llvm dev - Jan 2009 - [LLVMdev] RFA: Constant String c"\000"

The Constants.cpp file returns a ConstantAggregateZero object when you
pass it a c"\000" string. Here is the code:

Constant *ConstantArray::get(const ArrayType *Ty,
                             const std::vector<Constant*> &V) {
  // If this is an all-zero array, return a ConstantAggregateZero
object
  if (!V.empty()) {
    Constant *C = V[0];
    if (!C->isNullValue())
      return ArrayConstants->getOrCreate(Ty, V);
    for (unsigned i = 1, e = V.size(); i != e; ++i)
      if (V[i] != C)
        return ArrayConstants->getOrCreate(Ty, V);
  }
  return ConstantAggregateZero::get(Ty);
}

This seems wrong to me, and it's causing a problem in Objective-C,
which requires that an empty string be placed in the .cstring section.

Would it break everything or hurt performance to have a check in here
that if this is a [1 x i8] array where C->isNullValue(), we should
allow it to stay instead of creating the CAZ, which is translated into
a "zeroinitialier"?

-bw

On Wed, Jan 21, 2009 at 3:12 PM, Bill Wendling <isanbard at gmail.com>
wrote:
> This seems wrong to me, and it's causing a problem in Objective-C,
> which requires that an empty string be placed in the .cstring section.
What's wrong?  It is all zeros.  If some particular user requires it
to be in a special section, it should be marked as such; we shouldn't
be guaranteeing a different representation for two constructs which
fundamentally represent the same thing.

That said, I don't think allowing it would break anything...

-Eli

On Jan 21, 2009, at 3:12 PM, Bill Wendling wrote:
> The Constants.cpp file returns a ConstantAggregateZero object when you
> pass it a c"\000" string. Here is the code:
>
> Constant *ConstantArray::get(const ArrayType *Ty,
>                             const std::vector<Constant*> &V) {
>  // If this is an all-zero array, return a ConstantAggregateZero
> object
>  if (!V.empty()) {
>    Constant *C = V[0];
>    if (!C->isNullValue())
>      return ArrayConstants->getOrCreate(Ty, V);
>    for (unsigned i = 1, e = V.size(); i != e; ++i)
>      if (V[i] != C)
>        return ArrayConstants->getOrCreate(Ty, V);
>  }
>  return ConstantAggregateZero::get(Ty);
> }
>
> This seems wrong to me, and it's causing a problem in Objective-C,
> which requires that an empty string be placed in the .cstring section.
>
> Would it break everything or hurt performance to have a check in here
> that if this is a [1 x i8] array where C->isNullValue(), we should
> allow it to stay instead of creating the CAZ, which is translated into
> a "zeroinitialier"?
Bill and I spoke about this.  Using CAZ to represent an all-zeros  
array is a fine thing for the LLVM IR to do, and we should continue  
it.  If ObjC needs its string in a special place (not cstring) then  
the front-end should put the right section info on the globalvariable.

-Chris