llvm dev - Feb 2014 - [LLVMdev] How to codegen an LLVM-IR that has dynamic arrays in it?

Hi Fellows,

        Is there a way to allocate dynamic array within an LLVM IR file
from my code generator? Say., how to create an array type with a size
determined through a global variable. Symbolically, something like below:

                    Value *sz = Mod->getOrInsertGlobal("SIZE",
Int32Ty);
                    Type *ArrayTy = ArrayType::get(FloatTy, sz)
                    AllocaInst *AI = CreateAlloca(ArrayTy, 0, "");


Thanks,
Paul
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Call malloc()?

—Owen

On Feb 17, 2014, at 5:29 PM, Paul Vario <paul.paul.mit at gmail.com>
wrote:
> Hi Fellows, 
> 
>         Is there a way to allocate dynamic array within an LLVM IR file
from my code generator? Say., how to create an array type with a size determined
through a global variable. Symbolically, something like below:
> 
>                     Value *sz = Mod->getOrInsertGlobal("SIZE",
Int32Ty);
>                     Type *ArrayTy = ArrayType::get(FloatTy, sz)
>                     AllocaInst *AI = CreateAlloca(ArrayTy, 0,
"");
> 
> 
> Thanks,
> Paul        
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

Hi Paul,
>                     Value *sz = Mod->getOrInsertGlobal("SIZE",
Int32Ty);
>                     Type *ArrayTy = ArrayType::get(FloatTy, sz)
>                     AllocaInst *AI = CreateAlloca(ArrayTy, 0,
"");
You can't create dynamically varying types like that in LLVM, but an
AllocaInst can have a parameter saying how many elements to allocate,
so instead of trying to write:

   %numElts = load i32* @SIZE
   %val = alloca [@var x float] ; %val would have dynamic type
[%numElts x float]*. Not good.

You'd want:
    %numElts = load i32* @SIZE
    %val = alloca float, i32 %numElts ; %val now has type float*, but
%numElts elements can be accessed.

Roughly the same would happen if you took Owen up on using malloc
(%val would have to have type "float*" rather than a static array
type).

Cheers.

Tim.

I see. Problem solved! Thank you all for the clarification.

Best,
Paul


On Tue, Feb 18, 2014 at 4:24 AM, Tim Northover <t.p.northover at
gmail.com>wrote:
> Hi Paul,
>
> >                     Value *sz =
Mod->getOrInsertGlobal("SIZE", Int32Ty);
> >                     Type *ArrayTy = ArrayType::get(FloatTy, sz)
> >                     AllocaInst *AI = CreateAlloca(ArrayTy, 0,
"");
>
> You can't create dynamically varying types like that in LLVM, but an
> AllocaInst can have a parameter saying how many elements to allocate,
> so instead of trying to write:
>
>    %numElts = load i32* @SIZE
>    %val = alloca [@var x float] ; %val would have dynamic type
> [%numElts x float]*. Not good.
>
> You'd want:
>     %numElts = load i32* @SIZE
>     %val = alloca float, i32 %numElts ; %val now has type float*, but
> %numElts elements can be accessed.
>
> Roughly the same would happen if you took Owen up on using malloc
> (%val would have to have type "float*" rather than a static array
> type).
>
> Cheers.
>
> Tim.
>
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