[LLVMdev] Inserting a function call using LLVM

[Duncan Sands]

Hi Teja,

> I am new to LLVM and interested in using LLVM to work on a research project for
> my Master's degree.
> My idea is following -
>
> 1. I have a simple C program basic.c -
> #include <stdio.h>
> int main() {
>     printf("Hello World");
>     return 0;
> }
>
> 2. I will generate IR byte code for this basic.c using -
> llvm-gcc -emit-llvm -S basic.c
>
> This will give me basic.s which has IR byte code like this -
>
> ; ModuleID = 'basic.c'
> target datalayout =
> "e-p:64:64:64-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-s0:64:64-f80:128:128-n8:16:32:64"
> target triple = "x86_64-apple-darwin10.8"
>
> @.str = private constant [12 x i8] c"hello world\00", align 1 ; <[12 x i8]*>
> [#uses=1]
>
> define i32 @main() nounwind ssp {
> entry:
>    %retval = alloca i32                            ; <i32*> [#uses=2]
>    %0 = alloca i32                                 ; <i32*> [#uses=2]
>    %"alloca point" = bitcast i32 0 to i32          ; <i32> [#uses=0]
>    %1 = call i32 @puts(i8* getelementptr inbounds ([12 x i8]* @.str, i64 0, i64
> 0)) nounwind ; <i32> [#uses=0]
>    store i32 0, i32* %0, align 4
>    %2 = load i32* %0, align 4                      ; <i32> [#uses=1]
>    store i32 %2, i32* %retval, align 4
>    br label %return
>
> return:                                           ; preds = %entry
>    %retval1 = load i32* %retval                    ; <i32> [#uses=1]
>    ret i32 %retval1
> }
>
> declare i32 @puts(i8*)
>
> 3. I want to write a pass to insert a call foo(int a) after the printf() call,
> so that it looks like this -
> #include <stdio.h>
> int main() {
>     printf("Hello World");
>     foo(1);
>     return 0;
> }
>
> void foo(int a) {
>     printf("a + 5 is : %d", a + 5);
> }

you need to do:
(1) insert a declaration of "foo" into the module;
(2) find the spot in the IR where you want to put it, i.e. find the printf
(3) insert the call

For (1), use Function::Create.
For (2), use Module::getFunction to lookup "main", then iterate over the basic
blocks, and within each basic block iterate over instructions, looking for a
call instruction, using isa<CallInst> to test if an instruction is a call.
For (3), create an IRBuilder, set the insertion point to the instruction after
the printf, and use the CreateCall method.

Ciao, Duncan.


>
> How can I achieve this? I looked online but couldn't find a definite source
> which tells me how to do this.
> I think I can use Cloning.h and CloneFunction.cpp to clone a given function and
> then insert my call to foo().
> But I'm not sure if this is the right way to do this. The documentation
> obviously did not say this explicitly.
> Any thoughts / pointers ?
>
> Thanks in advance !
> - Teja
>
>
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