[LLD] [COFF] Don't error out on relocations to discarded sections in .eh_frame

[mstorsjo]

Summary

This allows linking code with dwarf exception handling.

[mstorsjo]

Updated a comment together in sync with the updated code.

[rnk]

lgtm, this is consistent with this comment in the ELF side:

// .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.

We might want to start using associative comdats to avoid making object files like this on win64, though.

[rnk]

Mark as accepted

[ruiu]

COFF/Chunks.h

192 ↗

(On Diff #118190)

I wonder if you actually need startswith. Why don't you use ==?

[mstorsjo]

COFF/Chunks.h

192 ↗

(On Diff #118190)

I remembered seeing section names like .eh_frame$funcName somewhere, but now I can't really find that (perhaps it even was in some tests on ELF), and linking succeeds with plain ==. So I can change it to ==, and I can see later if I ever need startswith.

[mstorsjo]

Changed the section name comparison to ==.

[ruiu]

LGTM

lld/trunk/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$");
   }
 
-  // True if this is a DWARF debug info chunk.
-  bool isDWARF() const { return SectionName.startswith(".debug_"); }
+  // True if this is a DWARF debug info or exception handling chunk.
+  bool isDWARF() const {
+    return SectionName.startswith(".debug_") || SectionName == ".eh_frame";
+  }
 
   // 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.

lld/trunk/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
+  .section  .eh_frame,"dr"
+  .quad f