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