llvm-mirror/docs/AddingConstrainedIntrinsics.rst

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How To Add A Constrained Floating-Point Intrinsic
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.. contents::
   :local:

.. 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

Update class ConstrainedFPIntrinsic to know about the intrinsics.::

  include/llvm/IR/IntrinsicInst.h

Functions like ConstrainedFPIntrinsic::isUnaryOp() or
ConstrainedFPIntrinsic::isTernaryOp() may need to know about the new
intrinsic.::

  lib/IR/IntrinsicInst.cpp

Update the IR verifier::

  lib/IR/Verifier.cpp

Add SelectionDAG node types
===========================

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

  include/llvm/CodeGen/ISDOpcodes.h

In class SDNode update isStrictFPOpcode()::

  include/llvm/CodeGen/SelectionDAGNodes.h

A mapping from the STRICT SDnode type to the non-STRICT is done in
TargetLoweringBase::getStrictFPOperationAction(). This allows STRICT
nodes to be legalized similarly to the non-STRICT node type.::

  include/llvm/CodeGen/TargetLowering.h

Building the SelectionDAG
-------------------------

The switch statement in SelectionDAGBuilder::visitIntrinsicCall() needs
to be updated to call SelectionDAGBuilder::visitConstrainedFPIntrinsic().
That function, in turn, needs to be updated to know how to create the
SDNode for the intrinsic. 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(). 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

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

  lib/CodeGen/SelectionDAG/SelectionDAGDumper.cpp

Add documentation and tests
===========================

::

  docs/LangRef.rst