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

[Steven Wu via llvm-dev]

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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