[llvm-objcopy] Dwarf decompression support.

[jakehehrlich]

So I was thinking that we might try and detect compressed section at section construction time instead. The we can implement decompression in terms of CompressedSection...the only blemish seems to be that CompressedSection always allocates. I'm not super sure what the right solution to that is.

On a much more complicated note, I'm thinking I have a nicer and more generic way of handling compressed sections but it's not something you should worry about in this change (unless it sounds awesome to you and you're up for a more complex challenge). If we "decompress" (air quotes explained later) every compressed section at construction regardless of weather it is a debug section then we can even handle compressed symbol tables or relocations! To avoid too much allocation and copying we can lazily decompress (e.g. the airquotes) only when needed. I've had an idea floating around in the back of my head about lazily constructing sections anyway. I'm going to need to think more about a design here but that's what I'm thinking would eventually be a better design.

tools/llvm-objcopy/Object.h

277

Why do these need to be moved into SectionBase?

tools/llvm-objcopy/llvm-objcopy.cpp

435

So section replacement is very tricky to do right. It's one of the fundamental operations (like remove and add) that I want on Object. I have not implemented it yet because, well, it's complicated. It's easy to do when no relocations, symbols, or other parts of the system reference the given section but we're almost never so lucky. In the previous change we dealt with relocations via this method but using a different, less generic name. It might be worth doing that again. So either the name should change, or we should bite the bullet and implement a proper replace method in Object. Implementing a proper replace method will require a wide scale change to llvm-objcopy but would have a lot of benefits. My thinking was that replace could be implemented by adding a new method to SectionBase called "replaceSection" much like "removeSection" works now. Architecturally that's pretty simple. The problem is that *every* section needs to be extended with this method and every detail has to be implemented correctly.

If you wanna do it, I'd be super happy to review it. If not, we can just rename this function to something less generic for now. Maybe something like "replaceDebugSections" since it really only handles relocations.

687

Instead of modifying OwnedDataSection can you just do the decompression here?

[alexander-shaposhnikov]

tools/llvm-objcopy/Object.h

366

I'm looking at the changes in OwnedDataSection, tbh, i'm not a big fan of them.
It looks like OwnedDataSection is not the right place for them (now we have a bool flag and a new branch everywhere + some details (like ".zdebug") have leaked into it).

[alexander-shaposhnikov]

tools/llvm-objcopy/Object.h

366

one potential approach might be to create DecompressedSection class. cc: @jakehehrlich
(to avoid misunderstanding: ubiquitous "if(DoCompression)" or "if(Sec.DoCompression)" all over the pipeline suggest that this dispatching mechanism should not be in OwnedDataSection)

[alexander-shaposhnikov]

tools/llvm-objcopy/Object.h

366

typo: DoDecompression

[jakehehrlich]

tools/llvm-objcopy/Object.h

366

+1 on this general view point. OwnedDataSection is not the right place.

[plotfi]

tools/llvm-objcopy/Object.h

277

Because I set them on "Sec" after makeSection is called. I think maybe if we reuse CompressedSection or add a DecompressedSection this could be moved elsewhere.

366

Sounds good. @jakehehrlich Would you prefer I reuse CompressedSection or write a new DecompressedSection class? I'll send out a new patch after you let me know.

tools/llvm-objcopy/llvm-objcopy.cpp

435

Yeah would be a good thing to think about. Alternatively I could maybe decompress the section into a "DecompressedSection" on read, and instead of replacing the decompressed sections I could possibly just flag them for decompressed write? I agree, better to not encourage people to reuse this outside of the context of debug sections.

687

Sure. Would you prefer I reuse CompressedSection somehow (adding some decompression flag) or make a whole new DecompressedSection class?

[plotfi]

Added DecompressedSection.

[alexander-shaposhnikov]

tools/llvm-objcopy/Object.h

382

oh, no, that's not what i meant tbh, and this looks kinda strange (i mean the class with this method), although maybe I'm missing smth.
When I said that probably it would make sense to introduce DecompressedSection I meant that the approach would be in some sense "dual" to what you did for compression in the previous review. What are your thoughts ?

[plotfi]

tools/llvm-objcopy/Object.h

382

I need to read and store the decompressed size at makeSection time. So either I need a decompressed size field on the SectionBase or I need to make the decompressed section at makeSection time. So next I could either replace DecompressedSection with a DecompressedSection that’s flagged for decompression or I could just flag it. Alternatively I guess if I had access to the args that HandleArgs gets I could create a decompressed section at makeSection time that always decompresses when the Arg is set.

[plotfi]

• using two new classes: DecompressableSection and DecompressedSection.
• When a compressed debug section is encountered in makeSection we create a DecompressableSection instead of a Section so that we can keep track of the DecompressedSize (which we need ELFT to read).
• in handleArgs if the Decompression command line arg is set then we replace all of the DecompressableSections with DecompressedSections. The difference being that the decompressed sections unset the SHF_COMPRESSED flag, undo the .zdebug* naming, and set the Section size (and alignment to DecompressedSize and DecompressedAlign. DecompressedSections Writer function also does the decompressing whereas DecompressableSection just writes the original uncompressed data.

[plotfi]

tools/llvm-objcopy/Object.h

400

Oops, I think I need to add Align = DecompressedAlign here. What do you think @jakehehrlich

[alexander-shaposhnikov]

this version looks much cleaner to me than the previous one!

