How To Add A Constrained Floating-Point Intrinsic

Warning

This is a work in progress.

Add the intrinsic

Multiple files need to be updated when adding a new constrained intrinsic.

Add the new intrinsic to the table of intrinsics:

include/llvm/IR/Intrinsics.td

Add SelectionDAG node types

Add the new STRICT version of the node type to the ISD::NodeType enum:

include/llvm/CodeGen/ISDOpcodes.h

Strict version name must be a concatenation of prefix STRICT_ and the name of corresponding non-strict node name. For instance, strict version of the node FADD must be STRICT_FADD.

Update mappings

Add new record to the mapping of instructions to constrained intrinsic and DAG nodes:

include/llvm/IR/ConstrainedOps.def

Follow instructions provided in this file.

Update IR components

Update the IR verifier:

lib/IR/Verifier.cpp

Update Selector components

Building the SelectionDAG

The function SelectionDAGBuilder::visitConstrainedFPIntrinsic builds DAG nodes using mappings specified in ConstrainedOps.def. If however this default build is not sufficient, the build can be modified, see how it is implemented for STRICT_FP_ROUND. The new STRICT node will eventually be converted to the matching non-STRICT node. For this reason it should have the same operands and values as the non-STRICT version but should also use the chain. This makes subsequent sharing of code for STRICT and non-STRICT code paths easier:

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

Most of the STRICT nodes get legalized the same as their matching non-STRICT counterparts. A new STRICT node with this property must get added to the switch in SelectionDAGLegalize::LegalizeOp().:

lib/CodeGen/SelectionDAG/LegalizeDAG.cpp

Other parts of the legalizer may need to be updated as well. Look for places where the non-STRICT counterpart is legalized and update as needed. Be careful of the chain since STRICT nodes use it but their counterparts often don’t.

The code to do the conversion or mutation of the STRICT node to a non-STRICT version of the node happens in SelectionDAG::mutateStrictFPToFP(). In most cases the function can do the conversion using information from ConstrainedOps.def. Be careful updating this function since some nodes have the same return type as their input operand, but some are different. Both of these cases must be properly handled:

lib/CodeGen/SelectionDAG/SelectionDAG.cpp

Whether the mutation may happens or not, depends on how the new node has been registered in TargetLoweringBase::initActions(). By default all strict nodes are registered with Expand action:

lib/CodeGen/TargetLoweringBase.cpp

To make debug logs readable it is helpful to update the SelectionDAG’s debug logger::

lib/CodeGen/SelectionDAG/SelectionDAGDumper.cpp