IR: Split Metadata from Value


IR: Split Metadata from Value

Split Metadata away from the Value class hierarchy, as part of
PR21532. Assembly and bitcode changes are in the wings, but this is the
bulk of the change for the IR C++ API.

I have a follow-up patch prepared for clang. If this breaks other
sub-projects, I apologize in advance :(. Help me compile it on Darwin
I'll try to fix it. FWIW, the errors should be easy to fix, so it may
be simpler to just fix it yourself.

This breaks the build for all metadata-related code that's out-of-tree.
Rest assured the transition is mechanical and the compiler should catch
almost all of the problems.

Here's a quick guide for updating your code:

  • Metadata is the root of a class hierarchy with three main classes: MDNode, MDString, and ValueAsMetadata. It is distinct from the Value class hierarchy. It is typeless -- i.e., instances do *not* have a Type.
  • MDNode's operands are all Metadata * (instead of Value *).
  • TrackingVH<MDNode> and WeakVH referring to metadata can be replaced with TrackingMDNodeRef and TrackingMDRef, respectively.

    If you're referring solely to resolved MDNodes -- post graph construction -- just use MDNode*.
  • MDNode (and the rest of Metadata) have only limited support for replaceAllUsesWith().

    As long as an MDNode is pointing at a forward declaration -- the result of MDNode::getTemporary() -- it maintains a side map of its uses and can RAUW itself. Once the forward declarations are fully resolved RAUW support is dropped on the ground. This means that uniquing collisions on changing operands cause nodes to become "distinct". (This already happened fairly commonly, whenever an operand went to null.)

    If you're constructing complex (non self-reference) MDNode cycles, you need to call MDNode::resolveCycles() on each node (or on a top-level node that somehow references all of the nodes). Also, don't do that. Metadata cycles (and the RAUW machinery needed to construct them) are expensive.
  • An MDNode can only refer to a Constant through a bridge called ConstantAsMetadata (one of the subclasses of ValueAsMetadata).

    As a side effect, accessing an operand of an MDNode that is known to be, e.g., ConstantInt, takes three steps: first, cast from Metadata to ConstantAsMetadata; second, extract the Constant; third, cast down to ConstantInt.

    The eventual goal is to introduce MDInt/MDFloat/etc. and have metadata schema owners transition away from using Constants when the type isn't important (and they don't care about referring to GlobalValues).

    In the meantime, I've added transitional API to the mdconst namespace that matches semantics with the old code, in order to avoid adding the error-prone three-step equivalent to every call site. If your old code was:

    MDNode *N = foo(); bar(isa <ConstantInt>(N->getOperand(0))); baz(cast <ConstantInt>(N->getOperand(1))); bak(cast_or_null <ConstantInt>(N->getOperand(2))); bat(dyn_cast <ConstantInt>(N->getOperand(3))); bay(dyn_cast_or_null<ConstantInt>(N->getOperand(4)));

    you can trivially match its semantics with:

    MDNode *N = foo(); bar(mdconst::hasa <ConstantInt>(N->getOperand(0))); baz(mdconst::extract <ConstantInt>(N->getOperand(1))); bak(mdconst::extract_or_null <ConstantInt>(N->getOperand(2))); bat(mdconst::dyn_extract <ConstantInt>(N->getOperand(3))); bay(mdconst::dyn_extract_or_null<ConstantInt>(N->getOperand(4)));

    and when you transition your metadata schema to MDInt:

    MDNode *N = foo(); bar(isa <MDInt>(N->getOperand(0))); baz(cast <MDInt>(N->getOperand(1))); bak(cast_or_null <MDInt>(N->getOperand(2))); bat(dyn_cast <MDInt>(N->getOperand(3))); bay(dyn_cast_or_null<MDInt>(N->getOperand(4)));
  • A CallInst -- specifically, intrinsic instructions -- can refer to metadata through a bridge called MetadataAsValue. This is a subclass of Value where getType()->isMetadataTy().

    MetadataAsValue is the *only* class that can legally refer to a LocalAsMetadata, which is a bridged form of non-Constant values like Argument and Instruction. It can also refer to any other Metadata subclass.

(I'll break all your testcases in a follow-up commit, when I propagate
this change to assembly.)


dexonsmithDec 9 2014, 10:38 AM
rL223801: AsmParser: Don't crash on malformed attribute groups