing symbol or create and insert a new one, then apply the given

lld/trunk/ELF/Symbols.h

public:
   bool isLazy() const {
     return SymbolKind == LazyArchiveKind || SymbolKind == LazyObjectKind;
   }
   bool isShared() const { return SymbolKind == SharedKind; }
   bool isLocal() const { return IsLocal; }
   bool isPreemptible() const;
 
   // Returns the symbol name.
-  StringRef getName() const {
-    assert(!isLocal());
-    return StringRef(Name.S, Name.Len);
-  }
+  StringRef getName() const;
   uint32_t getNameOffset() const {
     assert(isLocal());
     return NameOffset;
   }
 
   uint8_t getVisibility() const { return StOther & 0x3; }
 
   unsigned DynsymIndex = 0;
struct Symbol {
   unsigned IsUsedInRegularObj : 1;
 
   // If this flag is true and the symbol has protected or default visibility, it
   // will appear in .dynsym. This flag is set by interposable DSO symbols in
   // executables, by most symbols in DSOs and executables built with
   // --export-dynamic, and by dynamic lists.
   unsigned ExportDynamic : 1;
 
+  // If this flag is true then symbol name also contains version name. Such
+  // name can have single or double symbol @. Double means that version is
+  // used as default. Single signals about depricated symbol version.
+  unsigned VersionedName : 1;
+
   bool includeInDynsym() const;
   bool isWeak() const { return Binding == llvm::ELF::STB_WEAK; }
 
   // This field is used to store the Symbol's SymbolBody. This instantiation of
   // AlignedCharArrayUnion gives us a struct with a char array field that is
   // large and aligned enough to store any derived class of SymbolBody. We
   // assume that the size and alignment of ELF64LE symbols is sufficient for any
   // ELFT, and we verify this with the static_asserts in replaceBody.

lld/trunk/ELF/Symbols.cpp

: SymbolKind(K), NeedsCopyOrPltAddr(false), IsLocal(true),
       IsInGlobalMipsGot(false), Type(Type), StOther(StOther),
       NameOffset(NameOffset) {}
 
 SymbolBody::SymbolBody(Kind K, StringRef Name, uint8_t StOther, uint8_t Type)
     : SymbolKind(K), NeedsCopyOrPltAddr(false), IsLocal(false),
       IsInGlobalMipsGot(false), Type(Type), StOther(StOther),
       Name({Name.data(), Name.size()}) {}
 
+StringRef SymbolBody::getName() const {
+  assert(!isLocal());
+  StringRef S = StringRef(Name.S, Name.Len);
+  if (!symbol()->VersionedName)
+    return S;
+  return S.substr(0, S.find('@'));
+}
+
 // Returns true if a symbol can be replaced at load-time by a symbol
 // with the same name defined in other ELF executable or DSO.
 bool SymbolBody::isPreemptible() const {
   if (isLocal())
     return false;
 
   // Shared symbols resolve to the definition in the DSO. The exceptions are
   // symbols with copy relocations (which resolve to .bss) or preempt plt

lld/trunk/test/ELF/Inputs/verdef-defaultver.s

+b@LIBSAMPLE_1.0 = b_1
+b@@LIBSAMPLE_2.0 = b_2
+
+.globl a
+.type  a,@function
+a:
+retq
+
+.globl b_1
+.type  b_1,@function
+b_1:
+retq
+
+.globl b_2
+.type  b_2,@function
+b_2:
+retq
+
+.globl c
+.type  c,@function
+c:
+retq

