[llvm-dev] End-to-end -fembed-bitcode .llvmbc and .llvmcmd

[Sean Bartell via llvm-dev]

Hi Mircea,

If you use an ordinary linker that concatenates .llvmbc sections, you can use this code to get the size of each bitcode module. As far as I know, there's no clean way to separate the .llvmcmd sections without making assumptions about what options were used.

// Given a bitcode file followed by garbage, get the size of the actual
// bitcode. This only works correctly with some kinds of garbage (in
// particular, it will work if the bitcode file is followed by zeros, or if
// it's followed by another bitcode file).
size_t GetBitcodeSize(MemoryBufferRef Buffer) {
  const unsigned char *BufPtr =
      reinterpret_cast<const unsigned char *>(Buffer.getBufferStart());
  const unsigned char *EndBufPtr =
      reinterpret_cast<const unsigned char *>(Buffer.getBufferEnd());
  if (isBitcodeWrapper(BufPtr, EndBufPtr)) {
    const unsigned char *FixedBufPtr = BufPtr;
    if (SkipBitcodeWrapperHeader(FixedBufPtr, EndBufPtr, true))
      report_fatal_error("Invalid bitcode wrapper");
    return EndBufPtr - BufPtr;
  }

  if (!isRawBitcode(BufPtr, EndBufPtr))
    report_fatal_error("Invalid magic bytes; not a bitcode file?");

  BitstreamCursor Reader(Buffer);
  Reader.Read(32); // skip signature
  while (true) {
    size_t EntryStart = Reader.getCurrentByteNo();
    BitstreamEntry Entry =
        Reader.advance(BitstreamCursor::AF_DontAutoprocessAbbrevs);
    if (Entry.Kind == BitstreamEntry::SubBlock) {
      if (Reader.SkipBlock())
        report_fatal_error("Invalid bitcode file");
    } else {
      // We must have reached the end of the module.
      return EntryStart;
    }
  }
}

Sean

On Thu, Aug 27, 2020, at 13:17, Steven Wu via llvm-dev wrote:
> Hi Mircea 
> 
> From the RFC you mentioned, that is a Darwin specific implementation, which later got extended to support other targets. The main use case for the embed bitcode option is to allow compiler passing intermediate IR and command flags in the object file it produced for later use. For Darwin, it is used for bitcode recompilation, and some might use it to achieve other goals.
> 
> In order to use this information properly, you needs to have tools that understand the layout and sections for embedded bitcode. You can't just use an ordinary linker, because like you said, an ELF linker will just append the bitcode. Depending on what you are trying to achieve, you need to implement the downstream tools, like linker, binary analysis tools, etc. to understand this concept.
> 
> Steven
> 
>> On Aug 24, 2020, at 7:10 PM, Mircea Trofin via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>> 
>> Hello, 
>> 
>> I'm trying to understand how .llvmbc and .llvmcmd fit into an end-to-end story. From the RFC <http://lists.llvm.org/pipermail/llvm-dev/2016-February/094851.html>, and reading through the implementation, I'm piecing together that the goal was to enable capturing IR right after clang and before passing it to LLVM's optimization passes, as well as the command line options needed for later compiling that IR to the same native object it was compiled to originally (with the same compiler).
>> 
>> Here's what I don't understand: say you have a.o and b.o compiled with -fembed-bitcode=all. They are linked into a binary called my_binary. How do you re-create the corresponding IR for modules a and b (let's call them a.bc and b.bc), and their corresponding command lines? From what I can tell, the linker just concatenates the IR for a and b in my_binary's .llvmbc, and the same for the command line in .llvmcmd. Is there a separator maybe I missed? For .llvmcmd, I could see how *maybe* -cc1 could be that separator, what about the .llvmbc part? The magic number?
>> 
>> Thanks!
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> 
> *Attachments:*
>  * ATT00001.txt
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