[ELF/GC] Fix pending references to garbage collected sections

[davide]

Summary

The example reported in https://llvm.org/bugs/show_bug.cgi?id=30793 shows a case where gc reclaims a SHF_TLS section, but it doesn't recliam the section containing the debug info for it. This is, anyway, expected, as we do not reclaim non-alloc sections during the garbage collection phase (and this is not going to change anytime soon, at least this is what I gathered last I talked with Rafael about it).
So, we end up with a pending reference, thinking that the input was invalid (which is not true, as it's GC that removed the SH_TLS section, and therefore didn't create the PT_TLS *segment* for it). In cases like this, just assign a VA of zero at relocaiton time instead of error'ing out (this is what gold does as well, FWIW).

[Bigcheese]

ELF/InputSection.cpp

410–414

This can be simplified to:

uint64_t SymVA = 0;
if (!Sym.isTls() || Out<ELFT>::TlsPhdr)
  SymVA = SignExtend64<sizeof(uintX_t) * 8>(getSymVA<ELFT>(Type, Addend, AddrLoc, Sym, R_ABS));

test/ELF/gc-debuginfo-tls.s

20–29

This is a pretty obfuscated way to get a TLS symbol to a gced section. I think this can be simplified to something like:

  .section .tbss,"awT",@nobits
patatino:
  .long 0
  .section .noalloc,""
  .quad patatino

[davide]

test/ELF/gc-debuginfo-tls.s

20–29

This is reduced from the original testcase. But sure, I can add another test case if you want.

[Bigcheese]

test/ELF/gc-debuginfo-tls.s

20–29

Sure, just add a comment to the .cfi* one saying that it was reduced from a testcase and that the debug info directives are generating the reference.

[davide]

Addressed Spence's comments.

[Bigcheese]

lgtm

[• rafael]

LGTM

[phosek]

LGTM

ELF/InputSection.cpp

     }
 
     uintX_t AddrLoc = this->OutSec->getVA() + Offset;
-    uint64_t SymVA = SignExtend64<sizeof(uintX_t) * 8>(
-        getSymVA<ELFT>(Type, Addend, AddrLoc, Sym, R_ABS));
+    uint64_t SymVA = 0;
+    if (!Sym.isTls() || Out<ELFT>::TlsPhdr)
+      SymVA = SignExtend64<sizeof(uintX_t) * 8>(
+          getSymVA<ELFT>(Type, Addend, AddrLoc, Sym, R_ABS));
     Target->relocateOne(BufLoc, Type, SymVA);
   }
 }

test/ELF/gc-debuginfo-tls.s

+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+# RUN: ld.lld %t.o --gc-sections -shared -o %t1
+# RUN: ld.lld %t.o -shared -o %t2
+# RUN: llvm-readobj -symbols %t1 | FileCheck %s --check-prefix=GC
+# RUN: llvm-readobj -symbols %t2 | FileCheck %s --check-prefix=NOGC
+
+# NOGC: Symbol {
+# NOGC:   Name: .tbss
+# NOGC:   Value: 0x1000
+# NOGC:   Size: 0
+# NOGC:   Binding: Local
+# NOGC:   Type: TLS
+# NOGC:   Other: 0
+# NOGC:   Section: .tbss
+# NOGC: }
+
+# GC-NOT: tbss
+
+.section .tbss,"awT",@nobits
+patatino:
+  .long 0
+  .section .noalloc,""
+  .quad patatino