[alexander-shaposhnikov]

although I'd probably try to make the implementation more symmetric (i mean "compression" and "decompression"). Would that be possible ?

[plotfi]

In D51841#1234283, @alexshap wrote:

although I'd probably try to make the implementation more symmetric (i mean "compression" and "decompression"). Would that be possible ?

What does that mean?

[alexander-shaposhnikov]

@plotfi , oh, sorry, i should have added more context. The thing is, that I've been thinking about this patch more and more, and it seems to me, that it's possible to eliminate DecompressableSection by making the class CompressedSection a bit more robust (probably what you mentioned at some point). For example (not insisting on this approach, but just one potential option): CompressedSection can store (internally) either the original content, or the already compressed content + you can add one more constructor to that class etc. In this case the full picture will probably look a bit nicer and easier to perceive: we will have two classes: DecompressedSection and CompressedSection which are responsible for managing this sort of transformation (replacing a given section with the "compressed" version of it or, instead, with the "decompressed" version of it).

[jhenderson]

Please remember to include me as a reviewer on all llvm-objcopy reviews.

test/tools/llvm-objcopy/compress-debug-sections.test

6

As with other tests, this test (or possibly the specific compress and decompress tests) should show that the inputs are not modified when using the specified switch.

test/tools/llvm-objcopy/decompress-debug-sections-zlib-gnu.test

8 ↗

(On Diff #165331)

Why is this check and the following one operating on the compressed object? That's tested elsewhere. In fact, I'm not sure this test is really testing the decompression at all. It looks like almost a copy-and-paste job from the compressed test.

9 ↗

(On Diff #165331)

This check-prefix is badly named, since we are testing more than flags with it.

10 ↗

(On Diff #165331)

What is this check achieving?

test/tools/llvm-objcopy/decompress-debug-sections-zlib.test

1 ↗

(On Diff #165331)

The same comments apply to this test as the other test.

tools/llvm-objcopy/Object.cpp

155

No need to use strlen on a string literal. In general, use char arrays and sizeof. However, in this case, make Magic an ArrayRef, and use the size method and std::equals or similar to compare, rather than strncmp (assuming that ArrayRefs don't have a startswith method). It will read much better, and avoids unnecessary casting from uint8_t to const char.

163

Get rid of the blank lines in this function. They don't really improve readability, in my opinion.

164

Don't use unsigned. Use size_t, since that's the return type of sizeof.

dataOffset -> DataOffset
Please remember to follow LLVM style.

166

As "ZLIB" and its size are used everywhere, I suggest making it a constant global variable that you can query with .size(), as required. (See also my comments elsewhere about making it a uint8_t array/ArrayRef).

173

This may not be your fault, but SmallVector is clearly the wrong thing for this here, since this won't be Small for the vast majority of uses.

175

Is this explicit cast necessary? If it is (e.g. we support 32-bit hosts still), use static_cast.

1038

Please add braces here to this case, so that the local variables scope is tied to the case (see e.g. case SHT_SYMTAB_SHNDX).

1042

Don't use C-style casts. Also, this is the wrong thing to do here anyway. Compare the bytes against an array of uint8_t, rather than converting it to a string. Note that uint8_t does not necessarily convert trivially to char.

1043

I feel like most of this new code should be in a different function, which would add the section to the object, the call to which would be guarded by this if.

1044

Remove this blank line.

tools/llvm-objcopy/Object.h

400

What is DecompressedAlign actually used for? You should certainly be setting Align.

tools/llvm-objcopy/llvm-objcopy.cpp

436

doReplacement isn't clear what it means from the name alone. Perhaps shouldReplace would be clearer.

452

NS? Use a clearer name, please, e.g. NewSection

677–689

No need for the '{' and '}' here and in the else.

1023–1024

"... as well as --decompress-debug-sections at the same time" -> "... at the same time as --decompress-debug-sectons"

[jhenderson]

I may have missed this somewhere, but what happens if you specify --decompress/--compress-debug-sections on a system without zlib?

[plotfi]

In D51841#1234544, @jhenderson wrote:

I may have missed this somewhere, but what happens if you specify --decompress/--compress-debug-sections on a system without zlib?

For decompress right now it probably hits an unreachable, looking at the current code. But for compress it hits an error message.

What you're supposed to do is call zlib::isAvailable() to see if zlib is available and if not just print an error. llvm/lib/Support/Compression.cpp has unreachable versions of all the other zlib library functions.

tools/llvm-objcopy/Object.cpp

155

I don't want to use ArrayRefs like that again. They blow up when you compile with clang-8 and the bots fail. I'd rather just use StringRef.

173

the choices are:

Error uncompress(StringRef InputBuffer, char *UncompressedBuffer,
                 size_t &UncompressedSize);

Error uncompress(StringRef InputBuffer,
                 SmallVectorImpl<char> &UncompressedBuffer,
                 size_t UncompressedSize);

I dont want to use the one that passes UncompressedSize by reference.

175

Ah sorry about that.

[plotfi]

tools/llvm-objcopy/Object.cpp

166

I will not use an ArrayRef like that unless I can be convince something like ArrayRef<uint8_t> foo = { 'F', 'O', 'O' } points foo to an allocated array and that "{ 'F', 'O', 'O' }" isn't immediately deallocated. From what I can I have seen, this will not work on clang-8. I will use a std::vector for now.

[plotfi]

tools/llvm-objcopy/Object.h

400

I'm not actually completely sure. But it's in the ELF Chdr so I believe it's just encoding the alignment of the section from what be was before being compressed.

