[llvm-dev] Issue with inline assembly, function inlining, and position independent code

[Craig Topper via llvm-dev]

Trying to look at your example. Why is the constraint on the pointer
variable to the inline assembly "ri" instead of just "r". Changing it to
"r" seems to compile.

~Craig


On Thu, Jul 30, 2020 at 10:59 PM Dominic Chen via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Code: https://godbolt.org/z/T397fo
>
> I'm running some performance experiments on a x86-64 linux system, where
> I've modified LLVM to reserve a register, and I'd like to use that register
> in my code. Currently, I'm using %r12d, which is callee save, so I don't
> need to worry about compatibility with existing libraries or system calls.
> For security reasons, the generated binaries need to be position
> independent.
>
> To access the register, I need to use inline assembly; either for all
> dependent computations, or just for moving the register value into a C++
> variable. Since the latter results in a second register allocation, I'm
> just doing assembly for everything. So, I have a function `foo` that
> contains some inline assembly.
>
> The problem is that its input arguments can sometimes be pointers to
> constant data, and after function inlining, the result may no longer link
> because R_X86_64_32S relocations are not permitted. I need to get LLVM to
> introduce a load-effective address (`lea`) to move the pointer into a
> register, if applicable, but I haven't found a method that both works and
> is performant. I could modify LLVM locally to make this happen, but this
> seems pretty deep in the X86 backend and not straightforward?
>
>    1. Make the input argument `volatile` or introduce a `volatile`
>    variable. Works, but adds two moves to and from the stack.
>    2. Introduce a `register` variable equal to the input argument.
>    Results in an error with C++17, and doesn't seem to work otherwise.
>    3. Only use inline assembly to move %r12d into a C variable. Mostly
>    works, but the variable isn't incremented afterwards, requires an extra
>    register, and adds multiple extra moves.
>    4. Use the local register variable extension to force a C variable to
>    %r12d. Not supported by Clang/LLVM.
>
> Thanks,
>
> Dominic
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