llvm dev - Jul 2015 - [LLVMdev] Adding integer field to all C++ classes in LLVM

Hi,

I want to add an integer field to all C++ classes in LLVM. Where do I look
at in the LLVM source to make the necessary changes? As of now, what I
understand is that I need to make changes in Type.cpp
(llvm/lib/IR/Type.cpp) where I insert an integer field in the ArrayRef
passed to StructType::setBody?

Thanks!

-Mohit.
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I take it you mean that you want to add a field to all classes COMPILED  by
LLVM, and not all classes that LLVM consists of?

Still, not sure if that is a good idea - I'm fairly sure the compiler (e.g.
Clang) will make up its own classes that need to be of a certain size and
have certain content to match external functionst that are not compiled at
this time (e.g. data structures used to call OS system calls, C and C++
runtime functions, etc) [certainly my Pascal compiler produces internal
structures in this way, for example for FILE, STRING and SET], and adding
extra fields here would be pretty much guaranteed to cause things to go
wrong. So a better idea is probably to understand what data structures are
part of your actual source-code [not counting system header files such as
<iostream>, <stdio.h> or <string>], and add fields ONLY to
those structures
that "you own", not the ones that clang produced internally. This
would of
course mean you have to do this either at the source level or in the AST of
the compiler, not at LLVM level - but I'm fairly sure that doing this at
LLVM level is a bad idea.

Also, unless I'm terribly misinformed, adding something to an ArrayRef is
not going to work. ArrayRef, StringRef and such are "references to the
original data in the calling code", which means you have no right to modify
it.

I'm pretty sure you are asking what is called an XY question, you want to
do X, you think Y is the method of achieving that, and therefore ask how to
do Y. I'm pretty sure asking how to achieve whatever X is in your case will
provide you with better ideas of how to achieve what you want to do - as
knowing what the actual goal is will help a lot in providing a

--
Mats

On 16 July 2015 at 22:05, Mohit Mishra <mmishra23 at gmail.com> wrote:
> Hi,
>
> I want to add an integer field to all C++ classes in LLVM. Where do I look
> at in the LLVM source to make the necessary changes? As of now, what I
> understand is that I need to make changes in Type.cpp
> (llvm/lib/IR/Type.cpp) where I insert an integer field in the ArrayRef
> passed to StructType::setBody?
>
> Thanks!
>
> -Mohit.
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
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HI Mats,

Thanks for getting back to me.


I take it you mean that you want to add a field to all classes COMPILED 
by
> LLVM, and not all classes that LLVM consists of?
>
YES!
>
> Also, unless I'm terribly misinformed, adding something to an ArrayRef
is
> not going to work. ArrayRef, StringRef and such are "references to the
> original data in the calling code", which means you have no right to
modify
> it.
>
 What I want is to be able to randomize object layout; and hence adding of
integer fields to the classes might be a good option. While I did think so
about ArrayRef, I thought about to substitute that with vector something
like this:
vector<Type*> vecType = Elements.vec ();
vecType.insert (Type::getInt32Ty (Context))
ST->setBody(vecType, isPacked);

and modify the setBody function parameters replacing ArrayRef with the
vector. However, I'm still skeptical about this.

What do you propose to have a  fairly good object layout randomization?

Best,
Mohit.
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