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
}
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.
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?
Thanks
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.
Hi,
thanks for your answer
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@lists.llvm.org
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
–
Johannes Doerfert
Researcher
Argonne National Laboratory
Lemont, IL 60439, USA
jdoerfert@anl.gov
Thanks
Hi,
thanks for your answer
> > 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
Understanding the concept can be done either way, one is in my opinion
just unnecessarily complicated.
> > 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
This GEP computes the address of the 0th element of the "array" %6. The
result has the same address as the array and it also has the same type
so I'd say they are indistinguishable.
> > 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.
It seems the function was appropriately named 
I tried to get the value calling getOperandValue and basically
reproducing the usual LLVM behavior but i always have 0 as value.
What do you think "the usual LLVM behavior is"? Also consider that GEPs
can have multiple operands. What exactly do you want to achieve anyway?
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?
I'm not sure I understand what you are asking but I'm pretty sure you
will not.
Hi,
All good points and jokes in your email
My final goal is the get the value of %8 at runtime. So considering the program is called argv I would like to execute something like ./argv 22 and get the value of argv[0]. Is it possible? So far I have the address where the parameter at position 0 in store but I’d like to read the value and print it.
I hope it is clear now
Thanks again
I now get your intent but can’t really help you. You need to ask interpreter folks.
I’d suggest you send another email, state clearly what you want, and put a summary in the subject line.
You can also try the IRC, though during the week you might have more luck.
Hi,
No problem at all. Yes I think a good subject would help.
I didn’t know about the IRC channel
Thanks again, really appreciated
Hi,
I hope this can help someone else, I manually modified the IR in this way
…
%8 = load i8*, i8** %7, align 8
%9 = load i8, i8* %8, align 4
%10 = call i32 (i8*, …) @printf(i8* getelementptr inbounds ([4 x i8], [4 x i8]* @.str, i32 0, i32 0), i8* %8)
%11 = load i32, i32* %4, align 4
ret i32 %11
…
At this point it is enough to cast %9: const char *target = (const char *)v.PointerVal;
So it should be enough just to read from that memory address as expected
Thanks anyway