[plotfi]

test/tools/llvm-objcopy/compress-debug-sections.test

6

I don't understand what you mean here. Could you please explain?

test/tools/llvm-objcopy/decompress-debug-sections-zlib-gnu.test

8 ↗

(On Diff #165331)

Removed the test, simply added he decompression checks to an existing test that does most of this.

[plotfi]

-Cleanup: removing DecompressableSection, reusing CompressedSection.

[alexander-shaposhnikov]

tools/llvm-objcopy/Object.h

396

explicit

[jhenderson]

You've forgotten to include context in the latest diff. I can't review some aspects of this change without it.

test/tools/llvm-objcopy/compress-debug-sections.test

6

If you look at a lot of our other feature tests, we make a copy of the input before running llvm-objcopy and then compare %t.o against that copy, to show that llvm-objcopy didn't do an in-place modification of the file.

tools/llvm-objcopy/Object.cpp

139

This is okay, but you can actually do the following, which is probably even simpler:

static const uint8_t ZlibGnuMagic[4] = {'Z', 'L', 'I', 'B'};

You can then use sizeof to get the size, and std::begin/std::end for the start and end of the array.

172

You're still using unsigned. Use size_t or uint64_t not unsigned. This will fail for very large ELF files.

tools/llvm-objcopy/Object.h

400

Sorry, I meant, what is the DecompressedAlign member variable used for. Can we delete it?

tools/llvm-objcopy/llvm-objcopy.cpp

436

You should probably use function_ref instead of std::function, if I'm not mistaken. Also, I think that shouldReplace and addSection should be upper-case (i.e. ShouldReplace etc), since they are technically variables here.

1023

"can not" -> cannot

It feels to me like it would make sense to check both decompress and compress here, rather than in two completely different places, since they are closely related. I'd rather not rely on the library providing error messages, when we can error early and up-front.

1023–1024

You can remove the last part of the error message after "--decompress-debug-sections"

[jhenderson]

test/tools/llvm-objcopy/compress-debug-sections-zlib-gnu.test

11

This still isn't a good enough test. All it does is show that we have a section named .debug_foo after decompression. It shows nothing about the properties of the section header or that the section has been decompressed. You need to check both (e.g. by comparing the contents of the section/section header before and after decompression). If you bring across the comparisons in compress-debug-sections.test, then that at least will cover the contents aspect.

tools/llvm-objcopy/Object.h

369

Why this? You should just provide getters. Don't use friend here.

[plotfi]

test/tools/llvm-objcopy/compress-debug-sections-zlib-gnu.test

11

Added checks for flags and content. Lemme know if these are sufficient.

tools/llvm-objcopy/llvm-objcopy.cpp

1023

I could do that in an NFC commit after this one.

[alexander-shaposhnikov]

but, pls, also wait for Jake or James

[jhenderson]

Still a few comments to address, but this is mostly okay now.

test/tools/llvm-objcopy/compress-debug-sections-zlib-gnu.test

11

Thanks. One more suggestion: change "CHECK-FLAGS-DECOMPRESSED" to "CHECK-HEADER" and check the size field too. Then also add a check for this against the original object file too. That way, you'll be showing that the decompressed version is the same as the original version pre-compression.

test/tools/llvm-objcopy/compress-debug-sections.test

6

As with other tests, this test (or possibly the specific compress and decompress tests) should show that the inputs are not modified when using the specified switch.

This hasn't been addressed.

tools/llvm-objcopy/Object.h

400

Sorry, I meant, what is the DecompressedAlign member variable used for. Can we delete it?

This hasn't been addressed yet either. Is the DecompressedSection::DecompressedAlign variable used for anything?

Similarly, why do we need a separate DecompressedSize member variable for this section?

What is wrong with just using the Size and Align variables available in the base class?

tools/llvm-objcopy/llvm-objcopy.cpp

436

This comment hasn't been addressed yet either.

1023

If by that, you mean move the compress/zlib available check to the decompress check's location, that's fine.

[plotfi]

test/tools/llvm-objcopy/compress-debug-sections.test

6

I dont understand what you mean? Do you mean the decompressed section is the same as original?

[jhenderson]

test/tools/llvm-objcopy/compress-debug-sections.test

6

No. I mean you should look at the examples in the testsuite, such as discard-all.test and do something similar to show that the input file (%t.o) is not modified, by copying it first and comparing it against the copy after you have run llvm-objcopy.

[plotfi]

• changed std::function to llvm::function_ref
• added cp %t %t.o to tests

[plotfi]

test/tools/llvm-objcopy/compress-debug-sections.test

6

I hope this is what you meant.

[jhenderson]

LGTM, if you make the suggested test changes.

test/tools/llvm-objcopy/compress-debug-sections-zlib-gnu.test

4

Sorry, I guess I've confused you. You don't need to copy the input in this particular test, as long as one test (i.e. compress-debug-sections.test - see my inline comments) shows that llvm-objcopy does not change the input.

When you do llvm-readobj -relocations -s %t below, just do it on %t.o.

test/tools/llvm-objcopy/compress-debug-sections.test

5

Rename %t to %t.o, and copy it to %t.copy.o. This is more self-documenting as to the purpose, I think.

6

Nearly, I'll be more specific below.

