Add !associated metadata.

This is an ELF-specific thing that adds SHF_LINK_ORDER to the global's section
pointing to the metadata argument's section. The effect of that is a reverse dependency
between sections for the linker GC.

!associated does not change the behavior of global-dce. The global
may also need to be added to llvm.compiler.used.

Since SHF_LINK_ORDER is per-section, !associated effectively enables
fdata-sections for the affected globals, the same as comdats do.

Differential Revision: https://reviews.llvm.org/D29104

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@298157 91177308-0d34-0410-b5e6-96231b3b80d8

docs/LangRef.rst

@@ -5106,6 +5106,31 @@ Examples:
 
 See :doc:`TypeMetadata`.
 
+'``associated``' Metadata
+^^^^^^^^^^^^^^^^^^^
+
+The ``associated`` metadata may be attached to a global object
+declaration with a single argument that references another global object.
+
+This metadata prevents discarding of the global object in linker GC
+unless the referenced object is also discarded. The linker support for
+this feature is spotty. For best compatibility, globals carrying this
+metadata may also:
+
+- Be in a comdat with the referenced global.
+- Be in @llvm.compiler.used.
+- Have an explicit section with a name which is a valid C identifier.
+
+It does not have any effect on non-ELF targets.
+
+Example:
+
+.. code-block:: llvm
+    $a = comdat any
+    @a = global i32 1, comdat $a
+    @b = internal global i32 2, comdat $a, section "abc", !associated !0
+    !0 = !{i32* @a}
+
 
 Module Flags Metadata
 =====================

include/llvm/IR/LLVMContext.h

@@ -78,6 +78,7 @@ class LLVMContext {
     MD_type = 19,                     // "type"
     MD_section_prefix = 20,           // "section_prefix"
     MD_absolute_symbol = 21,          // "absolute_symbol"
+    MD_associated = 22,               // "associated"
   };
 
   /// Known operand bundle tag IDs, which always have the same value.  All

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

@@ -225,6 +225,20 @@ static const Comdat *getELFComdat(const GlobalValue *GV) {
   return C;
 }
 
+static const MCSymbolELF *getAssociatedSymbol(const GlobalObject *GO,
+                                              const TargetMachine &TM) {
+  MDNode *MD = GO->getMetadata(LLVMContext::MD_associated);
+  if (!MD)
+    return nullptr;
+
+  auto *VM = dyn_cast<ValueAsMetadata>(MD->getOperand(0));
+  if (!VM)
+    report_fatal_error("MD_associated operand is not ValueAsMetadata");
+
+  GlobalObject *OtherGO = dyn_cast<GlobalObject>(VM->getValue());
+  return OtherGO ? dyn_cast<MCSymbolELF>(TM.getSymbol(OtherGO)) : nullptr;
+}
+
 MCSection *TargetLoweringObjectFileELF::getExplicitSectionGlobal(
     const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const {
   StringRef SectionName = GO->getSection();
@@ -238,9 +252,23 @@ MCSection *TargetLoweringObjectFileELF::getExplicitSectionGlobal(
     Group = C->getName();
     Flags |= ELF::SHF_GROUP;
   }
-  return getContext().getELFSection(SectionName,
-                                    getELFSectionType(SectionName, Kind), Flags,
-                                    /*EntrySize=*/0, Group);
+
+  // A section can have at most one associated section. Put each global with
+  // MD_associated in a unique section.
+  unsigned UniqueID = MCContext::GenericSectionID;
+  const MCSymbolELF *AssociatedSymbol = getAssociatedSymbol(GO, TM);
+  if (AssociatedSymbol) {
+    UniqueID = NextUniqueID++;
+    Flags |= ELF::SHF_LINK_ORDER;
+  }
+
+  MCSectionELF *Section = getContext().getELFSection(
+      SectionName, getELFSectionType(SectionName, Kind), Flags,
+      /*EntrySize=*/0, Group, UniqueID, AssociatedSymbol);
+  // Make sure that we did not get some other section with incompatible sh_link.
+  // This should not be possible due to UniqueID code above.
+  assert(Section->getAssociatedSymbol() == AssociatedSymbol);
+  return Section;
 }
 
 /// Return the section prefix name used by options FunctionsSections and
@@ -262,11 +290,10 @@ static StringRef getSectionPrefixForGlobal(SectionKind Kind) {
   return ".data.rel.ro";
 }
 
-static MCSectionELF *
-selectELFSectionForGlobal(MCContext &Ctx, const GlobalObject *GO,
-                          SectionKind Kind, Mangler &Mang,
-                          const TargetMachine &TM, bool EmitUniqueSection,
-                          unsigned Flags, unsigned *NextUniqueID) {
+static MCSectionELF *selectELFSectionForGlobal(
+    MCContext &Ctx, const GlobalObject *GO, SectionKind Kind, Mangler &Mang,
+    const TargetMachine &TM, bool EmitUniqueSection, unsigned Flags,
+    unsigned *NextUniqueID, const MCSymbolELF *AssociatedSymbol) {
   unsigned EntrySize = 0;
   if (Kind.isMergeableCString()) {
     if (Kind.isMergeable2ByteCString()) {
@@ -333,7 +360,7 @@ selectELFSectionForGlobal(MCContext &Ctx, const GlobalObject *GO,
   if (Kind.isExecuteOnly())
     UniqueID = 0;
   return Ctx.getELFSection(Name, getELFSectionType(Name, Kind), Flags,
-                           EntrySize, Group, UniqueID);
+                           EntrySize, Group, UniqueID, AssociatedSymbol);
 }
 
