[cfe-dev] [LLVMdev] [RFC] Module Flags Metadata

Devang Patel dpatel at apple.com
Thu Jan 26 12:54:09 PST 2012

On Jan 26, 2012, at 11:15 AM, Dan Gohman wrote:

> On Jan 24, 2012, at 3:39 PM, Devang Patel wrote:
>>>> I have only one real comment -- this violates the contract and spirit of LLVM's metadata design. You're specifically encoding semantics in metadata, but the principle of metadata is that a program with all metadata stripped has the same behavior as one with the metadata still in place.
>> This is a simplified understanding of semantics. As I understand, the expected metadata design behavior is that optimizer/transformations are not responsible to preserve any _relationship_ between a User and a MDNode. For example, if a MDNode is  "using" a User then optimizer can remove the User without bothering about what happens to the MDNode.
> Right.
>> Same way, If MDNode is attached to an Instruction then optimizer can mutate, delete or replace the Instruction while completely ignoring attached MDNode. 
> However, this isn't necessarily true. For example, it would seem to be
> within the spirit of LLVM's metadata design to describe the range of values
> that a given instruction might have. However, if the optimizer mutates the
> instruction (and preserves program correctness by mutating its operand
> instructions to compensate), then that metadata could easily become
> incorrect. Right now, there aren't any rules about what metadata can do,

> or what optimizers must do to preserve it.

The number one reason behind metadata is to have a mechanism to track values while being completely transparent to optimizer. If you want a guarantee from the optimizer to preserve certain semantics about the way metadata is used (e.g. say to describe range of values) then metadata is not appropriate mechanism.


More information about the cfe-dev mailing list