llvm dev - Apr 2017 - How to get the address of a global variable in LLVM?

Let `x` be the address of a global variable `g` in a program at run-time.
LLVM IR produces a store instruction as shown below:

    store i32 30, i32* @g, align 4

I am writing an LLVM pass which will instrument the program such that `x`
is passed to an instrumentation function `func(int addr)` at run-time. I
can insert a call to `func` using `IRBuilder` successfully. What I am not
being able to do is to insert instrumentation to collect `x`.

    if (StoreInst *store_inst = dyn_cast<StoreInst>(&I)) {

                        Value* po = store_inst->getPointerOperand();
                        if(isa<GlobalVariable>(po)) {
                            errs() << "store [pointer]: "
<< *po << '\n';

                            Constant *instrument_func
F.getParent()->getOrInsertFunction("func", Type::getVoidTy(Ctx),
Type::getInt32Ty(Ctx), NULL);

                            IRBuilder<> builder(&I);
                            builder.SetInsertPoint(&B,
++builder.GetInsertPoint());

                            Value* args[] = {po};
                            builder.CreateCall(instrument_func, args);
                        }
                    }

The result of running `opt` is :

    Call parameter type does not match function signature!
    i32* @a
     i32  call void bitcast (void (i64)* @func to void (i32)*)(i32* @a)
    LLVM ERROR: Broken function found, compilation aborted!


-- 

Thanks & Regards,
Dipanjan
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20170428/459d855e/attachment.html>

On 4/28/17 3:20 PM, Dipanjan Das via llvm-dev wrote:
>
> Let `x` be the address of a global variable `g` in a program at
> run-time. LLVM IR produces a store instruction as shown below:
>
>     store i32 30, i32* @g, align 4
>
> I am writing an LLVM pass which will instrument the program such that
> `x` is passed to an instrumentation function `func(int addr)` at
> run-time. I can insert a call to `func` using `IRBuilder` successfully.
> What I am not being able to do is to insert instrumentation to collect `x`.
>
>     if (StoreInst *store_inst = dyn_cast<StoreInst>(&I)) {
>
>                         Value* po = store_inst->getPointerOperand();
>                         if(isa<GlobalVariable>(po)) {
>                             errs() << "store [pointer]: "
<< *po << '\n';
>
>                             Constant *instrument_func >
F.getParent()->getOrInsertFunction("func", Type::getVoidTy(Ctx),
> Type::getInt32Ty(Ctx), NULL);
>
>                             IRBuilder<> builder(&I);
>                             builder.SetInsertPoint(&B,
> ++builder.GetInsertPoint());
>
>                             Value* args[] = {po};
>                             builder.CreateCall(instrument_func, args);
>                         }
>                     }
>
> The result of running `opt` is :
>
>     Call parameter type does not match function signature!
>     i32* @a
>      i32  call void bitcast (void (i64)* @func to void (i32)*)(i32* @a)
>     LLVM ERROR: Broken function found, compilation aborted!
>
You need a load instruction since your function takes an i32, not an 
i32*.... A global's value in llvm is its address, not the numeric data 
it contains.


Jon
>
> --
>
> Thanks & Regards,
>
> Dipanjan
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
-- 
Jon Roelofs
jonathan at codesourcery.com
CodeSourcery / Mentor Embedded / Siemens

On 28 April 2017 at 14:32, Jonathan Roelofs via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> You need a load instruction since your function takes an i32, not an
> i32*.... A global's value in llvm is its address, not the numeric data
it
> contains.
I suspect he actually wants to make his function take an "i8*" and
bitcast his pointer to that type before calling it. [*]

Tim.

[*] Keeping it as an i32/i64 and using ptrtoint instead of bitcast
would also work, but be less portable and elegant.