9

Add the following here:

# RUN: cp %tz.o %tz.copy.o
# RUN: cp %tzg.o %tzg.copy.o

20

Add the following here:

# RUN: cmp %t.o %t.copy.o
# RUN: cmp %tz.o %tz.copy.o
# RUN: cmp %tzg.o %tzg.copy.o

test/tools/llvm-objcopy/compress-and-decompress-debug-sections-error.test

+# RUN: yaml2obj %p/Inputs/compress-debug-sections.yaml -o %t.o
+# RUN: not llvm-objcopy --compress-debug-sections=zlib --decompress-debug-sections %t.o 2>&1 | FileCheck %s
+
+# CHECK: Cannot specify --compress-debug-sections at the same time as --decompress-debug-sections at the same time.
+

test/tools/llvm-objcopy/compress-debug-sections-zlib-gnu.test

 # REQUIRES: zlib
 
-# RUN: yaml2obj %p/Inputs/compress-debug-sections.yaml -o %t.o
+# RUN: yaml2obj %p/Inputs/compress-debug-sections.yaml -o %t
+# RUN: cp %t %t.o
 # RUN: llvm-objcopy --compress-debug-sections=zlib-gnu %t.o %t-compressed.o
+# RUN: llvm-objcopy --decompress-debug-sections %t-compressed.o %t-decompressed.o
 
 # RUN: llvm-objdump -s %t.o -section=.debug_foo | FileCheck %s
 # RUN: llvm-objdump -s %t-compressed.o | FileCheck %s --check-prefix=CHECK-COMPRESSED
 # RUN: llvm-readobj -relocations -s %t-compressed.o | FileCheck %s --check-prefix=CHECK-FLAGS
+# RUN: llvm-readobj -relocations -s %t-decompressed.o | FileCheck %s --check-prefix=CHECK-FLAGS-DECOMPRESSED
+# RUN: llvm-objdump -s %t-decompressed.o -section=.debug_foo | FileCheck %s
 
 # CHECK: .debug_foo:
+# CHECK-NEXT: 0000 00000000 00000000
+
+# CHECK-FLAGS-DECOMPRESSED: Index: 1
+# CHECK-FLAGS-DECOMPRESSED-NEXT: Name: .debug_foo
+# CHECK-FLAGS-DECOMPRESSED-NEXT: Type: SHT_PROGBITS
+# CHECK-FLAGS-DECOMPRESSED-NEXT: Flags [
+# CHECK-FLAGS-DECOMPRESSED-NEXT: ]
 
 # CHECK-COMPRESSED: .zdebug_foo:
 # CHECK-COMPRESSED: ZLIB
 # CHECK-COMPRESSED: .notdebug_foo:
 
 # CHECK-FLAGS-NOT: Name: .debug_foo
 # CHECK-FLAGS: Index: 1
 # CHECK-FLAGS-NEXT: Name: .zdebug_foo

test/tools/llvm-objcopy/compress-debug-sections-zlib.test

 # REQUIRES: zlib
 
-# RUN: yaml2obj %p/Inputs/compress-debug-sections.yaml -o %t.o
+# RUN: yaml2obj %p/Inputs/compress-debug-sections.yaml -o %t
+# RUN: cp %t %t.o
 # RUN: llvm-objcopy --compress-debug-sections=zlib %t.o %t-compressed.o
+# RUN: llvm-objcopy --decompress-debug-sections %t-compressed.o %t-decompressed.o
 
 # RUN: llvm-objdump -s %t.o -section=.debug_foo | FileCheck %s
 # RUN: llvm-objdump -s %t-compressed.o | FileCheck %s --check-prefix=CHECK-COMPRESSED
 # RUN: llvm-readobj -relocations -s %t-compressed.o | FileCheck %s --check-prefix=CHECK-FLAGS
+# RUN: llvm-readobj -relocations -s %t-decompressed.o | FileCheck %s --check-prefix=CHECK-FLAGS-DECOMPRESSED
+# RUN: llvm-objdump -s %t-decompressed.o -section=.debug_foo | FileCheck %s
 
 # CHECK: .debug_foo:
+# CHECK-NEXT: 0000 00000000 00000000
+
+# CHECK-FLAGS-DECOMPRESSED: Index: 1
+# CHECK-FLAGS-DECOMPRESSED-NEXT: Name: .debug_foo
+# CHECK-FLAGS-DECOMPRESSED-NEXT: Type: SHT_PROGBITS
+# CHECK-FLAGS-DECOMPRESSED-NEXT: Flags [
+# CHECK-FLAGS-DECOMPRESSED-NEXT: ]
 
 # CHECK-COMPRESSED: .debug_foo:
 # CHECK-COMPRESSED: .notdebug_foo:
 
 # CHECK-FLAGS: Index: 1
 # CHECK-FLAGS-NEXT: Name: .debug_foo
 # CHECK-FLAGS-NEXT: Type: SHT_PROGBITS
 # CHECK-FLAGS-NEXT: Flags [

test/tools/llvm-objcopy/compress-debug-sections.test

+# REQUIRES: zlib
+
+# RUN: yaml2obj %p/Inputs/compress-debug-sections.yaml -o %t
+# RUN: cp %t %t.o
+
+# RUN: llvm-objcopy --compress-debug-sections=zlib     %t.o %tz.o
+# RUN: llvm-objcopy --compress-debug-sections=zlib-gnu %t.o %tzg.o
+
+# RUN: llvm-objcopy --decompress-debug-sections %tz.o  %t2.o
+# RUN: llvm-objcopy --decompress-debug-sections %tzg.o %t3.o
+
+# Using redirects to avoid llvm-objdump from printing the filename.
+# RUN: llvm-objdump -s -section=.debug_str - < %t  > %t.txt
+# RUN: llvm-objdump -s -section=.debug_str - < %t2.o > %t2.txt
+# RUN: llvm-objdump -s -section=.debug_str - < %t3.o > %t3.txt
+
+# RUN: diff %t.txt %t2.txt
+# RUN: diff %t.txt %t3.txt
+

