[llvm-dev] (no subject)

[Doerfert, Johannes Rudolf via llvm-dev]

On 01/20, Alberto Barbaro via llvm-dev wrote:
> Hi all,
> I have the following C code:
> 
> #include<stdio.h>
> 
> int main(int argc, char *argv[]) {
> 
>     printf("%s\n", argv[0]);
>     return argc;
> }
> 
> that generates the following IR for the main function:
> 
> ; Function Attrs: noinline nounwind optnone uwtable
> define i32 @main(i32, i8**) #0 {
>   %3 = alloca i32, align 4
>   %4 = alloca i32, align 4
>   %5 = alloca i8**, align 8
>   store i32 0, i32* %3, align 4
>   store i32 %0, i32* %4, align 4
>   store i8** %1, i8*** %5, align 8
>   %6 = load i8**, i8*** %5, align 8
>   %7 = getelementptr inbounds i8*, i8** %6, i64 0
>   %8 = load i8*, i8** %7, align 8
>   %9 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([4 x i8], [4
> x i8]* @.str, i32 0, i32 0), i8* %8)
>   %10 = load i32, i32* %4, align 4
>   ret i32 %10
> }

You should really run -mem2reg (without the optnone though). Also
-instcombine is afterwards often helpful to clean up clang generated
code, or maybe just run it with -O1.

> I think %8 will contain the address of argv[0], so %7 was storing a pointer
> to pointer to argv? I'm not sure of that.

Let's see: argc is %0 and argv == &argv[0] is %1. The latter is stored
in %5, which is loaded as %6. The GEP (%7) is a no-op, thus %6 == %7.
Finally, %8 is the value argv[0] of type char/i8.

> Considering that I can access the GenericValue set by
> visitGetElementPtrInst. How can I obtain the value of argv[0] at runtime
> using the Interpreter class?

Sorry can't help with that. I don't know the Interpreter class and I
guess "GenericValue" and "visitGetElementPtrInst" are somehow related to
it.

> Thanks

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> llvm-dev at lists.llvm.org
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-- 

Johannes Doerfert
Researcher

Argonne National Laboratory
Lemont, IL 60439, USA

jdoerfert at anl.gov
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