[llvm-dev] (no subject)

[Alberto Barbaro via llvm-dev]

Hi,
thanks for your answer

Il giorno ven 25 gen 2019 alle ore 06:23 Doerfert, Johannes Rudolf <
jdoerfert at anl.gov> ha scritto:

> 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.
>
>
thanks for the suggestion, I understand that the code is not clean but i'd
like to understand how it works in general for now :)


> > 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.
>
>
why do you consider the GET to be a no-op? %6 is a i8** instead %i7 is an
i8*. Thanks


> > 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.
>
>
No problem, visitGetElementPtrInst is jus that the function that is called
when a GEP instruction is found. I tried to get the value calling
getOperandValue and basically reproducing the usual LLVM behavior but i
always have 0 as value. Maybe a stupid question but when I see the
PointerVal of a pointer (e.g. 0x2281a30 ), if I read the bytes as that
specific address would I find the value or LLVM does not really store any
value in there and the address is just some sort of index within a
"emulated" memory?


> > Thanks
>
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
> --
>
> Johannes Doerfert
> Researcher
>
> Argonne National Laboratory
> Lemont, IL 60439, USA
>
> jdoerfert at anl.gov


Thanks
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