[llvm-objdump] Print "..." instead of random data for virtual sections

[thegameg]

Summary

When disassembling with -D, skip virtual sections by printing "..." for each symbol.

This patch also implements MachOObjectFile::isSectionVirtual.

Test case comes from:

.zerofill __DATA,__common,_data64unsigned,472,3

[JDevlieghere]

test/tools/llvm-objdump/macho-zerofill.test

1 ↗

(On Diff #143108)

Any reason we can't use an asm test for this?

[thegameg]

Thanks Jonas!

Added assembly test and a missing comment.

[JDevlieghere]

Thanks Francis, LGTM!

lib/Object/MachOObjectFile.cpp

return !(Flags & MachO::S_ATTR_PURE_INSTRUCTIONS) &&
           SectionType == MachO::S_GB_ZEROFILL);
 }
 
 unsigned MachOObjectFile::getSectionID(SectionRef Sec) const {
   return Sec.getRawDataRefImpl().d.a;
 }
 
 bool MachOObjectFile::isSectionVirtual(DataRefImpl Sec) const {
-  // FIXME: Unimplemented.
-  return false;
+  uint32_t Flags = getSectionFlags(*this, Sec);
+  unsigned SectionType = Flags & MachO::SECTION_TYPE;
+  return SectionType == MachO::S_ZEROFILL ||
+         SectionType == MachO::S_GB_ZEROFILL;
 }
 
 bool MachOObjectFile::isSectionBitcode(DataRefImpl Sec) const {
   StringRef SegmentName = getSectionFinalSegmentName(Sec);
   StringRef SectName;
   if (!getSectionName(Sec, SectName))
     return (SegmentName == "__LLVM" && SectName == "__bitcode");
   return false;

test/tools/llvm-objdump/AArch64/macho-zerofill.s

+// RUN: llvm-mc < %s -triple aarch64-macho -filetype=obj | llvm-objdump -triple=aarch64-- -D - | FileCheck %s
+
+
+// Check that we don't print garbage when we dump zerofill sections.
+
+.zerofill __DATA,__common,_data64unsigned,472,3
+// CHECK: Disassembly of section __DATA,__common:
+// CHECK: ltmp1:
+// CHECK-NEXT: ...

tools/llvm-objdump/llvm-objdump.cpp

for (unsigned si = 0, se = Symbols.size(); si != se; ++si) {
           while (End > Limit &&
             *reinterpret_cast<const support::ulittle32_t*>(&Bytes[End - 4]) == 0)
             End -= 4;
         }
       }
 
       outs() << '\n' << std::get<1>(Symbols[si]) << ":\n";
 
+      // Don't print raw contents of a virtual section. A virtual section
+      // doesn't have any contents in the file.
+      if (Section.isVirtual()) {
+        outs() << "...\n";
+        continue;
+      }
+
 #ifndef NDEBUG
       raw_ostream &DebugOut = DebugFlag ? dbgs() : nulls();
 #else
       raw_ostream &DebugOut = nulls();
 #endif
 
       for (Index = Start; Index < End; Index += Size) {
         MCInst Inst;