llvm dev - Dec 2017 - Removing constants from a constant array?

Hi,

I’m currently writing a LLVM pass and would like to modify a constant array
based on some basic analysis.

Is there any way to simply remove entries (`llvm::Constant`) from an array
(`llvm::ConstantArray`)? Replacing all uses with an undefined value doesn’t work
as the array is terminated by an undefined value, subsequent elements thus are
ignored at runtime. I’d rather avoid re-creating the entire array from scratch
and replace the existing array with it but I’m afraid there’s not many other
options.

Is there any solution I’m not aware of yet?

Thanks,
Nicolai

On 12/16/17 11:08 PM, Nicolai Davidsson via llvm-dev
wrote:
> Hi,
>
> I’m currently writing a LLVM pass and would like to modify a constant array
based on some basic analysis.
>
> Is there any way to simply remove entries (`llvm::Constant`) from an array
(`llvm::ConstantArray`)? Replacing all uses with an undefined value doesn’t work
as the array is terminated by an undefined value, subsequent elements thus are
ignored at runtime. I’d rather avoid re-creating the entire array from scratch
and replace the existing array with it but I’m afraid there’s not many other
options.
>
> Is there any solution I’m not aware of yet?
In many cases, the only way to "change" a value using the LLVM C++ API
is to create a new value and replace uses of the old value with the new 
value.  I haven't looked, but I would not be surprised if this is the 
only way to "modify" the number of elements in a ConstantArray.

I can see how removing a ConstantArray element or creating a new 
ConstantArray with fewer elements would be a bit of a pain: you would 
need to change the ConstantArray, all the GEPs for the constant array, 
and potentially handle code that casts the ConstantArray to another type.

What are you trying to achieve be "removing" an element from a 
ConstantArray?  Perhaps there is an easier way to do what you're trying 
to do.

Regards,

John Criswell
>
> Thanks,
> Nicolai
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

-- 
John Criswell
Assistant Professor
Department of Computer Science, University of Rochester
http://www.cs.rochester.edu/u/criswell

I can access the array through a GlobalVariable and set the variable's
initializer, creating a new array should be doable but I’d still rather avoid
doing so if possible.

The array contains pairs of strings and function pointers and is used by an
interpreter and I’d like to remove some of them to limit the interpreter’s
functionality.
 
If I replace the pair with an undefined value pair (i.e. both values are NULL),
the interpreter stops scanning the array and further pairs are ignored (I can’t
change that behavior). Removing only the string or function pointer doesn’t work
either, the interpreter checks if either of the values is NULL and throws a
runtime error.

Replacing the function pointer with a pointer to an empty function works but I’d
rather erase the pairs entirely.
I’ll probably try re-building the array and replacing the existing one and see
if that works.

Thanks,
Nicolai 
> On Dec 17, 2017, at 19:28, John Criswell wrote:
> 
> On 12/16/17 11:08 PM, Nicolai Davidsson via llvm-dev wrote:
>> Hi,
>> 
>> I’m currently writing a LLVM pass and would like to modify a constant
array based on some basic analysis.
>> 
>> Is there any way to simply remove entries (`llvm::Constant`) from an
array (`llvm::ConstantArray`)? Replacing all uses with an undefined value
doesn’t work as the array is terminated by an undefined value, subsequent
elements thus are ignored at runtime. I’d rather avoid re-creating the entire
array from scratch and replace the existing array with it but I’m afraid there’s
not many other options.
>> 
>> Is there any solution I’m not aware of yet?
> 
> In many cases, the only way to "change" a value using the LLVM
C++ API is to create a new value and replace uses of the old value with the new
value.  I haven't looked, but I would not be surprised if this is the only
way to "modify" the number of elements in a ConstantArray.
> 
> I can see how removing a ConstantArray element or creating a new
ConstantArray with fewer elements would be a bit of a pain: you would need to
change the ConstantArray, all the GEPs for the constant array, and potentially
handle code that casts the ConstantArray to another type.
> 
> What are you trying to achieve be "removing" an element from a
ConstantArray?  Perhaps there is an easier way to do what you're trying to
do.
> 
> Regards,
> 
> John Criswell
> 
>> 
>> Thanks,
>> Nicolai
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 
> 
> -- 
> John Criswell
> Assistant Professor
> Department of Computer Science, University of Rochester
> http://www.cs.rochester.edu/u/criswell
>