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
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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 -------------- next part -------------- A non-text attachment was scrubbed... Name: signature.asc Type: application/pgp-signature Size: 228 bytes Desc: not available URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190125/673150c3/attachment.sig>
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.govThanks -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190126/1a472568/attachment.html>
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 >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190127/f84593b3/attachment.html>