[llvm-mc] - Allow to set custom flags for debug sections.

[grimar]

Summary

I am experimenting with a single split dwarf (.dwo sections in .o files).
I want to make linker to ignore .dwo sections in .o, for that I am trying to add
SHF_EXCLUDE flag ("E") for them in my asm sample.

I found that currently, it is impossible to add any flag for debug sections using llvm-mc.

That happens because we have a set of predefined unique sections created early with default flags:
https://github.com/llvm-mirror/llvm/blob/master/lib/MC/MCObjectFileInfo.cpp#L391

This patch allows a user to add any flags he wants.

I had to edit TargetLoweringObjectFileImpl.cpp to set MetaData type for debug sections.
Their kind was Data by default (so they were allocatable) and so after changes introduced by
this patch the SHF_ALLOC flag was applied for them, what does not make sense for debug sections.
One of OrcJITTests tests failed because of that.

[JDevlieghere]

This looks reasonable, but then again I don't know much about ELF, so I'll leave it to someone with more expertise (looking at @dblaikie) to accept this.

[dblaikie]

Sure - seems plausible

[grimar]

It was reverted in rL340906 because of the following test case failure:
CodeGen/Mips/gpopt-explict-section.ll (http://lab.llvm.org:8011/builders/clang-ppc64be-linux/builds/23252)
Will take a look a bit later.

[grimar]

• Fix BB failure.

[grimar]

Abandoning, I believe D52303 is the better way to achieve what I want.

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

if (Name == ".tdata" ||
     return SectionKind::getThreadData();
 
   if (Name == ".tbss" ||
       Name.startswith(".tbss.") ||
       Name.startswith(".gnu.linkonce.tb.") ||
       Name.startswith(".llvm.linkonce.tb."))
     return SectionKind::getThreadBSS();
 
+  if (Name.startswith(".debug_"))
+    return SectionKind::getMetadata();
+
   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"))

lib/MC/MCContext.cpp

MCSectionELF *MCContext::getELFSection(const Twine &Section, unsigned Type,
                                        const MCSymbolELF *Associated) {
   StringRef Group = "";
   if (GroupSym)
     Group = GroupSym->getName();
   // Do the lookup, if we have a hit, return it.
   auto IterBool = ELFUniquingMap.insert(
       std::make_pair(ELFSectionKey{Section.str(), Group, UniqueID}, nullptr));
   auto &Entry = *IterBool.first;
+  MCSectionELF *&Sec = Entry.second;
+  if (!IterBool.second) {
+    // We want to let users add additional flags even for sections with
+    // defaults. For example, .debug_str has "MS" flags by default and user
+    // might want to add "E".
+    Sec->setFlags(Sec->getFlags() | Flags);
+    return Sec;
+  }
+
   if (!IterBool.second)
     return Entry.second;
 
   StringRef CachedName = Entry.first.SectionName;
 
   SectionKind Kind;
   if (Flags & ELF::SHF_ARM_PURECODE)
     Kind = SectionKind::getExecuteOnly();
   else if (Flags & ELF::SHF_EXECINSTR)
     Kind = SectionKind::getText();
   else
     Kind = SectionKind::getReadOnly();
 
-  MCSectionELF *Result = createELFSectionImpl(
-      CachedName, Type, Flags, Kind, EntrySize, GroupSym, UniqueID, Associated);
-  Entry.second = Result;
-  return Result;
+  Sec = createELFSectionImpl(CachedName, Type, Flags, Kind, EntrySize, GroupSym,
+                             UniqueID, Associated);
+  return Sec;
 }
 
 MCSectionELF *MCContext::createELFGroupSection(const MCSymbolELF *Group) {
   return createELFSectionImpl(".group", ELF::SHT_GROUP, 0,
                               SectionKind::getReadOnly(), 4, Group, ~0,
                               nullptr);
 }
 

test/MC/ELF/section-flags.s

+# RUN: llvm-mc -triple x86_64-pc-linux-gnu %s -filetype=obj -o %t.o
+# RUN: llvm-readobj -s --elf-output-style=GNU %t.o | FileCheck %s
+
+## Check we are able to set the custom flag ('E') for debug sections.
+# CHECK: .debug_info   {{.*}}  E
+# CHECK: .debug_str    {{.*}}  EMS
+
+.section .debug_info,"e"
+nop
+
+.section .debug_str,"eMS",@progbits,1
+nop