ELF: Create unique SHF_GNU_RETAIN sections for llvm.used global objects

If a global object is listed in `@llvm.used`, place it in a unique section with
the `SHF_GNU_RETAIN` flag. The section is a GC root under `ld --gc-sections`
with LLD>=13 or GNU ld>=2.36.

For front ends which do not expect to see multiple sections of the same name,
consider emitting `@llvm.compiler.used` instead of `@llvm.used`.

SHF_GNU_RETAIN is restricted to ELFOSABI_GNU and ELFOSABI_FREEBSD in
binutils. We don't do the restriction - see the rationale in D95749.

The integrated assembler has supported SHF_GNU_RETAIN since D95730.
GNU as>=2.36 supports section flag 'R'.
We don't need to worry about GNU ld support because older GNU ld just ignores
the unknown SHF_GNU_RETAIN.

With this change, `__attribute__((retain))` functions/variables emitted
by clang will get the SHF_GNU_RETAIN flag.

Differential Revision: https://reviews.llvm.org/D97448

1 files changed, 28 insertions, 8 deletions

diff --git a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
index 8f74866..f1729d1 100644
--- a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
@@ -293,6 +293,14 @@ void TargetLoweringObjectFileELF::Initialize(MCContext &Ctx,
   }
 }
 
+void TargetLoweringObjectFileELF::getModuleMetadata(Module &M) {
+  SmallVector<GlobalValue *, 4> Vec;
+  collectUsedGlobalVariables(M, Vec, false);
+  for (GlobalValue *GV : Vec)
+    if (auto *GO = dyn_cast<GlobalObject>(GV))
+      Used.insert(GO);
+}
+
 void TargetLoweringObjectFileELF::emitModuleMetadata(MCStreamer &Streamer,
                                                      Module &M) const {
   auto &C = getContext();
@@ -687,9 +695,15 @@ MCSection *TargetLoweringObjectFileELF::getExplicitSectionGlobal(
   // MD_associated in a unique section.
   unsigned UniqueID = MCContext::GenericSectionID;
   const MCSymbolELF *LinkedToSym = getLinkedToSymbol(GO, TM);
-  if (GO->getMetadata(LLVMContext::MD_associated)) {
+  const bool Associated = GO->getMetadata(LLVMContext::MD_associated);
+  const bool Retain = Used.count(GO);
+  if (Associated || Retain) {
     UniqueID = NextUniqueID++;
-    Flags |= ELF::SHF_LINK_ORDER;
+    if (Associated)
+      Flags |= ELF::SHF_LINK_ORDER;
+    if (Retain && (getContext().getAsmInfo()->useIntegratedAssembler() ||
+                   getContext().getAsmInfo()->binutilsIsAtLeast(2, 36)))
+      Flags |= ELF::SHF_GNU_RETAIN;
   } else {
     if (getContext().getAsmInfo()->useIntegratedAssembler() ||
         getContext().getAsmInfo()->binutilsIsAtLeast(2, 35)) {
@@ -802,13 +816,18 @@ static MCSectionELF *selectELFSectionForGlobal(
 
 static MCSection *selectELFSectionForGlobal(
     MCContext &Ctx, const GlobalObject *GO, SectionKind Kind, Mangler &Mang,
-    const TargetMachine &TM, bool EmitUniqueSection,  unsigned Flags,
-    unsigned *NextUniqueID) {
+    const TargetMachine &TM, bool Retain, bool EmitUniqueSection,
+    unsigned Flags, unsigned *NextUniqueID) {
   const MCSymbolELF *LinkedToSym = getLinkedToSymbol(GO, TM);
   if (LinkedToSym) {
     EmitUniqueSection = true;
     Flags |= ELF::SHF_LINK_ORDER;
   }
+  if (Retain && (Ctx.getAsmInfo()->useIntegratedAssembler() ||
+                 Ctx.getAsmInfo()->binutilsIsAtLeast(2, 36))) {
+    EmitUniqueSection = true;
+    Flags |= ELF::SHF_GNU_RETAIN;
+  }
 
   MCSectionELF *Section = selectELFSectionForGlobal(
       Ctx, GO, Kind, Mang, TM, EmitUniqueSection, Flags,
@@ -832,16 +851,17 @@ MCSection *TargetLoweringObjectFileELF::SelectSectionForGlobal(
   }
   EmitUniqueSection |= GO->hasComdat();
   return selectELFSectionForGlobal(getContext(), GO, Kind, getMangler(), TM,
-                                   EmitUniqueSection, Flags, &NextUniqueID);
+                                   Used.count(GO), EmitUniqueSection, Flags,
+                                   &NextUniqueID);
 }
 
 MCSection *TargetLoweringObjectFileELF::getUniqueSectionForFunction(
     const Function &F, const TargetMachine &TM) const {
   SectionKind Kind = SectionKind::getText();
   unsigned Flags = getELFSectionFlags(Kind);
-  return selectELFSectionForGlobal(getContext(), &F, Kind, getMangler(), TM,
-                                   /* EmitUniqueSection = */ true, Flags,
-                                   &NextUniqueID);
+  return selectELFSectionForGlobal(
+      getContext(), &F, Kind, getMangler(), TM, Used.count(&F),
+      /*EmitUniqueSection=*/true, Flags, &NextUniqueID);
 }
 
 MCSection *TargetLoweringObjectFileELF::getSectionForJumpTable(