llvm dev - Jul 2011 - [LLVMdev] print the memory address computed by getelementptr

Hi,

I want to print the memory locations computed by getelementptr. As I understood,
getelementptr does not access the memory, but it contains the address it 
computes. I want to print these addresses at runtime (or process them).  So, I 
try to build a function that takes as argument a pointer and prints its value. 
And to call this function, by sending the gep instruction as a parameter. 


Surely, I obtain an error for a type mismatch. Can I declare the function as 
taking an int64 as parameter and then (somehow) sending the value computed by 
getelementptr?

declare void @myFunction ( i64 )


%tmp22 = getelementptr inbounds %struct.linked* %tmp21, i32 0, i32 1

%tmp = convert_tmp22_to_i64_to_get_the_value_of_the_pointer

call void @myFunction(i64 %tmp)


Or could you suggest a better method to get the value computed by getelementptr 
?

Thanks,
Alexandra
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20110720/e2f43389/attachment.html>

On 7/20/11 10:02 AM, Jimborean Alexandra wrote:
> Hi,
>
> I want to print the memory locations computed by getelementptr. As I 
> understood, getelementptr does not access the memory, but it contains 
> the address it computes. I want to print these addresses at runtime 
> (or process them).  So, I try to build a function that takes as 
> argument a pointer and prints its value. And to call this function, by 
> sending the gep instruction as a parameter.
All you really need to do here is to cast the result of the 
getelementptr value to the type of the parameter of your function.  In 
this case, you need to insert a ptrtoint instruction that takes the GEP 
and casts it to an i64.

That said, instead of casting the GEP to an i64, it would be better if 
your run-time function took a void *.  In LLVM IR, a pointer to an i8 is 
a void *, and so you could insert a bitcast of the GEP to an i8 * 
instead of casting it to an i64.  That way, you don't have to worry 
about the pointer size if you start using your code on both 32-bit and 
64-bit targets.

-- John T.
>
> Surely, I obtain an error for a type mismatch. Can I declare the 
> function as taking an int64 as parameter and then (somehow) sending 
> the value computed by getelementptr?
>
> declare void @myFunction ( i64 )
>
>
> %tmp22 = getelementptr inbounds %struct.linked* %tmp21, i32 0, i32 1
>
> %tmp = convert_tmp22_to_i64_to_get_the_value_of_the_pointer
>
> call void @myFunction(i64 %tmp)
>
>
> Or could you suggest a better method to get the value computed by 
> getelementptr ?
>
> Thanks,
> Alexandra
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20110720/b6cd745e/attachment.html>

On Wed, Jul 20, 2011 at 8:02 AM, Jimborean Alexandra
<xinfinity_a at yahoo.com> wrote:
> Hi,
>
> I want to print the memory locations computed by getelementptr. As I
> understood, getelementptr does not access the memory, but it contains the
> address it computes. I want to print these addresses at runtime (or process
> them).  So, I try to build a function that takes as argument a pointer and
> prints its value. And to call this function, by sending the gep instruction
> as a parameter.
>
> Surely, I obtain an error for a type mismatch. Can I declare the function
as
> taking an int64 as parameter and then (somehow) sending the value computed
> by getelementptr?
>
> declare void @myFunction ( i64 )
>
>
> %tmp22 = getelementptr inbounds %struct.linked* %tmp21, i32 0, i32 1
>
> %tmp = convert_tmp22_to_i64_to_get_the_value_of_the_pointer
This instruction is called ptrtoint. :)

-Eli

Hi again,

Thank you, your suggestion worked well. I was looking for a pointer to void
actually. But it seems that I do not get the memory address when passing the
value of the GEP instruction. 

For instance, when I print the address accessed by the second operandof the
store instruction


store %struct.linked* %tmp23.reload8, %struct.linked** %curr.037.reg2mem
I obtain a memory address of the form

7fff29c28ac8

But when I try to get the address stored in the GEP

%tmp18 = getelementptr inbounds %struct.linked* %curr.025.reload, i64 0, i32 0

I obtain

2084a10

which is not the memory location I expect. Is this the computed memory location,
or I did not access it correctly?

Thank you.

Alexandra



________________________________
From: John Criswell <criswell at illinois.edu>
To: Jimborean Alexandra <xinfinity_a at yahoo.com>
Cc: llvmdev at cs.uiuc.edu
Sent: Wednesday, July 20, 2011 5:08 PM
Subject: Re: [LLVMdev] print the memory address computed by getelementptr


On 7/20/11 10:02 AM, Jimborean Alexandra wrote: 
 
>Hi,
>
>I want to print the memory locations computed by
          getelementptr. As I understood, getelementptr does not access
          the memory, but it contains the address it computes. I want to
          print these addresses at runtime (or process them).  So, I try
          to build a function that takes as argument a pointer and
          prints its value. And to call this function, by sending the
          gep instruction as a parameter. 
>
All you really need to do here is to cast the result of the
    getelementptr value to the type of the parameter of your function. 
    In this case, you need to insert a ptrtoint instruction that takes
    the GEP and casts it to an i64.

That said, instead of casting the GEP to an i64, it would be better
    if your run-time function took a void *.  In LLVM IR, a pointer to
    an i8 is a void *, and so you could insert a bitcast of the GEP to
    an i8 * instead of casting it to an i64.  That way, you don't have
    to worry about the pointer size if you start using your code on both
    32-bit and 64-bit targets.

-- John T.


>Surely, I obtain an error for a type mismatch. Can I declare
          the function as taking an int64 as parameter and then
          (somehow) sending the value computed by
getelementptr?
>
>declare void @myFunction ( i64 )
>
>
>%tmp22 = getelementptr inbounds %struct.linked* %tmp21, i32 0,
          i32 1
>
>%tmp = convert_tmp22_to_i64_to_get_the_value_of_the_pointer
>
>call void @myFunction(i64 %tmp)
>
>
>Or could you suggest a better method to get the value computed
          by getelementptr ?
>
>Thanks,
>Alexandra
>
>
>
>_______________________________________________
LLVM Developers mailing list LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20110725/30e73f2f/attachment.html>