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Alberto Barbaro via llvm-dev llvm-dev at lists.llvm.org
Sat Jan 26 17:07:26 PST 2019

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>

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