 MCSection *TargetLoweringObjectFileELF::SelectSectionForGlobal(
@@ -351,8 +378,17 @@ MCSection *TargetLoweringObjectFileELF::SelectSectionForGlobal(
   }
   EmitUniqueSection |= GO->hasComdat();
 
-  return selectELFSectionForGlobal(getContext(), GO, Kind, getMangler(), TM,
-                                   EmitUniqueSection, Flags, &NextUniqueID);
+  const MCSymbolELF *AssociatedSymbol = getAssociatedSymbol(GO, TM);
+  if (AssociatedSymbol) {
+    EmitUniqueSection = true;
+    Flags |= ELF::SHF_LINK_ORDER;
+  }
+
+  MCSectionELF *Section = selectELFSectionForGlobal(
+      getContext(), GO, Kind, getMangler(), TM, EmitUniqueSection, Flags,
+      &NextUniqueID, AssociatedSymbol);
+  assert(Section->getAssociatedSymbol() == AssociatedSymbol);
+  return Section;
 }
 
 MCSection *TargetLoweringObjectFileELF::getSectionForJumpTable(
@@ -365,8 +401,9 @@ MCSection *TargetLoweringObjectFileELF::getSectionForJumpTable(
     return ReadOnlySection;
 
   return selectELFSectionForGlobal(getContext(), &F, SectionKind::getReadOnly(),
-                                   getMangler(), TM, EmitUniqueSection, ELF::SHF_ALLOC,
-                                   &NextUniqueID);
+                                   getMangler(), TM, EmitUniqueSection,
+                                   ELF::SHF_ALLOC, &NextUniqueID,
+                                   /* AssociatedSymbol */ nullptr);
 }
 
 bool TargetLoweringObjectFileELF::shouldPutJumpTableInFunctionSection(

lib/IR/LLVMContext.cpp

@@ -58,6 +58,7 @@ LLVMContext::LLVMContext() : pImpl(new LLVMContextImpl(*this)) {
     {MD_type, "type"},
     {MD_section_prefix, "section_prefix"},
     {MD_absolute_symbol, "absolute_symbol"},
+    {MD_associated, "associated"},
   };
 
   for (auto &MDKind : MDKinds) {

test/CodeGen/X86/elf-associated.ll

@@ -0,0 +1,39 @@
+; RUN: llc -data-sections=1 -mtriple x86_64-pc-linux-gnu < %s | FileCheck %s
+; RUN: llc -data-sections=0 -mtriple x86_64-pc-linux-gnu < %s | FileCheck %s
+
+@a = global i32 1
+@b = global i32 2, !associated !0
+!0 = !{i32* @a}
+; CHECK-DAG: .section .data.b,"awm",@progbits,a
+
+; Loop is OK. Also, normally -data-sections=0 would place @c and @d in the same section. !associated prevents that.
+@c = global i32 2, !associated !2
+@d = global i32 2, !associated !1
+!1 = !{i32* @c}
+!2 = !{i32* @d}
+; CHECK-DAG: .section .data.c,"awm",@progbits,d
+; CHECK-DAG: .section .data.d,"awm",@progbits,c
+
+; BSS is OK.
+@e = global i32 0
+@f = global i32 0, !associated !3
+@g = global i32 1, !associated !3
+!3 = !{i32* @e}
+; CHECK-DAG: .section .bss.f,"awm",@nobits,e
+; CHECK-DAG: .section .data.g,"awm",@progbits,e
+
+; Explicit sections.
+@h = global i32 1, section "aaa"
+@i = global i32 1, section "bbb", !associated !4
+@j = global i32 1, section "bbb", !associated !4
+@k = global i32 1, !associated !4
+!4 = !{i32* @h}
+; CHECK-DAG: .section	aaa,"aw",@progbits
+; CHECK-DAG: .section	bbb,"awm",@progbits,h,unique,1
+; CHECK-DAG: .section	bbb,"awm",@progbits,h,unique,2
+; CHECK-DAG: .section	.data.k,"awm",@progbits,h
+
+; Non-GlobalObject metadata.
+@l = global i32 1, section "ccc", !associated !5
+!5 = !{i32* null}
+; CHECK-DAG: .section	ccc,"aw",@progbits

test/ThinLTO/X86/lazyload_metadata.ll

@@ -10,13 +10,13 @@
 ; RUN: llvm-lto -thinlto-action=import %t2.bc -thinlto-index=%t3.bc \
 ; RUN:          -o /dev/null -stats \
 ; RUN:  2>&1 | FileCheck %s -check-prefix=LAZY
-; LAZY: 49 bitcode-reader  - Number of Metadata records loaded
+; LAZY: 51 bitcode-reader  - Number of Metadata records loaded
 ; LAZY: 2 bitcode-reader  - Number of MDStrings loaded
 
 ; RUN: llvm-lto -thinlto-action=import %t2.bc -thinlto-index=%t3.bc \
 ; RUN:          -o /dev/null -disable-ondemand-mds-loading -stats \
 ; RUN:  2>&1 | FileCheck %s -check-prefix=NOTLAZY
-; NOTLAZY: 58 bitcode-reader  - Number of Metadata records loaded
+; NOTLAZY: 60 bitcode-reader  - Number of Metadata records loaded
 ; NOTLAZY: 7 bitcode-reader  - Number of MDStrings loaded
 
 
@@ -55,4 +55,4 @@ declare i1 @llvm.type.test(i8* %ptr, metadata %bitset) nounwind readnone
 !6 = !{!9}
 !7 = !{!"7"}
 !8 = !{!"8"}
-!9 = !{!6}
+!9 = !{!6}