[llvm-dev] How to pass the current function name at runtime without additional Global variables?

[David Blaikie via llvm-dev]

On Sun, Jan 9, 2022 at 12:42 AM Alberto Barbaro via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi all,
> I'm working on a little LLVM pass that should add a call to an external
> function ( C++ project ) and pass as a parameter the name of the function
> that is about to be called. I have a problem finding a nice way to pass the
> current function name. In fact, so far,  Unfortunately the only solution
> that I have found for passing the name is to create a global for each
> function, store the name there and pass it as a parameter. I don't really
> like this solution because the size of the final binary increases and the
> overall approach feels not  optimal.
>
> Assuming that I have a variable `I` which is the CallInst instruction, is
> it possible to pass the function name at runtime to my external library in
> a nice way? I thought about passing the address of the instruction and
> casting it back to the CallInst instruction but I don't know how to do it.
>

That doesn't quite line up to me - the "Instruction"/CallInst only exists
at compile-time, not runtime. So if you embedded the pointer value into the
program, casting that pointer back to a CallInst at runtime in the program
would only give you a corrupt pointer/garbage.

Any suggestions?
>

Not much to it that I can think of - one way or another you'd have to have
the string embedded in the program somewhere, or in a side-table (eg: your
compiler could produce a little csv file or something that lets you lookup
some ID number to the string value - then you just encode/pass around the
ID in the program, and in some post-processing step you can lookup the
string in the csv file given the ID). You could avoid a global variable by
generating instructions in the caller that could write the string value
into a buffer, then pass that buffer by-value to the function you're
calling - but that's a ton of extra instructions compared to passing a
pointer to a global/constant buffer.
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