lld/trunk/test/ELF/verdef-defaultver.s

+# REQUIRES: x86
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %p/Inputs/verdef-defaultver.s -o %t1
+# RUN: echo "LIBSAMPLE_1.0{               \
+# RUN:          global: a;                \
+# RUN:          local: *; };              \
+# RUN:       LIBSAMPLE_2.0{               \
+# RUN:          global: b; c;             \
+# RUN:       }LIBSAMPLE_1.0;" > %t.script
+# RUN: ld.lld -shared -soname shared %t1 --version-script %t.script -o %t.so
+# RUN: llvm-readobj -V -dyn-symbols %t.so | FileCheck --check-prefix=DSO %s
+
+# DSO:      DynamicSymbols [
+# DSO-NEXT:    Symbol {
+# DSO-NEXT:      Name: @
+# DSO-NEXT:      Value: 0x0
+# DSO-NEXT:      Size: 0
+# DSO-NEXT:      Binding: Local
+# DSO-NEXT:      Type: None
+# DSO-NEXT:      Other: 0
+# DSO-NEXT:      Section: Undefined
+# DSO-NEXT:    }
+# DSO-NEXT:    Symbol {
+# DSO-NEXT:      Name: a@@LIBSAMPLE_1.0
+# DSO-NEXT:      Value: 0x1000
+# DSO-NEXT:      Size: 0
+# DSO-NEXT:      Binding: Global
+# DSO-NEXT:      Type: Function
+# DSO-NEXT:      Other: 0
+# DSO-NEXT:      Section: .text
+# DSO-NEXT:    }
+# DSO-NEXT:    Symbol {
+# DSO-NEXT:      Name: b@@LIBSAMPLE_2.0
+# DSO-NEXT:      Value: 0x1002
+# DSO-NEXT:      Size: 0
+# DSO-NEXT:      Binding: Global
+# DSO-NEXT:      Type: Function
+# DSO-NEXT:      Other: 0
+# DSO-NEXT:      Section: .text
+# DSO-NEXT:    }
+# DSO-NEXT:    Symbol {
+# DSO-NEXT:      Name: b@LIBSAMPLE_1.0
+# DSO-NEXT:      Value: 0x1001
+# DSO-NEXT:      Size: 0
+# DSO-NEXT:      Binding: Global
+# DSO-NEXT:      Type: Function
+# DSO-NEXT:      Other: 0
+# DSO-NEXT:      Section: .text
+# DSO-NEXT:    }
+# DSO-NEXT:    Symbol {
+# DSO-NEXT:      Name: c@@LIBSAMPLE_2.0
+# DSO-NEXT:      Value: 0x1003
+# DSO-NEXT:      Size: 0
+# DSO-NEXT:      Binding: Global
+# DSO-NEXT:      Type: Function
+# DSO-NEXT:      Other: 0
+# DSO-NEXT:      Section: .text
+# DSO-NEXT:    }
+# DSO-NEXT:  ]
+# DSO-NEXT:  Version symbols {
+# DSO-NEXT:    Section Name: .gnu.version
+# DSO-NEXT:    Address: 0x240
+# DSO-NEXT:    Offset: 0x240
+# DSO-NEXT:    Link: 1
+# DSO-NEXT:    Symbols [
+# DSO-NEXT:      Symbol {
+# DSO-NEXT:        Version: 0
+# DSO-NEXT:        Name: @
+# DSO-NEXT:      }
+# DSO-NEXT:      Symbol {
+# DSO-NEXT:        Version: 2
+# DSO-NEXT:        Name: a@@LIBSAMPLE_1.0
+# DSO-NEXT:      }
+# DSO-NEXT:      Symbol {
+# DSO-NEXT:        Version: 3
+# DSO-NEXT:        Name: b@@LIBSAMPLE_2.0
+# DSO-NEXT:      }
+# DSO-NEXT:      Symbol {
+# DSO-NEXT:        Version: 2
+# DSO-NEXT:        Name: b@LIBSAMPLE_1.0
+# DSO-NEXT:      }
+# DSO-NEXT:      Symbol {
+# DSO-NEXT:        Version: 3
+# DSO-NEXT:        Name: c@@LIBSAMPLE_2.0
+# DSO-NEXT:      }
+# DSO-NEXT:    ]
+# DSO-NEXT:  }
+# DSO-NEXT:  SHT_GNU_verdef {
+# DSO-NEXT:    Definition {
+# DSO-NEXT:      Version: 1
+# DSO-NEXT:      Flags: Base
+# DSO-NEXT:      Index: 1
+# DSO-NEXT:      Hash: 127830196
+# DSO-NEXT:      Name: shared
+# DSO-NEXT:    }
+# DSO-NEXT:    Definition {
+# DSO-NEXT:      Version: 1
+# DSO-NEXT:      Flags: 0x0
+# DSO-NEXT:      Index: 2
+# DSO-NEXT:      Hash: 98457184
