[llvm-dev] Generating completely position agnostic code

[Peter Smith via llvm-dev]

I'm not sure if you are wanting to modify LLVM to achieve your goal,
or just use the functionality that already exists. If you are willing
to make changes there are a couple of options in the ARM backend
-fropi and -frwpi that are close, but unfortunately don't support C++.
My understanding is that there are constant data such as vtables
containing pointers that you would need quite a bit of work to turn
into something that wouldn't require some kind of relocation. The
initial RFC has an explanation
https://lists.llvm.org/pipermail/llvm-dev/2015-December/093022.html
there is a mention of a -fallow-unsupported option to allow c++ use,
but I expect that this would only work for a subset of C++.

I don't think that this is the same problem that you are trying to
solve here though. I'm guessing that you are providing a fixed address
libc external to the position independent code that you interface with
via a table of pointers? I have seen that being done, one way of doing
it is to provide the linker with the address of the the libc functions
via absolute symbols, the table of function pointers uses something
like the linker --wrap symbol to do the indirection.

Peter


On Mon, 1 Jul 2019 at 09:47, Hayden Livingston via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> It is wholly self-contained. It's code that has no references to
> anything beyond a set of pointers passed in as arguments to the
> function. This piece of code doesn't do any OS work at all. It is
> purely calling function pointers, doing math and allocating memory.
>
> On Mon, Jul 1, 2019 at 12:57 AM Jorg Brown <jorg.brown at gmail.com> wrote:
> >
> > Qs for you:
> >
> > The code that is being loaded from disk... is it wholly self-contained, or is your executable potentially made up of several pieces that each need to be loaded from disk?
> >
> > What does it mean to use the STL but not have global variables?  std::cout is a global variable, so you can't even do Hello World without globals.
> >
> > = = =
> >
> > Architectures such as 68K and PowerPC and RISC-V have a dedicated register for accessing global variables, rather than the PC-relative globals used in other architectures.  This makes them inherently more amenable to what you describe, since you can put the "array of function pointers" into global space, as part of setting up global space in general, and then load the code from disk, and go.  There is no relocation needed since all access to globals is done via the global register, not relative to wherever the program was loaded.  Of course, access to something like libc might normally need post-loading relocation, but if you do what you're talking about and use an "array of function pointers" to get to libc, no relocation would be needed.
> >
> > For what it's worth, the original 68K-based Macintosh used a scheme quite similar to this.  The big difference for the Mac was that to get to the OS (the equivalent of libc), it didn't use an array of function pointers, per se; it used a certain range of illegal instructions, which generated exceptions when used, and the (highly optimized) exception handlers would recover from the exception by dispatching to an OS routine determined by the specific bits in the illegal instruction.
> >
> > On Sun, Jun 30, 2019 at 9:07 PM Hayden Livingston via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> >>
> >> I'm on a mission to generate code that can be loaded from disk without
> >> any modifications. This means no relocations can occur.
> >>
> >> Trying to see if this can be done for C++ code that uses STL but has
> >> no global variables, and a single function, but of course Clang will
> >> generate more functions for STL code.
> >>
> >> I want to provide an array of function pointers so that for all
> >> interactions STL needs to do with LIBC that I'm able to just provide
> >> it via indirect calls.
> >>
> >> Has anyone had success with such a thing in LLVM?
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