tools/llvm-objcopy/ObjcopyOpts.td

defm output_target : Eq<"output-target">,
                      HelpText<"Format of the output file">,
                      Values<"binary">;
 def compress_debug_sections : Flag<["--", "-"], "compress-debug-sections">;
 def compress_debug_sections_eq : Joined<["--", "-"], "compress-debug-sections=">,
                                  MetaVarName<"[ zlib | zlib-gnu ]">,
                                  HelpText<"Compress DWARF debug sections using "
                                           "specified style. Supported styles: "
                                           "'zlib-gnu' and 'zlib'">;
+def decompress_debug_sections : Flag<["-", "--"], "decompress-debug-sections">,
+                                HelpText<"Decompress DWARF debug sections.">;
 def O : JoinedOrSeparate<["-"], "O">,
         Alias<output_target>;
 defm split_dwo : Eq<"split-dwo">,
                  MetaVarName<"dwo-file">,
                  HelpText<"Equivalent to extract-dwo on the input file to <dwo-file>, then strip-dwo on the input file">;
 
 def preserve_dates : Flag<[ "-", "--" ], "preserve-dates">,
                      HelpText<"Preserve access and modification timestamps">;

tools/llvm-objcopy/Object.h

 class StringTableSection;
 class SymbolTableSection;
 class RelocationSection;
 class DynamicRelocationSection;
 class GnuDebugLinkSection;
 class GroupSection;
 class SectionIndexSection;
 class CompressedSection;
+class DecompressedSection;
 class Segment;
 class Object;
 struct Symbol;
 
 class SectionTableRef {
   MutableArrayRef<std::unique_ptr<SectionBase>> Sections;
 
 public:
public:
   virtual void visit(const StringTableSection &Sec) = 0;
   virtual void visit(const SymbolTableSection &Sec) = 0;
   virtual void visit(const RelocationSection &Sec) = 0;
   virtual void visit(const DynamicRelocationSection &Sec) = 0;
   virtual void visit(const GnuDebugLinkSection &Sec) = 0;
   virtual void visit(const GroupSection &Sec) = 0;
   virtual void visit(const SectionIndexSection &Sec) = 0;
   virtual void visit(const CompressedSection &Sec) = 0;
+  virtual void visit(const DecompressedSection &Sec) = 0;
 };
 
 class SectionWriter : public SectionVisitor {
 protected:
   Buffer &Out;
 
 public:
   virtual ~SectionWriter(){};
 
   void visit(const Section &Sec) override;
   void visit(const OwnedDataSection &Sec) override;
   void visit(const StringTableSection &Sec) override;
   void visit(const DynamicRelocationSection &Sec) override;
   virtual void visit(const SymbolTableSection &Sec) override = 0;
   virtual void visit(const RelocationSection &Sec) override = 0;
   virtual void visit(const GnuDebugLinkSection &Sec) override = 0;
   virtual void visit(const GroupSection &Sec) override = 0;
   virtual void visit(const SectionIndexSection &Sec) override = 0;
   virtual void visit(const CompressedSection &Sec) override = 0;
+  virtual void visit(const DecompressedSection &Sec) override = 0;
 
   explicit SectionWriter(Buffer &Buf) : Out(Buf) {}
 };
 
 template <class ELFT> class ELFSectionWriter : public SectionWriter {
 private:
   using Elf_Word = typename ELFT::Word;
   using Elf_Rel = typename ELFT::Rel;
   using Elf_Rela = typename ELFT::Rela;
   using Elf_Sym = typename ELFT::Sym;
 
 public:
   virtual ~ELFSectionWriter() {}
   void visit(const SymbolTableSection &Sec) override;
   void visit(const RelocationSection &Sec) override;
   void visit(const GnuDebugLinkSection &Sec) override;
   void visit(const GroupSection &Sec) override;
   void visit(const SectionIndexSection &Sec) override;
   void visit(const CompressedSection &Sec) override;
+  void visit(const DecompressedSection &Sec) override;
 
   explicit ELFSectionWriter(Buffer &Buf) : SectionWriter(Buf) {}
 };
 
 #define MAKE_SEC_WRITER_FRIEND                                                 \
   friend class SectionWriter;                                                  \
   template <class ELFT> friend class ELFSectionWriter;
 
 class BinarySectionWriter : public SectionWriter {
 public:
   virtual ~BinarySectionWriter() {}
 
   void visit(const SymbolTableSection &Sec) override;
   void visit(const RelocationSection &Sec) override;
   void visit(const GnuDebugLinkSection &Sec) override;
   void visit(const GroupSection &Sec) override;
   void visit(const SectionIndexSection &Sec) override;
   void visit(const CompressedSection &Sec) override;
+  void visit(const DecompressedSection &Sec) override;
 
   explicit BinarySectionWriter(Buffer &Buf) : SectionWriter(Buf) {}
 };
 
