[LLVMdev] COFF on-disk section alignment Was: Re: [cfe-dev] [PATCH] Wrap clang modules inside Mach-O/ELF/COFF containers

[Adrian Prantl]

On Jan 30, 2015, at 6:04 PM, Adrian Prantl <aprantl at apple.com> wrote:
> The big drawback is that I can no longer influence the order in which the sections are emitted, which turns out to be a problem for COFF.
> Although COFF sections are marked with the correct alignment characteristics, they are not actually emitted aligned on disk. This is hopefully just a bug in our COFF backend. Since the DWARF sections are of arbitrary size and come before the serialized AST, this messes up the alignment for OnDiskHashTable. Until I figure out a solution for this problem, I’ve disabled module debug info ouput for COFF.

[CC’ing a bunch of people who recently touched the COFF streamer.]

I’m creating a new section via a global variable like this:

    auto *ASTSym = new llvm::GlobalVariable(
        *M, Ty, /*constant*/ true, llvm::GlobalVariable::InternalLinkage,
        Data, "__clang_ast");
    ASTSym->setAlignment(8);
    ASTSym->setSection("clangast”);

and llvm-readobj tells me it comes out as:

  Section {
    Number: 12
    Name: clangast (63 6C 61 6E 67 61 73 74)
    VirtualSize: 0x0
    VirtualAddress: 0x0
    RawDataSize: 148992
    PointerToRawData: 0x4FB   <-- 1275
    PointerToRelocations: 0x0
    PointerToLineNumbers: 0x0
    RelocationCount: 0
    LineNumberCount: 0
    Characteristics [ (0x40400040)
      IMAGE_SCN_ALIGN_8BYTES (0x400000)
      IMAGE_SCN_CNT_INITIALIZED_DATA (0x40)
      IMAGE_SCN_MEM_READ (0x40000000)
    ]
  }

or in assembly:

  ...
  Lsection_debug_loc:
        .section        .debug_ranges,"r"
  Ldebug_range:
        .section        clangast,"dr"
        .align  8
  ___clang_ast:
  ...

Even though the section has the IMAGE_SCN_ALIGN_8BYTES characteristic and the symbol is aligned, the ___clang_ast symbol is still emitted to disk immediately following the previous section and thus ends up at a random unaligned address. This breaks OnDiskHashtable when the object file is mmapped from disk later.

Am I using it wrong or is this a bug in the coff backend?
Any ideas how to fix it? 

-- adrian