[LLVMdev] Confuse on ptrtoint and load

[Quentin Colombet]

Hi Weixue,

On Sep 12, 2013, at 6:48 PM, weixuegong <weixuegong at gmail.com> wrote:

> Hi, all
> 
> When I read the document of IR on llvm.org,
> I found two instructions, "ptrtoint" and load.
> 
> Load, I think, is : when after allocating some
> bytes in memory and storing some data into it, we could
> use load to get the data, like this:
> 
> %0 = alloca i32
> store i32 5, i32* %0
> %1 = load i32* %0
> so, the type of %1 is i32, and the value is 5, right?
> 
> And "ptrtoint" is:we cast it from ptr to int?
> Just like
> "&" in c++ ?
More or less yes. More specifically, we convert the pointer to the defined type.

> When we are doing like this:
> 
> %0 = alloca i32
> store i32 5, i32* %0
> %1 = ptrtoint i32* %0 to i32
> No doubt, the type of %1 is i32. But what is the value of %1?
%1 contains the address of the alloca converted to i32.
The exact value depends on the architecture. For instance on x86-64, addresses are 64-bits, thus here you will truncate the address to a 32-bits value.

Does it make sense?

> Is it just like "load", loading the value stored, or becoming a "var"
> to store the address(not the real value stored in memory)?
> 
> If I want to get the value stored in memory, I should use load,
> and if I want to get the addr of the ptr, use ptrtoint, am I right?
Basically, yes :).
> 
> Could anyone tell me the difference between them clearly?
Hope it helps.

-Quentin
> 
> Best Regards
> 
> Weixue
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