llvm dev - Jul 2019 - Efficient way to identify an instruction

Hi all,
I would like to understand if there is an efficient way to identify the
instruction that "created" a specific variable. For example,

define i32 @main() #0 {
  %1 = alloca i32, align 4
  %2 = alloca %struct._IO_FILE*, align 8
  %3 = alloca [40 x i8], align 16
  store i32 0, i32* %1, align 4
  %4 = call %struct._IO_FILE* @fopen(i8* getelementptr inbounds ([51 x i8],
[51 x i8]* @.str, i32 0, i32 0), i8* getelementptr inbounds ([3 x i8], [3 x
i8]* @.str.1, i32 0, i32 0))
  store %struct._IO_FILE* %4, %struct._IO_FILE** %2, align 8
  %5 = getelementptr inbounds [40 x i8], [40 x i8]* %3, i32 0, i32 0
  %6 = load %struct._IO_FILE*, %struct._IO_FILE** %2, align 8
  %7 = call i64 @fread(i8* %5, i64 2, i64 1, %struct._IO_FILE* %6)
  %8 = load %struct._IO_FILE*, %struct._IO_FILE** %2, align 8
  %9 = call i32 @fclose(%struct._IO_FILE* %8)
  %10 = getelementptr inbounds [40 x i8], [40 x i8]* %3, i64 0, i64 0
  %11 = load i8, i8* %10, align 16
  *%12 = sext i8 %11 to i32*
  %13 = icmp eq i32 %12, 66
  br i1 %13, label %14, label %25
}

Having the reference I to the instruction in bold.Can i efficiently know
that the variable %11 was "created" by the %3 = alloca [40 x i8],
align 16.

I understand that the word "created" is not the most appropriate...

Any suggestion?

Thanks
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Hi Alberto,

On Sat, 27 Jul 2019 at 10:09, Alberto Barbaro via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Having the reference I to the instruction in bold.Can i efficiently know
that the variable %11 was "created" by the %3 = alloca [40 x i8],
align 16.
Yes, I.getOperand(0) *is* the AllocaInst in this case. So for example
isa<AllocaInst>(I.getOperand(0)) will return true. And if you care
about more details you can dyn_cast<AllocaInst> it and check any other
properties you want.

Cheers.

Tim.

Hi Tim,
as always thanks for your help. Unfortunately I made a mistake in my email
but apart from that I still have problems.

Il giorno sab 27 lug 2019 alle ore 11:53 Tim Northover <
t.p.northover at gmail.com> ha scritto:
> Hi Alberto,
>
> On Sat, 27 Jul 2019 at 10:09, Alberto Barbaro via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > Having the reference I to the instruction in bold.Can i efficiently
know
> that the variable %11 was "created" by the %3 = alloca [40 x i8],
align 16.
>
> Yes, I.getOperand(0) *is* the AllocaInst in this case. So for example
> isa<AllocaInst>(I.getOperand(0)) will return true. And if you care
> about more details you can dyn_cast<AllocaInst> it and check any
other
> properties you want.
>
>
I would like to use the approach you described considering I to be a
reference to the icmp instruction ( %13 = icmp eq i32 %12, 66 ). From what
I understood i should do something like:

Instruction* source;

if(source = dyn_cast<AllocaInst>(I.getOperand(0))) {
    cout << "Alloca Inst" << endl;
    I.dump();
    getchar();
}

I thought I.getOperand(0) was a reference to the instruction that have
created %12. What am I missing?

> Cheers.
>
> Tim.
>
Thanks again
Alberto
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