Mark ELF sections whose name start with .note as note

[phosek]

Summary

Previously, such sections would be marked as SHT_PROGBITS which makes it impossible to use an initialized C variable declaration to emit an (allocated) ELF note. The new behavior is also consistent with the ELF assembly parser.

There is a related GCC bug 77609.

[ruiu]

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

154 ↗

(On Diff #71716)

Shouldn't this be Name == ".note" || Name.startswith(".note.")?

[phosek]

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

154 ↗

(On Diff #71716)

That's definitely more strict so it's probably a better solution. The logic in lib/MC/MCParser/ELFAsmParser.cpp uses Name.startswith(".note"), shall I update that as well?

[ruiu]

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

154 ↗

(On Diff #71716)

I don't know the answer, but I *think* that's the right thing to do.

[phosek]

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

154 ↗

(On Diff #71716)

I checked the behavior of GNU assembler and it's equivalent to the current implementation, i.e. even .notefoo would end up with SHT_NOTE type.

[davide]

Side note, I almost always dislike relying on section name specific logic (in the assembler and in the linker), but this is the way things are. LGTM, modulo one comment.

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

154 ↗

(On Diff #71716)

Please add a comment here to explain what's going on (maybe add the link to the gcc bug, if you feel it's OK).

[davide]

lgtm

llvm/trunk/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

if (Name == ".tbss" ||
       Name.startswith(".llvm.linkonce.tb."))
     return SectionKind::getThreadBSS();
 
   return K;
 }
 
 
 static unsigned getELFSectionType(StringRef Name, SectionKind K) {
+  // Use SHT_NOTE for section whose name starts with ".note" to allow
+  // emitting ELF notes from C variable declaration.
+  // See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77609
+  if (Name.startswith(".note"))
+    return ELF::SHT_NOTE;
 
   if (Name == ".init_array")
     return ELF::SHT_INIT_ARRAY;
 
   if (Name == ".fini_array")
     return ELF::SHT_FINI_ARRAY;
 
   if (Name == ".preinit_array")

llvm/trunk/test/CodeGen/X86/note-sections.ll

+; RUN: llc -mtriple x86_64-pc-linux < %s | FileCheck %s
+
+%struct.note = type { %struct.Elf32_Nhdr, [7 x i8], %struct.payload }
+%struct.Elf32_Nhdr = type { i32, i32, i32 }
+%struct.payload = type { i16 }
+
+@foonote = internal constant %struct.note { %struct.Elf32_Nhdr { i32 7, i32 2, i32 17 }, [7 x i8] c"foobar\00", %struct.payload { i16 23 } }, section ".note.foo", align 4
+
+; CHECK:    .section  .note.foo,"a",@note
+; CHECK-NEXT: .p2align  2
+; CHECK-NEXT: foonote:
+; CHECK-NEXT:   .long 7
+; CHECK-NEXT:   .long 2
+; CHECK-NEXT:   .long 17
+; CHECK-NEXT:   .asciz  "foobar"
+; CHECK-NEXT:   .zero 1
+; CHECK-NEXT:   .short  23
+; CHECK-NEXT:   .zero 2
+; CHECK-NEXT:   .size foonote, 24