[ELF][PPC32] Don't report "relocation refers to a discarded section" for .got2

[MaskRay]

Summary

Similar to D63182 [ELF][PPC64] Don't report "relocation refers to a discarded section" for .toc

[Bdragon28]

This fixes my problem encountered trying to build kyua and atf for lld10-native FreeBSD ppc32.

lld/ELF/InputSection.cpp

if (sym.type == STT_SECTION) {
       // section data. We do that by adding to the Relocation vector.
 
       // .eh_frame is horribly special and can reference discarded sections. To
       // avoid having to parse and recreate .eh_frame, we just replace any
       // relocation in it pointing to discarded sections with R_*_NONE, which
       // hopefully creates a frame that is ignored at runtime. Also, don't warn
       // on .gcc_except_table and debug sections.
       //
-      // See the comment in maybeReportUndefined for PPC64 .toc .
+      // See the comment in maybeReportUndefined for PPC32 .got2 and PPC64 .toc
       auto *d = dyn_cast<Defined>(&sym);
       if (!d) {
         if (!isDebugSection(*sec) && sec->name != ".eh_frame" &&
-            sec->name != ".gcc_except_table" && sec->name != ".toc") {
+            sec->name != ".gcc_except_table" && sec->name != ".got2" &&
+            sec->name != ".toc") {
           uint32_t secIdx = cast<Undefined>(sym).discardedSecIdx;
           Elf_Shdr_Impl<ELFT> sec =
               CHECK(file->getObj().sections(), file)[secIdx];
           warn("relocation refers to a discarded section: " +
                CHECK(file->getObj().getSectionName(&sec), file) +
                "\n>>> referenced by " + getObjMsg(p->r_offset));
         }
         p->setSymbolAndType(0, 0, false);

lld/ELF/Relocations.cpp

static bool maybeReportUndefined(Symbol &sym, InputSectionBase &sec,
   if (config->unresolvedSymbols == UnresolvedPolicy::Ignore && canBeExternal)
     return false;
 
   // clang (as of 2019-06-12) / gcc (as of 8.2.1) PPC64 may emit a .rela.toc
   // which references a switch table in a discarded .rodata/.text section. The
   // .toc and the .rela.toc are incorrectly not placed in the comdat. The ELF
   // spec says references from outside the group to a STB_LOCAL symbol are not
   // allowed. Work around the bug.
-  if (config->emachine == EM_PPC64 &&
-      cast<Undefined>(sym).discardedSecIdx != 0 && sec.name == ".toc")
+  //
+  // PPC32 .got2 is similar but cannot be fixed. Multiple .got2 is infeasible
+  // because .LC0-.LTOC is not representable if the two labels are in different
+  // .got2
+  if (cast<Undefined>(sym).discardedSecIdx != 0 &&
+      (sec.name == ".got2" || sec.name == ".toc"))
     return false;
 
   bool isWarning =
       (config->unresolvedSymbols == UnresolvedPolicy::Warn && canBeExternal) ||
       config->noinhibitExec;
   undefs.push_back({&sym, {{&sec, offset}}, isWarning});
   return !isWarning;
 }

lld/test/ELF/comdat-discarded-ppc32.s

+# REQUIRES: ppc
+# RUN: llvm-mc -filetype=obj -triple=powerpc %s -o %t.o
+# RUN: ld.lld %t.o %t.o -o /dev/null
+# RUN: ld.lld -r --fatal-warnings %t.o %t.o -o /dev/null
+
+## Similar to PPC64, clang/gcc PPC32 may emit a .rela.got2 which references a local symbol
+## defined in a discarded .rodata section. Unfortunately, .got2 cannot be placed in a comdat
+## because for lwz 3, .LC0-.LTOC(30), we cannot define .LC0 in a different .got2 section.
+
+## Don't error "relocation refers to a discarded section".
+
+.section .text.foo,"axG",@progbits,foo,comdat
+.globl foo
+foo:
+ lwz 3, .LC0-.LTOC(30)
+.L0:
+
+.section .got2,"aw",@progbits
+.set .LTOC, .got2+32768
+.LC0:
+.long .L0