[LLVMdev] (no subject)

[James Molloy]

Hi Umesh,

The compiler generates an alloca (stack space) for every local variable defined in a function. Along with local variables this also includes space to store the function parameters and also the variable that holds the return value.

For example:

Int x(int a, int b) {
  Return a + b;
}

Would compile to something like this:

Define i32 @x(i32 %a, i32 %b) {
    %ret.addr = alloca i32
    %a.addr = alloca i32
    %b.addr = alloca i32

    Store i32 %a, i32 *%a.addr
    Store i32 %b, i32 *%b.addr

    %1 = load i32* %a.addr
    %2 = load i32* %b.addr

    %3 = add nsw i32 %1, %2

    Store i32 %3, i32* %ret.addr

    %4 = load i32* %ret.addr
    Ret %4
}

So firstly Clang would create variables on the stack to hold the parameters and the return value, then store the parameters there (function prologue).

Then, to perform the add, it would load the arguments from their local variable storage (in this case %a.addr and %b.addr), perform the add and write the result back to its local variable storage (%ret.addr)

Finally to return it will load the return value variable and return it.

Does this make things clearer?

Cheers,

James

From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of Umesh Kalappa
Sent: 02 February 2012 09:04
To: Duncan Sands
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] (no subject)

Hi Duncan,
Appreciate you response here and yeah any optimization  switch for clang will make this extra bytes  go away ...Was very curious to know why these extra bytes for  and can you please elaborate  more on "for storing the return value in
more complicated cases" ...That helps me understand the LLVM internals :)

Thanks
~Umesh

On Thu, Feb 2, 2012 at 2:19 PM, Duncan Sands <baldrick at free.fr<mailto:baldrick at free.fr>> wrote:
Hi Umesh,

> Again ,I'm newbie to LLVM  and please pardon me ..if you guys  feel that ,the
> below question is very basic :)
>
> Here i go ,compiled the below sample with clang i.e *clang enum.c -S -emit-llvm*
> and there respective file are
>
> $ cat enum.c
> int main()
> {
>   enum type{one=1,two,three} s;
>   s = one;
>   return s;
> }
>
> $ cat enum.s
> ; ModuleID = 'enum.c'
> target datalayout =
> "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-f80:32:32-n8:16:32-S32"
> target triple = "i386-pc-cygwin"
>
> define i32 @main() nounwind {
>    %1 = alloca i32, align 4
>    %s = alloca i32, align 4
>    store i32 0, i32* %1
>    store i32 1, i32* %s, align 4
>    %2 = load i32* %s, align 4
>    ret i32 %2
> }
>
> *Question :* Why there is  extra 4 bytes on stack i.e *"%1 = alloca i32, align
> 4"*  ???

I think this would normally be used for storing the return value in more
complicated cases.  If you compile with optimization it will go away.

Ciao, Duncan.
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