[PATCH] D52917: Avoid unnecessary buffer allocation and memcpy for compressed sections.

[George Rimar via Phabricator via llvm-commits]

grimar added a comment.

Idea by itself LG. My suggestions about implementation details are below.



================
Comment at: lld/ELF/InputSection.cpp:268
+  RawData = RawData.slice(sizeof(*Hdr));
 }
 
----------------
What do you think about still using the `Decompressor` here for parsing the
headers and checking `zlib` availability?
Like below.

```
  Decompressor Dec = check(Decompressor::create(Name, toStringRef(RawData),
                                                Config->IsLE, Config->Is64));
  UncompressedSize = Dec.getDecompressedSize();
  if (Name.startswith(".zdebug")) {
    RawData = RawData.slice(12);
    Name = Saver.save("." + Name.substr(2));
    return;
  }
  assert(Flags & SHF_COMPRESSED);
  Flags &= ~(uint64_t)SHF_COMPRESSED;

  // New-style 64-bit header
  if (Config->Is64)
    RawData = RawData.slice(sizeof(Elf64_Chdr));
  else
    RawData = RawData.slice(sizeof(Elf32_Chdr));
```

or, if we add `getCompressedData` method to `Decompressor` class, it can be:

```
  Decompressor Dec = check(Decompressor::create(Name, toStringRef(RawData),
                                                Config->IsLE, Config->Is64));
  UncompressedSize = Dec.getDecompressedSize();
  RawData = Dec.getCompressedData();

  if (Name.startswith(".zdebug")) {
    Name = Saver.save("." + Name.substr(2));
    return;
  }

  assert(Flags & SHF_COMPRESSED);
  Flags &= ~(uint64_t)SHF_COMPRESSED;
```


================
Comment at: lld/test/ELF/strip-debug.s:3
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
 # RUN: ld.lld %t -o %t2 --strip-debug
 # RUN: llvm-readobj -sections %t2 | FileCheck %s
----------------
If you remove `--strip-debug`, this will crash the linker because of
` llvm_unreachable("zlib::uncompress is unavailable");` on windows, where zlib is not available.

You may want to check `zlib::isAvailable()` or use `Decompressor`.


https://reviews.llvm.org/D52917