[LLVMdev] Alloca Instruction

[Chris Lattner]

On Jul 4, 2009, at 12:12 PM, kapil anand wrote:

> Hi all,
>
> I was reading the description of run-time stack and alloca  
> instructions in LLVM IR and have a question about the same. Do LLVM  
> alloca instructions allow the processing on stack across some alloca  
> blocks?
>
> For example, is following code valid?

No, that code is not valid.  Each alloca is considered a distinct  
object.  First example, if the first one was dead (no direct uses),  
the optimizer will delete it.  Accessing %0 from %4's pointer will not  
work well in general, and is undefined behavior.

-Chris

>
>
> define i32 @f()
> {
>        %0 = alloca i8, i32 16
>       %1 = ptrtoint i8* %0 to i32
>        %2 = add i32 %1, 4
>        %3 = inttoptr i32 %2 to i32*
>       store i32 5,    i32* %3
>
>       %4 = alloca i8, i32 20
>       %kkk = ptrtoint i8* %4 to i32
>        %5 = add i32 %kkk, 24                    -----> Access to  
> previous block
>       %6 = inttoptr i32 %5 to i32*
>        %7 = load i32* %6
>       ret i32 %7
>
>   }
>
> Is this kind of accessing across blocks valid? Are we guaranteed  
> that in final executable, this offest processing across blocks would  
> be taken care of properly. I have tried writing some LLVM IRs with  
> these kind of accesses and am getting correct answer( For example,  
> above code is working fine). But wanted to make sure that this is a  
> valid operation in IR.
>
> Thanks
>
> --Kapil
>
>
>
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