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Add !associated metadata.

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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
This commit is contained in:
Evgeniy Stepanov
2017-03-17 22:17:24 +00:00
parent 87552d6290
commit df808fe215
6 changed files with 119 additions and 16 deletions
Download Patch File Download Diff File Expand all files Collapse all files
lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+50 -13
View File
@@ -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(