[Support/ELF] - Added few constants and structs relative to compressed sections.

[grimar]

Summary

Patch adds few constants and structs to support compressed sections.

SHF_COMPRESSED intersects with platform specific XCORE_SHF_CP_SECTION,
both has value of 0x800U.

I took SHF_COMPRESSED and other values from glibc:
https://sourceware.org/git/?p=glibc.git;a=blob;f=elf/elf.h;hb=HEAD.

[emaste]

You can use http://www.sco.com/developers/gabi/latest/ch4.sheader.html as the canonical reference rather than glibc source

[ruiu]

As Ed wrote, please refer the official document in the commit message. With that, LGTM.

[davide]

LGTM after Ed's comment is addressed, thanks.

[grimar]

In D20209#428622, @emaste wrote:

You can use http://www.sco.com/developers/gabi/latest/ch4.sheader.html as the canonical reference rather than glibc source

Right. What I really wanted to say is that since "SHF_COMPRESSED intersects with platform specific XCORE_SHF_CP_SECTION," then SHF_COMPRESSED looks to be correct as glibc uses that right now. I am not sure what is XCORE_SHF_CP_SECTION and if it is correct that it has the same value.
llvm-readobj currently parses ELFs with compressed sections like:

  Section {
    Index: 16
    Name: .debug_str (163)
    Type: SHT_PROGBITS (0x1)
    Flags [ (0x830)
      SHF_MERGE (0x10)
      SHF_STRINGS (0x20)
      XCORE_SHF_CP_SECTION (0x800)
    ]
...

I think XCORE_SHF_CP_SECTION should be changed to SHF_COMPRESSED (this patch allows to do that), but I do not yet know will that affect on something that probably uses XCORE_SHF_CP_SECTION (I mean if some test check that constant name). I wonder if "_CP_" means ComPressed either ? Why the 'duplication' of the flag is requred then ?

[grimar]

Ah, according to comment CP is constant pool.

ELF.h

enum : unsigned {
   SHF_OS_NONCONFORMING = 0x100U,
 
   // This section is a member of a section group.
   SHF_GROUP = 0x200U,
 
   // This section holds Thread-Local Storage.
   SHF_TLS = 0x400U,
 
+  // Identifies a section containing compressed data.
+  SHF_COMPRESSED = 0x800U,
+
   // This section is excluded from the final executable or shared library.
   SHF_EXCLUDE = 0x80000000U,
 
   // Start of target-specific flags.
 
   /// XCORE_SHF_CP_SECTION - All sections with the "c" flag are grouped
   /// together by the linker to form the constant pool and the cp register is
   /// set to the start of the constant pool by the boot code.
enum {
   VER_NEED_CURRENT = 1
 };
 
 // SHT_NOTE section types
 enum {
   NT_GNU_BUILD_ID = 3
 };
 
+// Compressed section header for ELF32.
+struct Elf32_Chdr {
+  Elf32_Word ch_type;
+  Elf32_Word ch_size;
+  Elf32_Word ch_addralign;
+};
+
+// Compressed section header for ELF64.
+struct Elf64_Chdr {
+  Elf64_Word ch_type;
+  Elf64_Word ch_reserved;
+  Elf64_Xword ch_size;
+  Elf64_Xword ch_addralign;
+};
+
+// Legal values for ch_type field of compressed section header.
+enum {
+  ELFCOMPRESS_ZLIB = 1,            // ZLIB/DEFLATE algorithm.
+  ELFCOMPRESS_LOOS = 0x60000000,   // Start of OS-specific.
+  ELFCOMPRESS_HIOS = 0x6fffffff,   // End of OS-specific.
+  ELFCOMPRESS_LOPROC = 0x70000000, // Start of processor-specific.
+  ELFCOMPRESS_HIPROC = 0x7fffffff  // End of processor-specific.
+};
+
 } // end namespace ELF
 
 } // end namespace llvm
 
 #endif