llvm dev - Mar 2016 - Should @llvm.used be a NamedMDNode?

Hi,

Should @llvm.used be a NamedMDNode instead of a GlobalVariable?

I think this would make it easier to mark globals as used. The current method
involves (1) copying the existing elements of @llvm.used into a container, (2)
erasing @llvm.used, (3) appending the new global to the container, (4)
re-creating @llvm.used (thereby polluting the ConstantArray cache?), and (5)
calling LLVMUsed->setSection("llvm.metadata"). See
InstrProfiling::emitUses.

If we use NamedMDNode, this could be as simple as:

  auto *LLVMUsed = M->getOrInsertNamedMetadata("llvm.used");
  LLVMUsed->addOperand(ConstantAsMetadata::get(C));

I took a cursory look around and it doesn't seem like we really depend on
@llvm.used being a GlobalVariable.

Am I wrong about this? Other thoughts?

thanks
vedant

> On Mar 2, 2016, at 4:19 PM, Vedant Kumar <vsk at apple.com> wrote:
> 
> Hi,
> 
> Should @llvm.used be a NamedMDNode instead of a GlobalVariable?
> 
> I think this would make it easier to mark globals as used. The current
method
> involves (1) copying the existing elements of @llvm.used into a container,
(2)
> erasing @llvm.used, (3) appending the new global to the container, (4)
> re-creating @llvm.used (thereby polluting the ConstantArray cache?), and
(5)
> calling LLVMUsed->setSection("llvm.metadata"). See
InstrProfiling::emitUses.
> 
> If we use NamedMDNode, this could be as simple as:
> 
>  auto *LLVMUsed = M->getOrInsertNamedMetadata("llvm.used");
>  LLVMUsed->addOperand(ConstantAsMetadata::get(C));
Actually, the ConstantAsMetadata would have to be wrapped in an MDTuple,
because Metadata doesn't inherit from MDNode.

The point still stands.

vedant
> 
> I took a cursory look around and it doesn't seem like we really depend
on
> @llvm.used being a GlobalVariable.
> 
> Am I wrong about this? Other thoughts?
> 
> thanks
> vedant

> On Mar 2, 2016, at 4:19 PM, Vedant Kumar via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Hi,
> 
> Should @llvm.used be a NamedMDNode instead of a GlobalVariable?
> 
> I think this would make it easier to mark globals as used. The current
method
> involves (1) copying the existing elements of @llvm.used into a container,
(2)
> erasing @llvm.used, (3) appending the new global to the container, (4)
> re-creating @llvm.used (thereby polluting the ConstantArray cache?), and
(5)
> calling LLVMUsed->setSection("llvm.metadata"). See
InstrProfiling::emitUses.
> 
> If we use NamedMDNode, this could be as simple as:
> 
>  auto *LLVMUsed = M->getOrInsertNamedMetadata("llvm.used");
>  LLVMUsed->addOperand(ConstantAsMetadata::get(C));
> 
> I took a cursory look around and it doesn't seem like we really depend
on
> @llvm.used being a GlobalVariable.
> 
> Am I wrong about this? Other thoughts?
I’m not sure that this will work.  One of the reasons that it is a global
variable is that we *want* the global variable to show up in the uselist for the
“used” value (and thus appear to be used in an “unknown” way and treated
conservatively).  Metadata aren’t typically in the use list for a value.

-Chris