[COFF] Accept discarded relocations in DWARF debug sections

[smeenai]

Summary

DWARF debug sections can also contain relocations against symbols in
discared segments. LLD should accept such relocations.

[smeenai]

Add comment for isDWARF

[ruiu]

LGTM

COFF/Chunks.h

192–194

Please run clang-format-diff. I think it will rewrite it in one line.

[smeenai]

Correct formatting

[smeenai]

arc snafu

[smeenai]

COFF/Chunks.h

192–194

Whoops, fixed now. Sorry about that.

COFF/Chunks.h

public:
   void markDiscarded() { Discarded = true; }
 
   // True if this is a codeview debug info chunk. These will not be laid out in
   // the image. Instead they will end up in the PDB, if one is requested.
   bool isCodeView() const {
     return SectionName == ".debug" || SectionName.startswith(".debug$");
   }
 
+  bool isDWARF() const {
+    return SectionName.startswith(".debug_");
+  }
+
   // Allow iteration over the bodies of this chunk's relocated symbols.
   llvm::iterator_range<symbol_iterator> symbols() const {
     return llvm::make_range(symbol_iterator(File, Relocs.begin()),
                             symbol_iterator(File, Relocs.end()));
   }
 
   // Allow iteration over the associated child chunks for this section.
   ArrayRef<SectionChunk *> children() const { return AssocChildren; }

COFF/Chunks.cpp

for (const coff_relocation &Rel : Relocs) {
     OutputSection *OS = C ? C->getOutputSection() : nullptr;
 
     // Only absolute and __ImageBase symbols lack an output section. For any
     // other symbol, this indicates that the chunk was discarded.  Normally
     // relocations against discarded sections are an error.  However, debug info
     // sections are not GC roots and can end up with these kinds of relocations.
     // Skip these relocations.
     if (!OS && !isa<DefinedAbsolute>(Sym) && !isa<DefinedSynthetic>(Sym)) {
-      if (isCodeView())
+      if (isCodeView() || isDWARF())
         continue;
       fatal("relocation against symbol in discarded section: " +
             Sym->getName());
     }
     uint64_t S = Sym->getRVA();
 
     // Compute the RVA of the relocation for relative relocations.
     uint64_t P = RVA + Rel.VirtualAddress;

test/COFF/reloc-discarded-dwarf.s

+# RUN: llvm-mc -triple=x86_64-windows-msvc -filetype=obj -o %t1.obj %s
+# RUN: llvm-mc -triple=x86_64-windows-msvc -filetype=obj -o %t2.obj %s
+
+# LLD should not error on relocations in DWARF debug sections against symbols in
+# discarded sections.
+# RUN: lld-link -entry:main -debug %t1.obj %t2.obj
+
+  .section  .text,"xr",discard,main
+  .globl  main
+main:
+f:
+  retq
+
+  .section  .debug_info,"dr"
+  .quad f