[LLVMdev] mixing static/dynamic code

[Nick Lewycky]

Paul Martin wrote:
> 
> Nick,
> Thanks for the quick answer.
> Dereferencing the pointer does yield the same result in both cases but 
> that's not what I want to do. I want to instrument the program 
> dynamically and keep track of certain memory locations which is a 
> problem if the same variable has different addresses for the 
> static/dynamic code - as far I see this is what it's happening but I 
> have no clue why.

If you look at the LLVM IR you can see two distinct alloca instructions, 
the one your static code had named "%pi_addr" and the one you created 
named "%ptr32".

It sounds like what you want to do is instead of creating your own stack 
spot to store %pi into, you should look at the first user of %pi which 
should be a store of %pi into its stack space, as generated by the 
static compilation, and pull the stack slot out of there. Something like 
this:

   (Given Argument *A to look for:)
   Value *StackSlot = 0;
   if (StoreInst *SI = dyn_cast<StoreInst>(*A->use_begin()))
     StackSlot = SI->getPointerOperand();

though I haven't actually tried to compile or run that.

Nick

> Paul
> 
> On Sun, May 24, 2009 at 10:15 PM, Nick Lewycky <nicholas at mxc.ca 
> <mailto:nicholas at mxc.ca>> wrote:
> 
>     Paul Martin wrote:
>      > Hi,
>      > I have the following code, the lines preceded by `>` being added at
>      > runtime (the snipped was also printed at runtime)
>      >
>      > define i32 @myfunc(i32 %pi) {
>      > entry:
>      >         %pi_addr = alloca i32           ; <i32*> [#uses=3]
>      >         %retval = alloca i32            ; <i32*> [#uses=2]
>      >         %tmp = alloca i32               ; <i32*> [#uses=2]
>      >  >       %ptr32 = alloca i32             ; <i32*> [#uses=2]
>      >         %"alloca point" = bitcast i32 0 to i32          ; <i32>
>     [#uses=0]
>      >
>      >  >       store i32 %pi, i32* %ptr32
>      >  >       %ptr8 = bitcast i32* %ptr32 to i8*              ; <i8*>
>     [#uses=1]
>      >  >       call void @roc( i8* %ptr8 )
>      >
>      >         store i32 %pi, i32* %pi_addr
>      >         %pi_addr1 = bitcast i32* %pi_addr to i8*                ;
>     <i8*>
>      > [#uses=1]
>      >         call void @roc( i8* %pi_addr1 )
>      > ...
>      >
>      > void roc(void*) is a function that only prints its argument as an
>     integer.
> 
>     So it's printing the pointer.
> 
>      > I was expecting for the program to print the same thing twice (the
>      > address of the myfunc argument) because the code added at runtime
>     looks
>      > to me identical with the code that was compiled (3 lines each).
>      > However, that was not the case; what I got was:
>      >
>      > 147156320
>      > 147117168
>      >
>      > What am I missing?
> 
>     Make roc() dereference the pointers it's given and print that instead.
> 
>     Nick
> 
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