[llvm-dev] Generating completely position agnostic code

[Jorg Brown via llvm-dev]

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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