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176 lines
7.6 KiB
ReStructuredText
===============================
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How To Use Instruction Mappings
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===============================
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.. contents::
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:local:
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Introduction
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============
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This document contains information about adding instruction mapping support
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for a target. The motivation behind this feature comes from the need to switch
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between different instruction formats during various optimizations. One approach
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could be to use switch cases which list all the instructions along with formats
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they can transition to. However, it has large maintenance overhead
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because of the hardcoded instruction names. Also, whenever a new instruction is
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added in the .td files, all the relevant switch cases should be modified
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accordingly. Instead, the same functionality could be achieved with TableGen and
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some support from the .td files for a fraction of maintenance cost.
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``InstrMapping`` Class Overview
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===============================
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TableGen uses relationship models to map instructions with each other. These
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models are described using ``InstrMapping`` class as a base. Each model sets
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various fields of the ``InstrMapping`` class such that they can uniquely
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describe all the instructions using that model. TableGen parses all the relation
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models and uses the information to construct relation tables which relate
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instructions with each other. These tables are emitted in the
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``XXXInstrInfo.inc`` file along with the functions to query them. Following
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is the definition of ``InstrMapping`` class defined in Target.td file:
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.. code-block:: text
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class InstrMapping {
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// Used to reduce search space only to the instructions using this
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// relation model.
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string FilterClass;
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// List of fields/attributes that should be same for all the instructions in
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// a row of the relation table. Think of this as a set of properties shared
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// by all the instructions related by this relationship.
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list<string> RowFields = [];
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// List of fields/attributes that are same for all the instructions
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// in a column of the relation table.
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list<string> ColFields = [];
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// Values for the fields/attributes listed in 'ColFields' corresponding to
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// the key instruction. This is the instruction that will be transformed
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// using this relation model.
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list<string> KeyCol = [];
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// List of values for the fields/attributes listed in 'ColFields', one for
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// each column in the relation table. These are the instructions a key
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// instruction will be transformed into.
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list<list<string> > ValueCols = [];
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}
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Sample Example
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--------------
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Let's say that we want to have a function
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``int getPredOpcode(uint16_t Opcode, enum PredSense inPredSense)`` which
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takes a non-predicated instruction and returns its predicated true or false form
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depending on some input flag, ``inPredSense``. The first step in the process is
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to define a relationship model that relates predicated instructions to their
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non-predicated form by assigning appropriate values to the ``InstrMapping``
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fields. For this relationship, non-predicated instructions are treated as key
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instruction since they are the one used to query the interface function.
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.. code-block:: text
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def getPredOpcode : InstrMapping {
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// Choose a FilterClass that is used as a base class for all the
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// instructions modeling this relationship. This is done to reduce the
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// search space only to these set of instructions.
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let FilterClass = "PredRel";
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// Instructions with same values for all the fields in RowFields form a
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// row in the resulting relation table.
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// For example, if we want to relate 'ADD' (non-predicated) with 'Add_pt'
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// (predicated true) and 'Add_pf' (predicated false), then all 3
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// instructions need to have same value for BaseOpcode field. It can be any
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// unique value (Ex: XYZ) and should not be shared with any other
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// instruction not related to 'add'.
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let RowFields = ["BaseOpcode"];
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// List of attributes that can be used to define key and column instructions
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// for a relation. Key instruction is passed as an argument
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// to the function used for querying relation tables. Column instructions
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// are the instructions they (key) can transform into.
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//
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// Here, we choose 'PredSense' as ColFields since this is the unique
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// attribute of the key (non-predicated) and column (true/false)
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// instructions involved in this relationship model.
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let ColFields = ["PredSense"];
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// The key column contains non-predicated instructions.
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let KeyCol = ["none"];
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// Two value columns - first column contains instructions with
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// PredSense=true while second column has instructions with PredSense=false.
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let ValueCols = [["true"], ["false"]];
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}
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TableGen uses the above relationship model to emit relation table that maps
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non-predicated instructions with their predicated forms. It also outputs the
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interface function
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``int getPredOpcode(uint16_t Opcode, enum PredSense inPredSense)`` to query
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the table. Here, Function ``getPredOpcode`` takes two arguments, opcode of the
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current instruction and PredSense of the desired instruction, and returns
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predicated form of the instruction, if found in the relation table.
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In order for an instruction to be added into the relation table, it needs
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to include relevant information in its definition. For example, consider
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following to be the current definitions of ADD, ADD_pt (true) and ADD_pf (false)
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instructions:
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.. code-block:: text
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def ADD : ALU32_rr<(outs IntRegs:$dst), (ins IntRegs:$a, IntRegs:$b),
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"$dst = add($a, $b)",
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[(set (i32 IntRegs:$dst), (add (i32 IntRegs:$a),
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(i32 IntRegs:$b)))]>;
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def ADD_Pt : ALU32_rr<(outs IntRegs:$dst),
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(ins PredRegs:$p, IntRegs:$a, IntRegs:$b),
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"if ($p) $dst = add($a, $b)",
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[]>;
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def ADD_Pf : ALU32_rr<(outs IntRegs:$dst),
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(ins PredRegs:$p, IntRegs:$a, IntRegs:$b),
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"if (!$p) $dst = add($a, $b)",
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[]>;
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In this step, we modify these instructions to include the information
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required by the relationship model, <tt>getPredOpcode</tt>, so that they can
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be related.
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.. code-block:: text
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def ADD : PredRel, ALU32_rr<(outs IntRegs:$dst), (ins IntRegs:$a, IntRegs:$b),
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"$dst = add($a, $b)",
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[(set (i32 IntRegs:$dst), (add (i32 IntRegs:$a),
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(i32 IntRegs:$b)))]> {
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let BaseOpcode = "ADD";
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let PredSense = "none";
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}
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def ADD_Pt : PredRel, ALU32_rr<(outs IntRegs:$dst),
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(ins PredRegs:$p, IntRegs:$a, IntRegs:$b),
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"if ($p) $dst = add($a, $b)",
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[]> {
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let BaseOpcode = "ADD";
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let PredSense = "true";
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}
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def ADD_Pf : PredRel, ALU32_rr<(outs IntRegs:$dst),
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(ins PredRegs:$p, IntRegs:$a, IntRegs:$b),
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"if (!$p) $dst = add($a, $b)",
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[]> {
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let BaseOpcode = "ADD";
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let PredSense = "false";
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}
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Please note that all the above instructions use ``PredRel`` as a base class.
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This is extremely important since TableGen uses it as a filter for selecting
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instructions for ``getPredOpcode`` model. Any instruction not derived from
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``PredRel`` is excluded from the analysis. ``BaseOpcode`` is another important
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field. Since it's selected as a ``RowFields`` of the model, it is required
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to have the same value for all 3 instructions in order to be related. Next,
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``PredSense`` is used to determine their column positions by comparing its value
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with ``KeyCol`` and ``ValueCols``. If an instruction sets its ``PredSense``
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value to something not used in the relation model, it will not be assigned
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a column in the relation table.
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