llvm dev - Mar 2019 - Cast a function parameter to GEP

Thanks Tim,
I'll try to solve my problem ASAP, if I cannot maybe I'll some other
clarifications.

Thanks again

On Sat, Mar 9, 2019, 06:03 Tim Northover <t.p.northover at gmail.com>
wrote:
> Hi Alberto,
>
> On Sat, 9 Mar 2019 at 05:50, Alberto Barbaro via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > i8* getelementptr inbounds ([7 x i8], [7 x i8]* @.str1, i32 0, i32 0)
>
> Because all of its inputs are Constants, this is actually a
> GetElementPtrConstantExpr instead of a GetElementPtrInst (the easiest
> way to tell on sight without context is the extra parens, which the
> instruction variant never has).
>
> The main practical difference between the two is that instances of
> Constant don't live in a BasicBlock independently, but are referenced
> directly by each user. When printing the IR this is shown by
> "inlining" them into the parent Instruction.
>
> LLVM has a GEPOperator class to help deal with this issue; it can be
> used to access the common features of Instructions and Constants.
> There are similar Operators for other operations because this is quite
> a common situation.
>
> > I think it does not work because str1 is a "private
constant". Is there
> a way to solve it?
>
> Broadly true, but the same would apply to any global, not just ones
> declared constant.
>
> Cheers.
>
> Tim.
>
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Sorry Tim,
I now need to find the value of the constant expression and I found online
a solution like: string value
cast<ConstantDataArray>(cast<GlobalVariable>(CallInstruction->getOperand(0))->getInitializer())->getAsCString();

This one works but I feel the GEPOperator should be used based on your
comments, so I tried to cast the operator and get the value but I'm not
able to do it.

I cast using GEPOperator *op =
dyn_cast<GEPOperator>(CI->getOperand(0));

Could you tell me if it is correct and how to extract the value please?

Thanks

Il giorno sab 9 mar 2019 alle ore 06:26 Alberto Barbaro <
barbaro.alberto at gmail.com> ha scritto:
> Thanks Tim,
> I'll try to solve my problem ASAP, if I cannot maybe I'll some
other
> clarifications.
>
> Thanks again
>
> On Sat, Mar 9, 2019, 06:03 Tim Northover <t.p.northover at gmail.com>
wrote:
>
>> Hi Alberto,
>>
>> On Sat, 9 Mar 2019 at 05:50, Alberto Barbaro via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>> > i8* getelementptr inbounds ([7 x i8], [7 x i8]* @.str1, i32 0, i32
0)
>>
>> Because all of its inputs are Constants, this is actually a
>> GetElementPtrConstantExpr instead of a GetElementPtrInst (the easiest
>> way to tell on sight without context is the extra parens, which the
>> instruction variant never has).
>>
>> The main practical difference between the two is that instances of
>> Constant don't live in a BasicBlock independently, but are
referenced
>> directly by each user. When printing the IR this is shown by
>> "inlining" them into the parent Instruction.
>>
>> LLVM has a GEPOperator class to help deal with this issue; it can be
>> used to access the common features of Instructions and Constants.
>> There are similar Operators for other operations because this is quite
>> a common situation.
>>
>> > I think it does not work because str1 is a "private
constant". Is there
>> a way to solve it?
>>
>> Broadly true, but the same would apply to any global, not just ones
>> declared constant.
>>
>> Cheers.
>>
>> Tim.
>>
>
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On Sat, 9 Mar 2019 at 07:22, Alberto Barbaro <barbaro.alberto at
gmail.com> wrote:
> I now need to find the value of the constant expression and I found online
a solution like: string value =
cast<ConstantDataArray>(cast<GlobalVariable>(CallInstruction->getOperand(0))->getInitializer())->getAsCString();
> This one works but I feel the GEPOperator should be used based on your
comments, so I tried to cast the operator and get the value but I'm not able
to do it.
If that one works it sounds like you don't have any kind of GEP at
that point. Otherwise the cast to GlobalVariable would fail. Have you
tried calling Module::dump while that code is executing to see what
you're really dealing with?
> I cast using GEPOperator *op =
dyn_cast<GEPOperator>(CI->getOperand(0));
That looks right to me if you have a GEP.  Once you've got that you'd
have to carefully check the operands of the GEP to make sure no-one is
doing anything tricksy like

    printf(LocalVariable); //
cast<GlobalVariable>(MyGEP->getPointerOperand) will fail so use
dyn_cast and check the result.
    printf(&"MwahahahaHello world\n"[9]); // The GEP will have
non-zero index operands. They may not even be constant.

Cheers.

Tim.