MC: Rename isVirtualSection to isBssSection

The term BSS (Block Started by Symbol) is a standard, widely recognized
term, available in the a.out object file format and adopted by formats
like COFF, XCOFF, Mach-O (called S_ZEROFILL while `__bss` is also used),
and ELF. To avoid introducing unfamiliar terms, we should use
isBSSSection instead of isVirtualSection.

Author: MaskRay

llvm/include/llvm/MC/MCSection.h

@@ -92,8 +92,7 @@ class LLVM_ABI MCSection {
   bool IsRegistered : 1;
 
   bool IsText : 1;
-
-  bool IsVirtual : 1;
+  bool IsBss : 1;
 
   /// Whether the section contains linker-relaxable fragments. If true, the
   /// offset between two locations may not be fully resolved.
@@ -177,9 +176,9 @@ class LLVM_ABI MCSection {
   /// instead of 0s.
   virtual bool useCodeAlign() const = 0;
 
-  /// Check whether this section is "virtual", that is has no actual object
-  /// file contents.
-  bool isVirtualSection() const { return IsVirtual; }
+  /// Return true if this is a BSS section (e.g., ELF .bss or .tbss) that does
+  /// not store content and is typically initialized to zeroes by the runtime.
+  bool isBssSection() const { return IsBss; }
 
   virtual StringRef getVirtualSectionKind() const;
 };

llvm/include/llvm/MC/MCSectionGOFF.h

@@ -111,7 +111,7 @@ class LLVM_ABI MCSectionGOFF final : public MCSection {
 
   // Returns the text style for a section. Only defined for ED and PR sections.
   GOFF::ESDTextStyle getTextStyle() const {
-    assert((isED() || isPR() || isVirtualSection()) && "Expect ED or PR section");
+    assert((isED() || isPR() || isBssSection()) && "Expect ED or PR section");
     if (isED())
       return EDAttributes.TextStyle;
     if (isPR())

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -810,7 +810,7 @@ void AsmPrinter::emitGlobalVariable(const GlobalVariable *GV) {
 
   // If we have a bss global going to a section that supports the
   // zerofill directive, do so here.
-  if (GVKind.isBSS() && MAI->isMachO() && TheSection->isVirtualSection()) {
+  if (GVKind.isBSS() && MAI->isMachO() && TheSection->isBssSection()) {
     if (Size == 0)
       Size = 1; // zerofill of 0 bytes is undefined.
     emitLinkage(GV, GVSym);

llvm/lib/MC/MCAssembler.cpp

@@ -362,7 +362,7 @@ uint64_t MCAssembler::getSectionAddressSize(const MCSection &Sec) const {
 
 uint64_t MCAssembler::getSectionFileSize(const MCSection &Sec) const {
   // Virtual sections have no file size.
-  if (Sec.isVirtualSection())
+  if (Sec.isBssSection())
     return 0;
   return getSectionAddressSize(Sec);
 }
@@ -559,7 +559,7 @@ void MCAssembler::writeSectionData(raw_ostream &OS,
                                    const MCSection *Sec) const {
   assert(getBackendPtr() && "Expected assembler backend");
 
-  if (Sec->isVirtualSection()) {
+  if (Sec->isBssSection()) {
     assert(getSectionFileSize(*Sec) == 0 && "Invalid size for section!");
 
     // Ensure no fixups or non-zero bytes are written to BSS sections, catching

llvm/lib/MC/MCMachOStreamer.cpp

@@ -401,7 +401,7 @@ void MCMachOStreamer::emitZerofill(MCSection *Section, MCSymbol *Symbol,
   // On darwin all virtual sections have zerofill type. Disallow the usage of
   // .zerofill in non-virtual functions. If something similar is needed, use
   // .space or .zero.
-  if (!Section->isVirtualSection()) {
+  if (!Section->isBssSection()) {
     getContext().reportError(
         Loc, "The usage of .zerofill is restricted to sections of "
              "ZEROFILL type. Use .zero or .space instead.");

llvm/lib/MC/MCParser/AsmParser.cpp

@@ -3404,7 +3404,7 @@ bool AsmParser::parseDirectiveAlign(bool IsPow2, uint8_t ValueSize) {
   const MCSection *Section = getStreamer().getCurrentSectionOnly();
   assert(Section && "must have section to emit alignment");
 
