[ELF] - Make IR symbols be visible when doing relocatable link.

[grimar]

Summary

This is PR33097.
Previously when doing relocatable link, all IR symbols were absent
in result object file. Patch makes external symbols to be exported.

[davide]

This is fine, but maybe too conservative.
You just need to keep externals, not everything in a relocatable (unless being in a relocatable implies being external, in which case, that needs a comment :)
I proposed an alternative https://reviews.llvm.org/D36959 (feel free to take a look and review/reply)

[davide]

Apologies, this should be fine.
Peter made me realize we should never see STB_LOCAL here.

[pcc]

LGTM with a comment explaining why we must preserve these symbols.

lld/trunk/ELF/LTO.cpp

for (const lto::InputFile::Symbol &ObjSym : Obj.symbols()) {
 
     // Ideally we shouldn't check for SF_Undefined but currently IRObjectFile
     // reports two symbols for module ASM defined. Without this check, lld
     // flags an undefined in IR with a definition in ASM as prevailing.
     // Once IRObjectFile is fixed to report only one symbol this hack can
     // be removed.
     R.Prevailing = !ObjSym.isUndefined() && B->getFile() == &F;
 
-    R.VisibleToRegularObj = Sym->IsUsedInRegularObj ||
+    // We ask LTO to preserve following global symbols:
+    // 1) All symbols when doing relocatable link, so that them can be used
+    //    for doing final link.
+    // 2) Symbols that are used in regular objects.
+    // 3) C named sections if we have corresponding __start_/__stop_ symbol.
+    // 4) Symbols that are defined in bitcode files and used for dynamic linking.
+    R.VisibleToRegularObj = Config->Relocatable || Sym->IsUsedInRegularObj ||
                             (R.Prevailing && Sym->includeInDynsym()) ||
                             UsedStartStop.count(ObjSym.getSectionName());
     if (R.Prevailing)
       undefine(Sym);
     R.LinkerRedefined = Config->RenamedSymbols.count(Sym);
   }
   checkError(LTOObj->add(std::move(F.Obj), Resols));
 }

lld/trunk/test/ELF/lto/relocatable.ll

+; REQUIRES: x86
+; RUN: llvm-as %s -o %t1.o
+; RUN: ld.lld %t1.o -r -o %t
+; RUN: llvm-readobj -symbols %t | FileCheck %s
+
+; CHECK:       Symbols [
+; CHECK-NEXT:   Symbol {
+; CHECK-NEXT:     Name:
+; CHECK-NEXT:     Value: 0x0
+; CHECK-NEXT:     Size: 0
+; CHECK-NEXT:     Binding: Local
+; CHECK-NEXT:     Type: None
+; CHECK-NEXT:     Other: 0
+; CHECK-NEXT:     Section: Undefined
+; CHECK-NEXT:   }
+; CHECK-NEXT:   Symbol {
+; CHECK-NEXT:     Name:
+; CHECK-NEXT:     Value: 0x0
+; CHECK-NEXT:     Size: 0
+; CHECK-NEXT:     Binding: Local
+; CHECK-NEXT:     Type: Section
+; CHECK-NEXT:     Other: 0
+; CHECK-NEXT:     Section: .text
+; CHECK-NEXT:   }
+; CHECK-NEXT:   Symbol {
+; CHECK-NEXT:     Name:
+; CHECK-NEXT:     Value: 0x0
+; CHECK-NEXT:     Size: 0
+; CHECK-NEXT:     Binding: Local
+; CHECK-NEXT:     Type: Section
+; CHECK-NEXT:     Other: 0
+; CHECK-NEXT:     Section: .text.foo
+; CHECK-NEXT:   }
+; CHECK-NEXT:   Symbol {
+; CHECK-NEXT:     Name: foo
+; CHECK-NEXT:     Value: 0x0
+; CHECK-NEXT:     Size: 1
+; CHECK-NEXT:     Binding: Global
+; CHECK-NEXT:     Type: Function
+; CHECK-NEXT:     Other: 0
+; CHECK-NEXT:     Section: .text.foo
+; CHECK-NEXT:   }
+; CHECK-NEXT: ]
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @foo() {
+  call void @bar()
+  ret void
+}
+
+define internal void @bar() {
+  ret void
+}