[lld] r285735 - [ELF/GC] Fix pending references to garbage collected sections.

[Davide Italiano via llvm-commits]

Author: davide
Date: Tue Nov  1 15:11:01 2016
New Revision: 285735

URL: http://llvm.org/viewvc/llvm-project?rev=285735&view=rev
Log:
[ELF/GC] Fix pending references to garbage collected sections.

The example reported in PR30793 shows a case where gc reclaims
a SHF_TLS section, but it doesn't reclaim the section containing
the debug info for it.
This is 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
SHT_TLS section, and therefore didn't create the PT_TLS *segment*
for it). In cases like this, just assign a VA of zero at relocation
time instead of error'ing out (this is what gold does as well, FWIW).

Differential Revision:  https://reviews.llvm.org/D26201

Added:
    lld/trunk/test/ELF/gc-debuginfo-tls.s
Modified:
    lld/trunk/ELF/InputSection.cpp

Modified: lld/trunk/ELF/InputSection.cpp
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/InputSection.cpp?rev=285735&r1=285734&r2=285735&view=diff
==============================================================================
--- lld/trunk/ELF/InputSection.cpp (original)
+++ lld/trunk/ELF/InputSection.cpp Tue Nov  1 15:11:01 2016
@@ -411,8 +411,10 @@ void InputSection<ELFT>::relocateNonAllo
     }
 
     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);
   }
 }

Added: lld/trunk/test/ELF/gc-debuginfo-tls.s
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/gc-debuginfo-tls.s?rev=285735&view=auto
==============================================================================
--- lld/trunk/test/ELF/gc-debuginfo-tls.s (added)
+++ lld/trunk/test/ELF/gc-debuginfo-tls.s Tue Nov  1 15:11:01 2016
@@ -0,0 +1,23 @@
+# 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", at nobits
+patatino:
+  .long 0
+  .section .noalloc,""
+  .quad patatino