[llvm-dev] How to get the address of a global variable in LLVM?

[Jonathan Roelofs via llvm-dev]

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
>
>
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-- 
Jon Roelofs
jonathan at codesourcery.com
CodeSourcery / Mentor Embedded / Siemens