 // The class Buffer abstracts out the common interface of FileOutputBuffer and
 // WritableMemoryBuffer so that the hierarchy of Writers depends on this
 // abstract interface and doesn't depend on a particular implementation.
 // TODO: refactor the buffer classes in LLVM to enable us to use them here
public:
   uint32_t EntrySize = 0;
   uint64_t Flags = 0;
   uint64_t Info = 0;
   uint64_t Link = ELF::SHN_UNDEF;
   uint64_t NameIndex = 0;
   uint64_t Offset = 0;
   uint64_t Size = 0;
   uint64_t Type = ELF::SHT_NULL;
   ArrayRef<uint8_t> OriginalData;
 
   SectionBase() = default;
   SectionBase(const SectionBase &) = default;
 
   virtual ~SectionBase() = default;
 
   virtual void initialize(SectionTableRef SecTable);
   virtual void finalize();
OwnedDataSection(StringRef SecName, ArrayRef<uint8_t> Data)
     Name = SecName.str();
     Type = ELF::SHT_PROGBITS;
     Size = Data.size();
     OriginalOffset = std::numeric_limits<uint64_t>::max();
   }
 
   void accept(SectionVisitor &Sec) const override;
 };
 
 class CompressedSection : public SectionBase {
   MAKE_SEC_WRITER_FRIEND
 
   DebugCompressionType CompressionType;
   uint64_t DecompressedSize;
   uint64_t DecompressedAlign;
   SmallVector<char, 128> CompressedData;
 
 public:
   CompressedSection(const SectionBase &Sec,
                     DebugCompressionType CompressionType);
+  CompressedSection(ArrayRef<uint8_t> CompressedData, uint64_t DecompressedSize,
+                    uint64_t DecompressedAlign);
+
+  uint64_t getDecompressedSize() const { return DecompressedSize; }
+  uint64_t getDecompressedAlign() const { return DecompressedAlign; }
+
+  void accept(SectionVisitor &Visitor) const override;
+
+  static bool classof(const SectionBase *S) {
+    return (S->Flags & ELF::SHF_COMPRESSED) ||
+           (StringRef(S->Name).startswith(".zdebug"));
+  }
+};
+
+class DecompressedSection : public SectionBase {
+  MAKE_SEC_WRITER_FRIEND
+
+public:
+  explicit DecompressedSection(const CompressedSection &Sec)
+      : SectionBase(Sec) {
+    Size =  Sec.getDecompressedSize();
+    Align = Sec.getDecompressedAlign();
+    Flags = (Flags & ~ELF::SHF_COMPRESSED);
+    if (StringRef(Name).startswith(".zdebug"))
+      Name = "." + Name.substr(2);
+  }
+
   void accept(SectionVisitor &Visitor) const override;
 };
 
 // There are two types of string tables that can exist, dynamic and not dynamic.
 // In the dynamic case the string table is allocated. Changing a dynamic string
 // table would mean altering virtual addresses and thus the memory image. So
 // dynamic string tables should not have an interface to modify them or
 // reconstruct them. This type lets us reconstruct a string table. To avoid

tools/llvm-objcopy/Object.cpp

 
 void Section::accept(SectionVisitor &Visitor) const { Visitor.visit(*this); }
 
 void SectionWriter::visit(const OwnedDataSection &Sec) {
   uint8_t *Buf = Out.getBufferStart() + Sec.Offset;
   std::copy(std::begin(Sec.Data), std::end(Sec.Data), Buf);
 }
 
+static const std::vector<uint8_t> ZlibGnuMagic = {'Z', 'L', 'I', 'B'};
+
+static bool isDataGnuCompressed(ArrayRef<uint8_t> Data) {
+  return Data.size() > ZlibGnuMagic.size() &&
+         std::equal(ZlibGnuMagic.begin(), ZlibGnuMagic.end(), Data.data());
+}
+
+template <class ELFT>
+static std::tuple<uint64_t, uint64_t>
+getDecompressedSizeAndAlignment(ArrayRef<uint8_t> Data) {
+  const bool IsGnuDebug = isDataGnuCompressed(Data);
+  const uint64_t DecompressedSize =
+      IsGnuDebug
+          ? support::endian::read64be(reinterpret_cast<const uint64_t *>(
+                Data.data() + ZlibGnuMagic.size()))
+          : reinterpret_cast<const Elf_Chdr_Impl<ELFT> *>(Data.data())->ch_size;
+  const uint64_t DecompressedAlign =
+      IsGnuDebug ? 1
+                 : reinterpret_cast<const Elf_Chdr_Impl<ELFT> *>(Data.data())
+                       ->ch_addralign;
+
+  return std::make_tuple(DecompressedSize, DecompressedAlign);
+}
+
+template <class ELFT>
+void ELFSectionWriter<ELFT>::visit(const DecompressedSection &Sec) {
+  uint8_t *Buf = Out.getBufferStart() + Sec.Offset;
+
+  if (!zlib::isAvailable()) {
+    std::copy(Sec.OriginalData.begin(), Sec.OriginalData.end(), Buf);
+    return;
+  }
+
+  const size_t DataOffset =
+      isDataGnuCompressed(Sec.OriginalData)
+          ? (ZlibGnuMagic.size() + sizeof(Sec.Size))
+          : sizeof(Elf_Chdr_Impl<ELFT>);
+
+  StringRef CompressedContent(
+      reinterpret_cast<const char *>(Sec.OriginalData.data()) + DataOffset,
+      Sec.OriginalData.size() - DataOffset);
+
+  SmallVector<char, 128> DecompressedContent;
+  if (Error E = zlib::uncompress(CompressedContent, DecompressedContent,
+                                 static_cast<size_t>(Sec.Size)))
+    reportError(Sec.Name, std::move(E));
+
+  std::copy(DecompressedContent.begin(), DecompressedContent.end(), Buf);
+}
+
+void BinarySectionWriter::visit(const DecompressedSection &Sec) {
+  error("Cannot write compressed section '" + Sec.Name + "' ");
+}
+
+void DecompressedSection::accept(SectionVisitor &Visitor) const {
+  Visitor.visit(*this);
+}
+
 void OwnedDataSection::accept(SectionVisitor &Visitor) const {
   Visitor.visit(*this);
 }
 
