[llvm-dev] Position independent code writes absolute pointer

[Gaier, Bjoern via llvm-dev]

I wanted to add an thought to this:

Could it be possible to modify the code on the IR-Level to store PIC/offset address and not absolute address? I'm not familiar with the LLVM IR so I don't know what is possible and how it effects the code at all.

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Gaier, Bjoern via llvm-dev
Sent: 08 January 2020 16:29
To: llvm-dev at lists.llvm.org
Subject: [llvm-dev] Position independent code writes absolute pointer

Hello everyone,

I have an issue with some code that I jit/load as position independent code. I have a feeling that it is not possible to solve the issue but I wanted to give it a try.

#include <stdio.h>

int magicValue  = 123;
int magicValue2 = 321;

volatile int *pValue = &magicValue;

void printMagicValue()
{
       printf("Planschi...\n");
       printf("The magic value is %i 0x%p && 0x%p\n", magicValue, &magicValue, pValue);
}

void setMagicValue(int value)
{
       magicValue = value;
}

This is the code which I will load as PIC, for the JTMB I use the following settings:
JTMB->setRelocationModel(llvm::Reloc::PIC_);
JTMB->setCodeModel(llvm::CodeModel::Small);
The code will be loaded into a shared memory. Two process will execute the memory from there, calling "printMagicValue", "setMagicValue(120)" and "printMagicValue" again. Only the first process will JIT the code, every other process will access it from the shared memory.

The first Process will say:
Planschi...
The magic value is 123 0x00000270BB090038 && 0x00000270BB090038
Planschi...
The magic value is 120 0x00000270BB090038 && 0x00000270BB090038

The second Process will say:
Planschi...
The magic value is 120 0x00000237A5DE0038 && 0x00000270BB090038
Planschi...
The magic value is 120 0x00000237A5DE0038 && 0x00000270BB090038

The values will be read correctly! Hurray! But my problem is, that the pointer 'pValue' was written with an absolute value and not with a PIC conform value. The second process will now print the address from the first process. I hoped, that - since the code is PIC - that also the pointers are written PIC like. I think I understand why this is not the case, but can I somehow change this behaviour without calculating the offset myself? My overall goal is to share the entire code between two processes.

I hope my question is somewhat understandable and I hope even more, that there is a solution to this...

Thank you for any help in advance and kind greetings
Björn
Als GmbH eingetragen im Handelsregister Bad Homburg v.d.H. HRB 9816, USt.ID-Nr. DE 114 165 789 Geschäftsführer: Dr. Hiroshi Nakamura, Dr. Robert Plank, Markus Bode, Heiko Lampert, Takashi Nagano, Takeshi Fukushima. Junichi Tajika
Als GmbH eingetragen im Handelsregister Bad Homburg v.d.H. HRB 9816, USt.ID-Nr. DE 114 165 789 Geschäftsführer: Dr. Hiroshi Nakamura, Dr. Robert Plank, Markus Bode, Heiko Lampert, Takashi Nagano, Takeshi Fukushima. Junichi Tajika
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