llvm dev - Nov 2019 - Understanding AllocaInst

Hello,

Hopefully this is the correct place for this sort of question. I have been
trying to do some simple instrumentation of LLVM IR, and I have been
confused by the return type of alloca. When passing an AllocaInst as a
parameter to a function call, the parameter type is i32**, which is not
what I would expect. Really I would like the address of the allocated
memory. What am I missing? And is it possible to represent the result of
alloca as an integer? Thanks for any help.

Cheers,

Avery
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The type of an alloca is a pointer to the type of the allocated object. For
instance:

This C++ source code:

  int i;
  float f;
  f1(i, f);

compiles to this LLVM IR:

 %i = alloca i32, align 4
  %f = alloca float, align 4
  %0 = load i32, i32* %i, align 4
  %1 = load float, float* %f, align 4
  call void @_Z2f1if(i32 %0, float %1)

So you can see the type of an alloca of i32 is i32* and the type of an
alloca of float is float*.

You can convert pointers to integers with a ptrtoint instruction. eg: %0
ptrtoint i32* %i to i64

On Fri, Nov 8, 2019 at 11:05 AM Avery Laird via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hello,
>
> Hopefully this is the correct place for this sort of question. I have been
> trying to do some simple instrumentation of LLVM IR, and I have been
> confused by the return type of alloca. When passing an AllocaInst as a
> parameter to a function call, the parameter type is i32**, which is not
> what I would expect. Really I would like the address of the allocated
> memory. What am I missing? And is it possible to represent the result of
> alloca as an integer? Thanks for any help.
>
> Cheers,
>
> Avery
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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David Blaikie via llvm-dev <llvm-dev at lists.llvm.org> writes:
> The type of an alloca is a pointer to the type of the allocated object. For
> instance:
>
> This C++ source code:
>
>   int i;
>   float f;
>   f1(i, f);
>
> compiles to this LLVM IR:
>
>  %i = alloca i32, align 4
>   %f = alloca float, align 4
>   %0 = load i32, i32* %i, align 4
>   %1 = load float, float* %f, align 4
>   call void @_Z2f1if(i32 %0, float %1)
>
> So you can see the type of an alloca of i32 is i32* and the type of an
> alloca of float is float*.

And the type of an alloca of i32* is i32** which is where that comes
from.  So Avery is doing an alloca of an i32*, a pointer rather than an
integer.

HTH,

                 -David