[llvm-objcopy] - Remove an excessive zlib::isAvailable() check and dead code.

[grimar]

Summary

There are 2 places where llvm-objcopy creates CompressedSection:

1. For --compress-debug-sections. It might create the compressed section from

regular here:
https://github.com/llvm-mirror/llvm/blob/master/tools/llvm-objcopy/ELF/ELFObjcopy.cpp#L486

2. All initially compressed sections are created as CompressedSection during reading the sections

from an object:
https://github.com/llvm-mirror/llvm/blob/master/tools/llvm-objcopy/ELF/Object.cpp#L1118
Those have DebugCompressionType::None type and a different constructor.

Case 1 has the following code in its constructor:

if (!zlib::isAvailable()) {
  CompressionType = DebugCompressionType::None;
  return;
}

(https://github.com/llvm-mirror/llvm/blob/master/tools/llvm-objcopy/ELF/Object.cpp#L267)

We can never reach that code with because would report an error much earlier:
https://github.com/llvm-mirror/llvm/blob/master/tools/llvm-objcopy/CopyConfig.cpp#L480

So the code I am removing is dead. Landing this will address the issue mentioned in https://bugs.llvm.org/show_bug.cgi?id=40886.

[jhenderson]

tools/llvm-objcopy/ELF/Object.cpp

265

Maybe you should have an assertion here so that we don't try to create CompressedSection if there is no zlib?

[grimar]

tools/llvm-objcopy/ELF/Object.cpp

265

FTR: we still want to create it for the case 2 from the description, but it uses a different constructor.
I'll update the patch with an assert here.

[jhenderson]

LGTM, even without the assertion.

tools/llvm-objcopy/ELF/Object.cpp

265

Right, yes, but as I just realised in D59017, no need for the assertion as zlib::compress already does it.

tools/llvm-objcopy/ELF/Object.cpp

void ELFSectionWriter<ELFT>::visit(const CompressedSection &Sec) {
   }
 
   std::copy(Sec.CompressedData.begin(), Sec.CompressedData.end(), Buf);
 }
 
 CompressedSection::CompressedSection(const SectionBase &Sec,
                                      DebugCompressionType CompressionType)
     : SectionBase(Sec), CompressionType(CompressionType),
       DecompressedSize(Sec.OriginalData.size()), DecompressedAlign(Sec.Align) {
-
-  if (!zlib::isAvailable()) {
-    CompressionType = DebugCompressionType::None;
-    return;
-  }
-
   if (Error E = zlib::compress(
           StringRef(reinterpret_cast<const char *>(OriginalData.data()),
                     OriginalData.size()),
           CompressedData))
     reportError(Name, std::move(E));
 
   size_t ChdrSize;
   if (CompressionType == DebugCompressionType::GNU) {