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

[David Majnemer]

On Wed, Feb 11, 2015 at 11:39 AM, Adrian Prantl <aprantl at apple.com> wrote:

>
> 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?
>

I don't think this is a bug.  Here is the relevant text from the COFF
specification for the PointerToRawData field:
For executable images, this must be a multiple of FileAlignment from the
optional header.
For object files, the value should be aligned on a 4byte boundary for best
performance.

We aren't required to align the section contents in object files.


> Any ideas how to fix it?
>

Nothing would stop us from having a little padding before the start of the
section contents.


>
> -- adrian
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