-  if (HasFillExpr && FillExpr != 0 && Section->isVirtualSection()) {
+  if (HasFillExpr && FillExpr != 0 && Section->isBssSection()) {
     ReturnVal |=
         Warning(FillExprLoc, "ignoring non-zero fill value in " +
                                  Section->getVirtualSectionKind() +

llvm/lib/MC/MCSection.cpp

@@ -21,7 +21,7 @@ using namespace llvm;
 MCSection::MCSection(SectionVariant V, StringRef Name, bool IsText,
                      bool IsVirtual, MCSymbol *Begin)
     : Begin(Begin), HasInstructions(false), IsRegistered(false), IsText(IsText),
-      IsVirtual(IsVirtual), LinkerRelaxable(false), Name(Name), Variant(V) {
+      IsBss(IsVirtual), LinkerRelaxable(false), Name(Name), Variant(V) {
   // The initial subsection number is 0. Create a fragment list.
   CurFragList = &Subsections.emplace_back(0u, FragList{}).second;
 }

llvm/lib/MC/MachObjectWriter.cpp

@@ -134,7 +134,7 @@ uint64_t MachObjectWriter::getPaddingSize(const MCAssembler &Asm,
     return 0;
 
   const MCSection &NextSec = *SectionOrder[Next];
-  if (NextSec.isVirtualSection())
+  if (NextSec.isBssSection())
     return 0;
   return offsetToAlignment(EndAddr, NextSec.getAlign());
 }
@@ -277,7 +277,7 @@ void MachObjectWriter::writeSection(const MCAssembler &Asm,
   const MCSectionMachO &Section = cast<MCSectionMachO>(Sec);
 
   // The offset is unused for virtual sections.
-  if (Section.isVirtualSection()) {
+  if (Section.isBssSection()) {
     assert(Asm.getSectionFileSize(Sec) == 0 && "Invalid file size!");
     FileOffset = 0;
   }
@@ -692,13 +692,13 @@ void MachObjectWriter::computeSectionAddresses(const MCAssembler &Asm) {
   unsigned i = 0;
   // Compute the section layout order. Virtual sections must go last.
   for (MCSection &Sec : Asm) {
-    if (!Sec.isVirtualSection()) {
+    if (!Sec.isBssSection()) {
       SectionOrder.push_back(&Sec);
       cast<MCSectionMachO>(Sec).setLayoutOrder(i++);
     }
   }
   for (MCSection &Sec : Asm) {
-    if (Sec.isVirtualSection()) {
+    if (Sec.isBssSection()) {
       SectionOrder.push_back(&Sec);
       cast<MCSectionMachO>(Sec).setLayoutOrder(i++);
     }
@@ -895,7 +895,7 @@ void MachObjectWriter::writeMachOHeader(MCAssembler &Asm) {
 
     VMSize = std::max(VMSize, Address + Size);
 
-    if (Sec.isVirtualSection())
+    if (Sec.isBssSection())
       continue;
 
     SectionDataSize = std::max(SectionDataSize, Address + Size);
@@ -938,7 +938,7 @@ void MachObjectWriter::writeMachOHeader(MCAssembler &Asm) {
     unsigned Flags = Sec.getTypeAndAttributes();
     if (Sec.hasInstructions())
       Flags |= MachO::S_ATTR_SOME_INSTRUCTIONS;
-    if (!cast<MCSectionMachO>(Sec).isVirtualSection() &&
+    if (!cast<MCSectionMachO>(Sec).isBssSection() &&
         !isUInt<32>(SectionStart)) {
       getContext().reportError(
           SMLoc(), "cannot encode offset of section; object file too large");

llvm/lib/ObjCopy/MachO/MachOObject.h

@@ -64,14 +64,14 @@ struct Section {
     return static_cast<MachO::SectionType>(Flags & MachO::SECTION_TYPE);
   }
 
-  bool isVirtualSection() const {
+  bool isBssSection() const {
     return (getType() == MachO::S_ZEROFILL ||
             getType() == MachO::S_GB_ZEROFILL ||
             getType() == MachO::S_THREAD_LOCAL_ZEROFILL);
   }
 
   bool hasValidOffset() const {
-    return !(isVirtualSection() || OriginalOffset == 0);
+    return !(isBssSection() || OriginalOffset == 0);
   }
 };
 

llvm/lib/ObjCopy/MachO/MachOWriter.cpp

@@ -112,7 +112,7 @@ size_t MachOWriter::totalSize() const {
     for (const std::unique_ptr<Section> &S : LC.Sections) {
       if (!S->hasValidOffset()) {
         assert((S->Offset == 0) && "Skipped section's offset must be zero");
-        assert((S->isVirtualSection() || S->Size == 0) &&
+        assert((S->isBssSection() || S->Size == 0) &&
                "Non-zero-fill sections with zero offset must have zero size");
         continue;
       }
@@ -240,7 +240,7 @@ void MachOWriter::writeSections() {
     for (const std::unique_ptr<Section> &Sec : LC.Sections) {
       if (!Sec->hasValidOffset()) {
         assert((Sec->Offset == 0) && "Skipped section's offset must be zero");
-        assert((Sec->isVirtualSection() || Sec->Size == 0) &&
+        assert((Sec->isBssSection() || Sec->Size == 0) &&
                "Non-zero-fill sections with zero offset must have zero size");
         continue;
       }