1==================================================
2How To Add A Constrained Floating-Point Intrinsic
3==================================================
4
5.. contents::
6   :local:
7
8.. warning::
9  This is a work in progress.
10
11Add the intrinsic
12=================
13
14Multiple files need to be updated when adding a new constrained intrinsic.
15
16Add the new intrinsic to the table of intrinsics::
17
18  include/llvm/IR/Intrinsics.td
19
20Add SelectionDAG node types
21===========================
22
23Add the new STRICT version of the node type to the ISD::NodeType enum::
24
25  include/llvm/CodeGen/ISDOpcodes.h
26
27Strict version name must be a concatenation of prefix ``STRICT_`` and the name
28of corresponding non-strict node name. For instance, strict version of the
29node FADD must be STRICT_FADD.
30
31Update mappings
32===============
33
34Add new record to the mapping of instructions to constrained intrinsic and
35DAG nodes::
36
37  include/llvm/IR/ConstrainedOps.def
38
39Follow instructions provided in this file.
40
41Update IR components
42====================
43
44Update the IR verifier::
45
46  lib/IR/Verifier.cpp
47
48Update Selector components
49==========================
50
51Building the SelectionDAG
52-------------------------
53
54The function SelectionDAGBuilder::visitConstrainedFPIntrinsic builds DAG nodes
55using mappings specified in ConstrainedOps.def. If however this default build is
56not sufficient, the build can be modified, see how it is implemented for
57STRICT_FP_ROUND. The new STRICT node will eventually be converted
58to the matching non-STRICT node. For this reason it should have the same
59operands and values as the non-STRICT version but should also use the chain.
60This makes subsequent sharing of code for STRICT and non-STRICT code paths
61easier::
62
63  lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
64
65Most of the STRICT nodes get legalized the same as their matching non-STRICT
66counterparts. A new STRICT node with this property must get added to the
67switch in SelectionDAGLegalize::LegalizeOp().::
68
69  lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
70
71Other parts of the legalizer may need to be updated as well. Look for
72places where the non-STRICT counterpart is legalized and update as needed.
73Be careful of the chain since STRICT nodes use it but their counterparts
74often don't.
75
76The code to do the conversion or mutation of the STRICT node to a non-STRICT
77version of the node happens in SelectionDAG::mutateStrictFPToFP(). In most cases
78the function can do the conversion using information from ConstrainedOps.def. Be
79careful updating this function since some nodes have the same return type
80as their input operand, but some are different. Both of these cases must
81be properly handled::
82
83  lib/CodeGen/SelectionDAG/SelectionDAG.cpp
84
85Whether the mutation may happens or not, depends on how the new node has been
86registered in TargetLoweringBase::initActions(). By default all strict nodes are
87registered with Expand action::
88
89  lib/CodeGen/TargetLoweringBase.cpp
90
91To make debug logs readable it is helpful to update the SelectionDAG's
92debug logger:::
93
94  lib/CodeGen/SelectionDAG/SelectionDAGDumper.cpp
95
96Add documentation and tests
97===========================
98
99::
100
101  docs/LangRef.rst
102