Don't set dso_local flag in LTO resolutions for absolute symbols defined in ELF objects

[dmikulin]

Summary

Setting dso_local flag for absolute symbols defined in real ELF objects forces codegen to make incorrect assumptions about code sequences it can use to access those symbols. It leads to linker errors:
ld.lld: error: relocation R_X86_64_PC32 cannot refer to absolute symbol: blah

defined in a.o
referenced by ld-temp.o

lto.tmp:(_start)

[dmikulin]

Addressed review comments

[dmikulin]

Minor tweaks.

lld/trunk/ELF/LTO.cpp

for (const lto::InputFile::Symbol &ObjSym : Obj.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());
+    const auto *DR = dyn_cast<Defined>(Sym);
     R.FinalDefinitionInLinkageUnit =
-        Sym->isDefined() && (IsExecutable || Sym->Visibility != STV_DEFAULT);
+        (IsExecutable || Sym->Visibility != STV_DEFAULT) && DR &&
+        // Skip absolute symbols from ELF objects, otherwise PC-rel relocations
+        // will be generated by for them, triggering linker errors.
+        // Symbol section is always null for bitcode symbols, hence the check
+        // for isElf().
+        !(Sym->File && Sym->File->isElf() && DR->Section == nullptr);
 
     if (R.Prevailing)
       undefine(Sym);
 
     // We tell LTO to not apply interprocedural optimization for wrapped
     // (with --wrap) symbols because otherwise LTO would inline them while
     // their values are still not final.
     R.LinkerRedefined = !Sym->CanInline;

lld/trunk/test/ELF/lto/Inputs/absolute.s

+.globl blah
+ blah = 0xdeadbeef

lld/trunk/test/ELF/lto/abs-resol.ll

+; REQUIRES: x86
+
+; RUN: llvm-as %s -o %t.o
+; RUN: llvm-mc -triple=x86_64-pc-linux %p/Inputs/absolute.s -o %t2.o -filetype=obj
+; RUN: ld.lld %t.o %t2.o -o %t.out -pie
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@blah = external global i8, align 1
+
+define i8* @_start() {
+ ret i8* @blah
+}