[llvm-dev] [RFC] Support embedding bitcodes in LLD with LTO

[Josef Eisl via llvm-dev]

Thanks for sharing your thoughts!

On 31/01/2019 20:04, Rui Ueyama wrote:
> That feature is probably too specific to your project. Most projects 
> that use LTO are using LTO just because it generates better code. Your 
> project is special as your program itself can also interpret LLVM 
> bitcode, but that's not the case for most other programs.

I see that the requirement somewhat specific. On the other hand, the 
same feature is for example supported on Darwin via the __bundle 
section, so I'd see it more as a feature parity measure than something 
that is only of use to our project. (My view might be biased, though ;)

> I think the option that's closest to the one you are looking for is 
> `--plugin-opt=emit-llvm`. That option makes lld to make an output file 
> in the bitcode file format (so lld doesn't do LTO if the option is given 
> and instead writes a raw bitcode as an output). With that option, I 
> don't think it's too hard to embed bitcode file to your executable. Run 
> the linker twice, with and without `--plugin-opt=emit-llvm`, and embed 
> the generated bitcode file using objcopy.
 >
 > Does that work for you?
 >

Sure it does and I fully agree that it is currently possible to get to 
the result. Actually, there are many different ways to accomplish it 
using combinations of wllvm, gllvm, llvm-link, --plugin-opt=emit-llvm, 
objcopy, etc. We are currently using these but my feelings about the 
approaches are mixed. I see two downsides.
First, they require modifications to the build scripts. We want to 
support a variety of different build systems and make it as easy as 
possible for users to compile their projects for Sulong. Running the 
linker twice is not easy to accomplish in general, especially if the 
source project is not under our control. However, adding a linker flag 
is simpler in most cases.
Second, the approaches add new dependencies and their portability is 
limited. E.g. what about objcopy on Darwin? Anyway, as mentioned above, 
Darwin does support embedding linked bitcode, but then we have two 
distinct workflows for Linux and Darwin, which I think is not very user 
friendly.

That all said, I understand that there is some hesitation of bringing 
new, non-mainstream features to a project that needs to be stable and 
maintainable. Anyhow, the prototype we are currently experimenting with 
did no require too much work. It reuses existing code from clang (but 
unrelated to clang), which we moved to a common place in llvm. The rest 
of the patch is mainly option handling. There is hardly any duplication 
of logic. I see it more of an addition to existing functionality (i.e., 
to `--plugin-opt=emit-llvm`). All the pieces are already there.

Whatever the outcome of this RFC is, I guess I need thank you and all 
other contributors for providing such a stable and mature platform that 
allows us to do these kind of changes easily. Thanks. :)


> On Thu, Jan 31, 2019 at 4:18 AM Josef Eisl <josef.eisl at oracle.com 
> <mailto:josef.eisl at oracle.com>> wrote:
> 
>     Thanks for your response!
> 
>     On 30/01/2019 20:18, Rui Ueyama wrote:
>      > Hi Josef,
>      >
>      > Let me clarify my understanding. Do you want to keep original
>     bitcode
>      > files in the output executable when doing LTO, so that the resulting
>      > executable contains both compiled bitcode (which is in native
>     machine
>      > instructions) and original bitcode files?
> 
>     Exactly! Kind of analogous to what `clang -fembed-bitcode -c` does, but
>     for executables.
> 
>      >
>      > Did you try embedding bitcode files into existing ELF files using
>      > objcopy or linker option `--format=binary`?
> 
>     Yes, that is the alternative. However, having support in the linker for
>     that would require less tweaking of exiting build systems. Adding an
>     option to CFLAGS/LDFLAGS would then be sufficient.
> 
>      >
>      > On Mon, Jan 28, 2019 at 6:41 AM Josef Eisl via llvm-dev
>      > <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>     <mailto:llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>>
>     wrote:
>      >
>      >     Hi everybody!
>      >
>      >     I'm Josef and I'm working at Oracle Labs on Sulong [1,2], the
>     LLVM IR
>      >     execution engine in GraalVM [3]. In addition to executing
>     bare bitcode
>      >     files, Sulong also accepts ELF files with embedded bitcode
>     sections.
>      >     Therefore, it would be great if LLD in (Full)LTO mode would
>     support
>      >     embedding bitcode sections to the resulting object file. Is that
>      >     something that would be considered useful and worth contributing?
>      >
>      >     Thanks,
>      >     Josef
>      >
>