[llvm-dev] Calling virtual elf functions under windows -> Adding ASM code

[via llvm-dev]

Hello everyone,

I'm sorry for "flooding" your inbox, but I wanted to extend my message 
which is shown below. The problem is still not solved and I'm more 
confused then before.
I compiled the code - the one seen in the pictures  to elf bc and to msvc 
(pecoff) bc. Then I used llc to generate assembler output.

In my opinion the involved functions are acting exactly the same. There 
are only some registers swapped. I should mention: when using the msvc bc 
code, then the calls via the interface are working.

I attach the assembly code of the involved code to. Could please anyone 
explain me the difference, which could cause the crash?

Kind regards
Björn





From:   Bjoern Gaier/HE/HORIBA
To:     llvm-dev at lists.llvm.org, cfe-dev at lists.llvm.org
Date:   07.02.2018 11:47
Subject:        Calling virtual elf functions under windows


Hello everyone reading this,

I'm totally confused and need an explanation. I'm also not sure if it's a 
clang or a LLVM subject. 

I'm working with Visual Studio 2015, LLVM 5.0.1 and Windows 7 64bit.

I have a simple project. With 2 Header files and 1 cpp file.
"Interface.h"
This file defines a simple class with two pure virtual member functions.

"SimpleResult.h"
This file defines a struct holding an integer

"CM_Elf.cpp"
This file defines a class (CM_Elf) implementing the interface from 
Interface.h
Also there is an instance of the class (named "elf").

I compile the CM_Elf.cpp to a CM_Elf.bc file via clang-cl but with target 
"x86_64-pc-windows-elf"


In another application I use LLVM to parse the BC file and use the 
ExecutionEngine to get some addresses.
What I want to archive is:
I want to call the implemented member functions of "elf".

1.) I execute all the global constructors collected by the ExecutionEngine
-> The constructor of elf gets called
2.) I use ExecutionEngine to get the address of "elf"
3.) I use the interface to execute Interface::init
-> Init gets called
4.) I use the interface to execute Interface::shutdown
-> CRASH!

When I swap the calls to init and shutdown, then everything works. So the 
virtual call to init seems to corrupt my stack - maybe it's the return 
value. Could it be a calling convention problem? But my target is still 
"windows" only msvc changed to elf...
When I compile CM_Elf for target "x86_64-pc-windows-msvc19.0.24215" then 
it works.

When I get the address of CM_Elf::init and CM_Elf::shutdown via the 
ExecutionEngine and call them without using the virtual table, then again 
everything works.

SO! Is it not possible to call virtual elf functions via there interface? 
But why does normal functions work? Also I reach the functions which are 
stored in the vTable of CM_Elf so the vTable is correct.

I append some pictures of the code... I hope someone could help me with 
this.

Kind regards
Björn





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