+# DSO-NEXT:      Name: LIBSAMPLE_1.0
+# DSO-NEXT:    }
+# DSO-NEXT:    Definition {
+# DSO-NEXT:      Version: 1
+# DSO-NEXT:      Flags: 0x0
+# DSO-NEXT:      Index: 3
+# DSO-NEXT:      Hash: 98456416
+# DSO-NEXT:      Name: LIBSAMPLE_2.0
+# DSO-NEXT:      Predecessor: LIBSAMPLE_1.0
+# DSO-NEXT:    }
+# DSO-NEXT:  }
+
+## Check that we can link against DSO produced.
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t2
+# RUN: ld.lld %t2 %t.so -o %t3
+# RUN: llvm-readobj -V -dyn-symbols %t3 | FileCheck --check-prefix=EXE %s
+
+# EXE:      DynamicSymbols [
+# EXE-NEXT:    Symbol {
+# EXE-NEXT:      Name: @
+# EXE-NEXT:      Value: 0x0
+# EXE-NEXT:      Size: 0
+# EXE-NEXT:      Binding: Local
+# EXE-NEXT:      Type: None
+# EXE-NEXT:      Other: 0
+# EXE-NEXT:      Section: Undefined
+# EXE-NEXT:    }
+# EXE-NEXT:    Symbol {
+# EXE-NEXT:      Name: a@LIBSAMPLE_1.0
+# EXE-NEXT:      Value: 0x11020
+# EXE-NEXT:      Size: 0
+# EXE-NEXT:      Binding: Global
+# EXE-NEXT:      Type: Function
+# EXE-NEXT:      Other: 0
+# EXE-NEXT:      Section: Undefined
+# EXE-NEXT:    }
+# EXE-NEXT:    Symbol {
+# EXE-NEXT:      Name: b@LIBSAMPLE_2.0
+# EXE-NEXT:      Value: 0x11030
+# EXE-NEXT:      Size: 0
+# EXE-NEXT:      Binding: Global
+# EXE-NEXT:      Type: Function
+# EXE-NEXT:      Other: 0
+# EXE-NEXT:      Section: Undefined
+# EXE-NEXT:    }
+# EXE-NEXT:    Symbol {
+# EXE-NEXT:      Name: c@LIBSAMPLE_2.0
+# EXE-NEXT:      Value: 0x11040
+# EXE-NEXT:      Size: 0
+# EXE-NEXT:      Binding: Global
+# EXE-NEXT:      Type: Function
+# EXE-NEXT:      Other: 0
+# EXE-NEXT:      Section: Undefined
+# EXE-NEXT:    }
+# EXE-NEXT:  ]
+# EXE-NEXT:  Version symbols {
+# EXE-NEXT:    Section Name: .gnu.version
+# EXE-NEXT:    Address: 0x10228
+# EXE-NEXT:    Offset: 0x228
+# EXE-NEXT:    Link: 1
+# EXE-NEXT:    Symbols [
+# EXE-NEXT:      Symbol {
+# EXE-NEXT:        Version: 0
+# EXE-NEXT:        Name: @
+# EXE-NEXT:      }
+# EXE-NEXT:      Symbol {
+# EXE-NEXT:        Version: 2
+# EXE-NEXT:        Name: a@LIBSAMPLE_1.0
+# EXE-NEXT:      }
+# EXE-NEXT:      Symbol {
+# EXE-NEXT:        Version: 3
+# EXE-NEXT:        Name: b@LIBSAMPLE_2.0
+# EXE-NEXT:      }
+# EXE-NEXT:      Symbol {
+# EXE-NEXT:        Version: 3
+# EXE-NEXT:        Name: c@LIBSAMPLE_2.0
+# EXE-NEXT:      }
+# EXE-NEXT:    ]
+# EXE-NEXT:  }
+# EXE-NEXT:  SHT_GNU_verdef {
+# EXE-NEXT:  }
+# EXE-NEXT:  SHT_GNU_verneed {
+# EXE-NEXT:    Dependency {
+# EXE-NEXT:      Version: 1
+# EXE-NEXT:      Count: 2
+# EXE-NEXT:      FileName: shared
+# EXE-NEXT:      Entry {
+# EXE-NEXT:        Hash: 98457184
+# EXE-NEXT:        Flags: 0x0
+# EXE-NEXT:        Index: 2
+# EXE-NEXT:        Name: LIBSAMPLE_1.0
+# EXE-NEXT:      }
+# EXE-NEXT:      Entry {
+# EXE-NEXT:        Hash: 98456416
+# EXE-NEXT:        Flags: 0x0
+# EXE-NEXT:        Index: 3
+# EXE-NEXT:        Name: LIBSAMPLE_2.0
+# EXE-NEXT:      }
+# EXE-NEXT:    }
+# EXE-NEXT:  }
+
+# RUN: not ld.lld -shared %t1 -o %terr.so 2>&1 | \
+# RUN:   FileCheck -check-prefix=ERR1 %s
+# ERR1: symbol b@@LIBSAMPLE_2.0 has undefined version LIBSAMPLE_2.0
+
+.globl _start
+_start:
+  callq a
+  callq b
+  callq c