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

[Adrian Prantl]

> On Feb 11, 2015, at 12:57 PM, David Majnemer <david.majnemer at gmail.com> wrote:
> 
> 
> 
> On Wed, Feb 11, 2015 at 11:39 AM, Adrian Prantl <aprantl at apple.com <mailto:aprantl at apple.com>> wrote:
> 
> On Jan 30, 2015, at 6:04 PM, Adrian Prantl <aprantl at apple.com <mailto: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.

I see. In this particular case, however, it is not even aligned on a 4-byte boundary (0x4FB==1275). Technically I think 4-byte alignment is all that OnDiskHashTable is asking for, so if we could get that right, it would be good enough for my use-case.

>  
> Any ideas how to fix it?
> 
> Nothing would stop us from having a little padding before the start of the section contents.

Do you have a pointer where to look for?

thanks!
adrian
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