 void BinarySectionWriter::visit(const CompressedSection &Sec) {
   error("Cannot write compressed section '" + Sec.Name + "' ");
 }
 
ChdrSize =
                           sizeof(object::Elf_Chdr_Impl<object::ELF64BE>)),
                  std::max(sizeof(object::Elf_Chdr_Impl<object::ELF32LE>),
                           sizeof(object::Elf_Chdr_Impl<object::ELF32BE>)));
   }
   Size = ChdrSize + CompressedData.size();
   Align = 8;
 }
 
+CompressedSection::CompressedSection(ArrayRef<uint8_t> CompressedData,
+                                     uint64_t DecompressedSize,
+                                     uint64_t DecompressedAlign)
+    : CompressionType(DebugCompressionType::None),
+      DecompressedSize(DecompressedSize), DecompressedAlign(DecompressedAlign) {
+  OriginalData = CompressedData;
+}
+
 void CompressedSection::accept(SectionVisitor &Visitor) const {
   Visitor.visit(*this);
 }
 
 void StringTableSection::addString(StringRef Name) {
   StrTabBuilder.add(Name);
   Size = StrTabBuilder.getSize();
 }
SectionBase &ELFBuilder<ELFT>::makeSection(const Elf_Shdr &Shdr) {
   }
   case SHT_SYMTAB_SHNDX: {
     auto &ShndxSection = Obj.addSection<SectionIndexSection>();
     Obj.SectionIndexTable = &ShndxSection;
     return ShndxSection;
   }
   case SHT_NOBITS:
     return Obj.addSection<Section>(Data);
-  default:
+  default: {
     Data = unwrapOrError(ElfFile.getSectionContents(&Shdr));
+
+    if (isDataGnuCompressed(Data) || (Shdr.sh_flags & ELF::SHF_COMPRESSED)) {
+      uint64_t DecompressedSize, DecompressedAlign;
+      std::tie(DecompressedSize, DecompressedAlign) =
+          getDecompressedSizeAndAlignment<ELFT>(Data);
+      return Obj.addSection<CompressedSection>(Data, DecompressedSize,
+                                               DecompressedAlign);
+    }
+
     return Obj.addSection<Section>(Data);
   }
   }
+}
 
 template <class ELFT> void ELFBuilder<ELFT>::readSectionHeaders() {
   uint32_t Index = 0;
   for (const auto &Shdr : unwrapOrError(ElfFile.sections())) {
     if (Index == 0) {
       ++Index;
       continue;
     }

tools/llvm-objcopy/llvm-objcopy.cpp

struct CopyConfig {
   bool StripAll = false;
   bool StripAllGNU = false;
   bool StripDWO = false;
   bool StripDebug = false;
   bool StripNonAlloc = false;
   bool StripSections = false;
   bool StripUnneeded = false;
   bool Weaken = false;
+  bool DecompressDebugSections = false;
   DebugCompressionType CompressionType = DebugCompressionType::None;
 };
 
 // Configuration for the overall invocation of this tool. When invoked as
 // objcopy, will always contain exactly one CopyConfig. When invoked as strip,
 // will contain one or more CopyConfigs.
 struct DriverConfig {
   SmallVector<CopyConfig, 1> CopyConfigs;
return StringRef(Section.Name).startswith(".zdebug") ||
          (Section.Flags & ELF::SHF_COMPRESSED);
 }
 
 static bool isCompressable(const SectionBase &Section) {
   return !isCompressed(Section) && isDebugSection(Section) &&
          Section.Name != ".gdb_index";
 }
 
-static void compressSections(const CopyConfig &Config, Object &Obj,
-                             SectionPred &RemovePred) {
-  SmallVector<SectionBase *, 13> ToCompress;
+static void replaceDebugSections(
+    const CopyConfig &Config, Object &Obj, SectionPred &RemovePred,
+    function_ref<bool(const SectionBase &)> shouldReplace,
+    function_ref<SectionBase *(const SectionBase *)> addSection) {
+  SmallVector<SectionBase *, 13> ToReplace;
   SmallVector<RelocationSection *, 13> RelocationSections;
   for (auto &Sec : Obj.sections()) {
     if (RelocationSection *R = dyn_cast<RelocationSection>(&Sec)) {
-      if (isCompressable(*R->getSection()))
+      if (shouldReplace(*R->getSection()))
         RelocationSections.push_back(R);
       continue;
     }
 
-    if (isCompressable(Sec))
-      ToCompress.push_back(&Sec);
+    if (shouldReplace(Sec))
+      ToReplace.push_back(&Sec);
   }
 
