[ELF] - Fixed linkage error when using -g --gc-sections together.

[grimar]

Summary

r282444 introduced new issue, sample program below
fails to link on

assert(Piece.Live);

int main() { return 0; }

clang test.cpp -c -o out.o -g
ld.lld -flavor gnu --gc-sections out.o -o out

Problem is that .debug_info contains relocations to .debug_str:
Section (7) .rela.debug_info {
..

0xC R_X86_64_32 .debug_str 0x0
0x12 R_X86_64_32 .debug_str 0x37

..
But we do not preserve .debug_str in a right way now.

To fix this we should ignore relocations from non-allocatable sections to allocatable
to allow GC work at full power, but still should proccess relocations from non-allocatable to non-allocatable sections
as usual to mark some parts of debug sections alive to keep them so we do not end
up with such assert when trying to access dead pieces. That looks like what gold/ld do, they do
not strip .debug_str section from what I saw using sample provided.

Thanks to Evgeny Leviant for suggestions about how to fix this.

[grimar]

Looks this should help to https://llvm.org/bugs/show_bug.cgi?id=30539

[• rafael]

LGTM with a test nit.

test/ELF/debug-gc.s

18

Add a third, unused string and check if it is kept or not.

[grimar]

test/ELF/debug-gc.s

18

Hmm, interesting that we GC it (that I think is what we want), but ld/gold do not..

ELF/MarkLive.cpp

 }
 
 template <class ELFT>
 static typename ELFT::uint getAddend(InputSectionBase<ELFT> &Sec,
                                      const typename ELFT::Rela &Rel) {
   return Rel.r_addend;
 }
 
+template <class ELFT> static bool IsAlloc(InputSectionBase<ELFT> &Sec) {
+  return (&Sec != &InputSection<ELFT>::Discarded) &&
+         (Sec.getSectionHdr()->sh_flags & SHF_ALLOC);
+}
+
 template <class ELFT, class RelT>
 static ResolvedReloc<ELFT> resolveReloc(InputSectionBase<ELFT> &Sec,
                                         RelT &Rel) {
   SymbolBody &B = Sec.getFile()->getRelocTargetSym(Rel);
   auto *D = dyn_cast<DefinedRegular<ELFT>>(&B);
   if (!D || !D->Section)
     return {nullptr, 0};
+  if (!IsAlloc<ELFT>(Sec) && IsAlloc<ELFT>(*D->Section))
+    return {nullptr, 0};
   typename ELFT::uint Offset = D->Value;
   if (D->isSection())
     Offset += getAddend(Sec, Rel);
   return {D->Section->Repl, Offset};
 }
 
 // Calls Fn for each section that Sec refers to via relocations.
 template <class ELFT>
auto Enqueue = [&](ResolvedReloc<ELFT> R) {
     // So we explicitly tell it which offset is in use.
     if (auto *MS = dyn_cast<MergeInputSection<ELFT>>(R.Sec))
       MS->markLiveAt(R.Offset);
 
     if (R.Sec->Live)
       return;
     R.Sec->Live = true;
     if (InputSection<ELFT> *S = dyn_cast<InputSection<ELFT>>(R.Sec))
-      if (S->getSectionHdr()->sh_flags & SHF_ALLOC)
-        Q.push_back(S);
   };
 
   auto MarkSymbol = [&](const SymbolBody *Sym) {
     if (auto *D = dyn_cast_or_null<DefinedRegular<ELFT>>(Sym))
       Enqueue({D->Section, D->Value});
   };
 
   // Add GC root symbols.

test/ELF/debug-gc.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+# RUN: ld.lld %t.o -o %t1 --gc-sections
+# RUN: llvm-objdump -s %t1 | FileCheck %s
+
+# CHECK:      Contents of section .debug_str:
+# CHECK-NEXT:  0000 41414100 42424200   AAA.BBB.
+# CHECK:      Contents of section .debug_info:
+# CHECK-NEXT:  0000 00000000 04000000
+
+.globl _start
+_start:
+
+.section .debug_str,"MS",@progbits,1
+.Linfo_string0:
+  .asciz "AAA"
+.Linfo_string1:
+  .asciz "BBB"
+
+.section  .debug_info,"",@progbits
+  .long .Linfo_string0
+  .long .Linfo_string1