[llvm-dev] (no subject)

Sun Jan 27 01:26:10 PST 2019

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

Il giorno dom 27 gen 2019 alle ore 08:03 Alberto Barbaro <
barbaro.alberto at gmail.com> ha scritto:

> 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
>
>
> On Sun, Jan 27, 2019, 01:25 Doerfert, Johannes Rudolf <jdoerfert at anl.gov
> wrote:
>
>> 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.
>> ------------------------------
>> *From:* Alberto Barbaro <barbaro.alberto at gmail.com>
>> *Sent:* Saturday, January 26, 2019 7:07:26 PM
>> *To:* Doerfert, Johannes Rudolf
>> *Cc:* llvm-dev
>> *Subject:* Re: [llvm-dev] (no subject)
>>
>> 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
>>
>>
>> On Fri, Jan 25, 2019, 06:23 Doerfert, Johannes Rudolf <jdoerfert at anl.gov
>> wrote:
>>
>> 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
>>
>> > _______________________________________________
>> > 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
>>
>>
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