llvm dev - Jul 2008 - [LLVMdev] allocating an array

> if the length is %n, you can alloca %n i8's:
>   %x = alloca i8, i32 %n
If the array element type is i32 (like the size
field), then you may as well use an alloca i32;
if you use an alloca i8 then you would have to
use 4*%n rather than %n in order to get the right
amount of memory.

Ciao,

Duncan.

Thanks, Bill and Duncan, for your suggestions. I've changed my
approach, and use a pointer instead of an array now.

So this code:

  var size : nat = 9
  var x : [size] int

now becomes:

%size = alloca i32              ; <i32*> [#uses=2]
store i32 9, i32* %size
%x = alloca { i32, i32* }               ; <{ i32, i32* }*> [#uses=6]
load i32* %size         ; <i32>:1 [#uses=1]
getelementptr { i32, i32* }* %x, i32 0, i32 0  ; <i32*>:2 [#uses=1]
store i32 %1, i32* %2
%arr_storage = alloca i32, i32 %1            ; <i32*> [#uses=1]
%arr_dataptr_addr = getelementptr { i32, i32* }* %x, i32 0, i32 1 ;
<i32**> [#uses=1]
store i32* %arr_storage, i32** %arr_dataptr_addr

(did some manual optimizations on this code, as my front end generates
really dumb code)

So I need two allocas now, one for the structure and one for the
storage of the elements.

Now I'm wondering: do you think it's a bad idea to use alloca for
allocating an unknown number of elements? In that case, should I just
always use the malloc instruction instead, or do a runtime decision to
choose between alloca and malloc?


Regards,
Kevin André

Hi,
> Now I'm wondering: do you think it's a bad idea to use alloca for
> allocating an unknown number of elements? In that case, should I just
> always use the malloc instruction instead, or do a runtime decision to
> choose between alloca and malloc?
on some operating systems there is a limit to how much you can allocate
on the stack.  This is 16k on windows and something bigger but not huge
on Sun OS (IIRC).  On the other hand, allocating things on the stack is
usually more efficient than using malloc.

Ciao,

Duncan.

PS: Not to mention that if you use malloc then
you will need to explicitly reclaim memory using
free when the variable goes out of scope, while
this happens automatically if you use alloca.