[PATCH] D96636: [lld][ELF] Support for zero flag section groups

[Petr Hosek via Phabricator via llvm-commits]

phosek created this revision.
phosek added reviewers: MaskRay, jhenderson, mcgrathr, ruiu.
Herald added subscribers: arichardson, emaste.
phosek requested review of this revision.
Herald added a project: LLVM.
Herald added a subscriber: llvm-commits.

This change introduces support for zero flag ELF section groups to lld.
lld already supports COMDAT sections, which in ELF are a special type of
ELF section groups. These are generally useful to enable linker GC where
you want a group of sections to always travel together, that is to be
either retained or discarded as a whole, but without the COMDAT
semantics. Other ELF linkers already support zero flag ELF section
groups and this change helps us reach feature parity.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D96636

Files:
  lld/ELF/InputFiles.cpp
  lld/test/ELF/gc-sections-group-debug.s


Index: lld/test/ELF/gc-sections-group-debug.s
===================================================================
--- lld/test/ELF/gc-sections-group-debug.s
+++ lld/test/ELF/gc-sections-group-debug.s
@@ -8,5 +8,5 @@
 
 # CHECK: Name: .debug_types
 
-.section .debug_types,"G", at progbits,abcd,comdat
+.section .debug_types,"G", at progbits,abcd
 .quad .debug_types
Index: lld/ELF/InputFiles.cpp
===================================================================
--- lld/ELF/InputFiles.cpp
+++ lld/ELF/InputFiles.cpp
@@ -609,24 +609,17 @@
       StringRef signature = getShtGroupSignature(objSections, sec);
       this->sections[i] = &InputSection::discarded;
 
-
       ArrayRef<Elf_Word> entries =
           CHECK(obj.template getSectionContentsAsArray<Elf_Word>(sec), this);
       if (entries.empty())
         fatal(toString(this) + ": empty SHT_GROUP");
 
-      // The first word of a SHT_GROUP section contains flags. Currently,
-      // the standard defines only "GRP_COMDAT" flag for the COMDAT group.
-      // An group with the empty flag doesn't define anything; such sections
-      // are just skipped.
-      if (entries[0] == 0)
-        continue;
-
-      if (entries[0] != GRP_COMDAT)
+      Elf_Word flag = entries[0];
+      if (flag && flag != GRP_COMDAT)
         fatal(toString(this) + ": unsupported SHT_GROUP format");
 
       bool isNew =
-          ignoreComdats ||
+          (flag & GRP_COMDAT) == 0 || ignoreComdats ||
           symtab->comdatGroups.try_emplace(CachedHashStringRef(signature), this)
               .second;
       if (isNew) {


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