[MC] Move ELFWriter::createMemtagRelocs to AArch64ELFStreamer::finishImpl

Follow-up to https://reviews.llvm.org/D128958

* Move target-specific code away from the generic ELFWriter.
* All sections should have been created before MCAssembler::layout.
* Remove one `registerSection` use, which should be considered private to MCAssembler.

Author: MaskRay

llvm/lib/MC/ELFObjectWriter.cpp

@@ -193,8 +193,6 @@ struct ELFWriter {
   MCSectionELF *createRelocationSection(MCContext &Ctx,
                                         const MCSectionELF &Sec);
 
-  void createMemtagRelocs(MCAssembler &Asm);
-
   void writeSectionHeader(const MCAsmLayout &Layout,
                           const SectionIndexMapTy &SectionIndexMap,
                           const SectionOffsetsTy &SectionOffsets);
@@ -616,23 +614,6 @@ bool ELFWriter::isInSymtab(const MCAsmLayout &Layout, const MCSymbolELF &Symbol,
   return true;
 }
 
-void ELFWriter::createMemtagRelocs(MCAssembler &Asm) {
-  MCSectionELF *MemtagRelocs = nullptr;
-  for (const MCSymbol &Sym : Asm.symbols()) {
-    const auto &SymE = cast<MCSymbolELF>(Sym);
-    if (!SymE.isMemtag())
-      continue;
-    if (MemtagRelocs == nullptr) {
-      MemtagRelocs = OWriter.TargetObjectWriter->getMemtagRelocsSection(Asm.getContext());
-      if (MemtagRelocs == nullptr)
-        report_fatal_error("Tagged globals are not available on this architecture.");
-      Asm.registerSection(*MemtagRelocs);
-    }
-    ELFRelocationEntry Rec(0, &SymE, ELF::R_AARCH64_NONE, 0, nullptr, 0);
-    OWriter.Relocations[MemtagRelocs].push_back(Rec);
-  }
-}
-
 void ELFWriter::computeSymbolTable(
     MCAssembler &Asm, const MCAsmLayout &Layout,
     const SectionIndexMapTy &SectionIndexMap, const RevGroupMapTy &RevGroupMap,
@@ -1094,8 +1075,6 @@ uint64_t ELFWriter::writeObject(MCAssembler &Asm, const MCAsmLayout &Layout) {
       Ctx.getELFSection(".strtab", ELF::SHT_STRTAB, 0);
   StringTableIndex = addToSectionTable(StrtabSection);
 
-  createMemtagRelocs(Asm);
-
   RevGroupMapTy RevGroupMap;
   SectionIndexMapTy SectionIndexMap;
 

llvm/lib/Target/AArch64/MCTargetDesc/AArch64ELFStreamer.cpp

@@ -28,7 +28,7 @@
 #include "llvm/MC/MCExpr.h"
 #include "llvm/MC/MCInst.h"
 #include "llvm/MC/MCObjectWriter.h"
-#include "llvm/MC/MCSection.h"
+#include "llvm/MC/MCSectionELF.h"
 #include "llvm/MC/MCStreamer.h"
 #include "llvm/MC/MCSubtargetInfo.h"
 #include "llvm/MC/MCSymbolELF.h"
@@ -183,7 +183,7 @@ class AArch64ELFStreamer : public MCELFStreamer {
                       std::move(Emitter)),
         MappingSymbolCounter(0), LastEMS(EMS_None) {}
 
-  void changeSection(MCSection *Section, uint32_t Subsection) override {
+  void changeSection(MCSection *Section, uint32_t Subsection = 0) override {
     // We have to keep track of the mapping symbol state of any sections we
     // use. Each one should start off as EMS_None, which is provided as the
     // default constructor by DenseMap::lookup.
@@ -248,6 +248,9 @@ class AArch64ELFStreamer : public MCELFStreamer {
     emitDataMappingSymbol();
     MCObjectStreamer::emitFill(NumBytes, FillValue, Loc);
   }
+
+  void finishImpl() override;
+
 private:
   enum ElfMappingSymbol {
     EMS_None,
@@ -284,6 +287,27 @@ class AArch64ELFStreamer : public MCELFStreamer {
   ElfMappingSymbol LastEMS;
 };
 
+void AArch64ELFStreamer::finishImpl() {
+  MCContext &Ctx = getContext();
+  auto &Asm = getAssembler();
+  MCSectionELF *MemtagSec = nullptr;
+  const auto *Zero = MCConstantExpr::create(0, Ctx);
+  for (const MCSymbol &Symbol : Asm.symbols()) {
+    const auto &Sym = cast<MCSymbolELF>(Symbol);
+    if (!Sym.isMemtag())
+      continue;
+    if (!MemtagSec) {
+      MemtagSec = Ctx.getELFSection(".memtag.globals.static",
+                                    ELF::SHT_AARCH64_MEMTAG_GLOBALS_STATIC, 0);
+      switchSection(MemtagSec);
+    }
+    auto *SRE = MCSymbolRefExpr::create(&Sym, MCSymbolRefExpr::VK_None, Ctx);
+    (void)MCObjectStreamer::emitRelocDirective(
+        *Zero, "BFD_RELOC_NONE", SRE, SMLoc(), *Ctx.getSubtargetInfo());
+  }
+  MCELFStreamer::finishImpl();
+}
+
 } // end anonymous namespace
 
 AArch64ELFStreamer &AArch64TargetELFStreamer::getStreamer() {