-  for (SectionBase *S : ToCompress) {
-    CompressedSection &CS =
-        Obj.addSection<CompressedSection>(*S, Config.CompressionType);
+  for (SectionBase *S : ToReplace) {
+    SectionBase *NewSection = addSection(S);
 
     for (RelocationSection *RS : RelocationSections) {
       if (RS->getSection() == S)
-        RS->setSection(&CS);
+        RS->setSection(NewSection);
     }
   }
 
-  RemovePred = [RemovePred](const SectionBase &Sec) {
-    return isCompressable(Sec) || RemovePred(Sec);
+  RemovePred = [shouldReplace, RemovePred](const SectionBase &Sec) {
+    return shouldReplace(Sec) || RemovePred(Sec);
   };
 }
 
 // This function handles the high level operations of GNU objcopy including
 // handling command line options. It's important to outline certain properties
 // we expect to hold of the command line operations. Any operation that "keeps"
 // should keep regardless of a remove. Additionally any removal should respect
 // any previous removals. Lastly whether or not something is removed shouldn't
if ((!Config.SymbolsToKeep.empty() || Config.KeepFileSymbols) &&
     RemovePred = [&Obj, RemovePred](const SectionBase &Sec) {
       if (&Sec == Obj.SymbolTable || &Sec == Obj.SymbolTable->getStrTab())
         return false;
       return RemovePred(Sec);
     };
   }
 
   if (Config.CompressionType != DebugCompressionType::None)
-    compressSections(Config, Obj, RemovePred);
+    replaceDebugSections(Config, Obj, RemovePred, isCompressable,
+                         [&Config, &Obj](const SectionBase *S) {
+                           return &Obj.addSection<CompressedSection>(
+                               *S, Config.CompressionType);
+                         });
+  else if (Config.DecompressDebugSections)
+    replaceDebugSections(
+        Config, Obj, RemovePred,
+        [](const SectionBase &S) { return isa<CompressedSection>(&S); },
+        [&Obj](const SectionBase *S) {
+          auto CS = cast<CompressedSection>(S);
+          return &Obj.addSection<DecompressedSection>(*CS);
+        });
 
   Obj.removeSections(RemovePred);
 
   if (!Config.SectionsToRename.empty()) {
     for (auto &Sec : Obj.sections()) {
       const auto Iter = Config.SectionsToRename.find(Sec.Name);
       if (Iter != Config.SectionsToRename.end()) {
         const SectionRename &SR = Iter->second;
static DriverConfig parseObjcopyOptions(ArrayRef<const char *> ArgsArr) {
   Config.StripNonAlloc = InputArgs.hasArg(OBJCOPY_strip_non_alloc);
   Config.StripUnneeded = InputArgs.hasArg(OBJCOPY_strip_unneeded);
   Config.ExtractDWO = InputArgs.hasArg(OBJCOPY_extract_dwo);
   Config.LocalizeHidden = InputArgs.hasArg(OBJCOPY_localize_hidden);
   Config.Weaken = InputArgs.hasArg(OBJCOPY_weaken);
   Config.DiscardAll = InputArgs.hasArg(OBJCOPY_discard_all);
   Config.OnlyKeepDebug = InputArgs.hasArg(OBJCOPY_only_keep_debug);
   Config.KeepFileSymbols = InputArgs.hasArg(OBJCOPY_keep_file_symbols);
+  Config.DecompressDebugSections =
+      InputArgs.hasArg(OBJCOPY_decompress_debug_sections);
   for (auto Arg : InputArgs.filtered(OBJCOPY_localize_symbol))
     Config.SymbolsToLocalize.push_back(Arg->getValue());
   for (auto Arg : InputArgs.filtered(OBJCOPY_keep_global_symbol))
     Config.SymbolsToKeepGlobal.push_back(Arg->getValue());
   for (auto Arg : InputArgs.filtered(OBJCOPY_keep_global_symbols))
     addGlobalSymbolsFromFile(Config.SymbolsToKeepGlobal, Arg->getValue());
   for (auto Arg : InputArgs.filtered(OBJCOPY_globalize_symbol))
     Config.SymbolsToGlobalize.push_back(Arg->getValue());
   for (auto Arg : InputArgs.filtered(OBJCOPY_weaken_symbol))
     Config.SymbolsToWeaken.push_back(Arg->getValue());
   for (auto Arg : InputArgs.filtered(OBJCOPY_strip_symbol))
     Config.SymbolsToRemove.push_back(Arg->getValue());
   for (auto Arg : InputArgs.filtered(OBJCOPY_keep_symbol))
     Config.SymbolsToKeep.push_back(Arg->getValue());
 
   Config.PreserveDates = InputArgs.hasArg(OBJCOPY_preserve_dates);
 
   DriverConfig DC;
   DC.CopyConfigs.push_back(std::move(Config));
+  if (Config.DecompressDebugSections &&
+      Config.CompressionType != DebugCompressionType::None) {
+    error("Cannot specify --compress-debug-sections at the same time as "
+          "--decompress-debug-sections at the same time");
+  }
+
+  if (Config.DecompressDebugSections && !zlib::isAvailable())
+    error("LLVM was not compiled with LLVM_ENABLE_ZLIB: cannot decompress.");
+
   return DC;
 }
 
 // ParseStripOptions returns the config and sets the input arguments. If a
 // help flag is set then ParseStripOptions will print the help messege and
 // exit.
 static DriverConfig parseStripOptions(ArrayRef<const char *> ArgsArr) {
   StripOptTable T;