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568 lines
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568 lines
22 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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"http://www.w3.org/TR/html4/strict.dtd">
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<html>
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<head>
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<title>TableGen Fundamentals</title>
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<link rel="stylesheet" href="llvm.css" type="text/css">
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</head>
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<body>
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<div class="doc_title">TableGen Fundamentals</div>
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<div class="doc_text">
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<ul>
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<li><a href="#introduction">Introduction</a>
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<ol>
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<li><a href="#concepts">Basic concepts</a></li>
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<li><a href="#example">An example record</a></li>
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<li><a href="#running">Running TableGen</a></li>
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</ol></li>
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<li><a href="#syntax">TableGen syntax</a>
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<ol>
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<li><a href="#primitives">TableGen primitives</a>
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<ol>
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<li><a href="#comments">TableGen comments</a></li>
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<li><a href="#types">The TableGen type system</a></li>
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<li><a href="#values">TableGen values and expressions</a></li>
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</ol></li>
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<li><a href="#classesdefs">Classes and definitions</a>
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<ol>
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<li><a href="#valuedef">Value definitions</a></li>
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<li><a href="#recordlet">'let' expressions</a></li>
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<li><a href="#templateargs">Class template arguments</a></li>
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</ol></li>
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<li><a href="#filescope">File scope entities</a>
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<ol>
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<li><a href="#include">File inclusion</a></li>
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<li><a href="#globallet">'let' expressions</a></li>
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</ol></li>
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</ol></li>
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<li><a href="#backends">TableGen backends</a>
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<ol>
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<li><a href="#">todo</a></li>
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</ol></li>
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</ul>
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</div>
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<div class="doc_author">
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<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"><a name="introduction">Introduction</a></div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>TableGen's purpose is to help a human develop and maintain records of
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domain-specific information. Because there may be a large number of these
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records, it is specifically designed to allow writing flexible descriptions and
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for common features of these records to be factored out. This reduces the
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amount of duplication in the description, reduces the chance of error, and
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makes it easier to structure domain specific information.</p>
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<p>The core part of TableGen <a href="#syntax">parses a file</a>, instantiates
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the declarations, and hands the result off to a domain-specific "<a
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href="#backends">TableGen backend</a>" for processing. The current major user
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of TableGen is the <a href="CodeGenerator.html">LLVM code generator</a>.</p>
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<p>Note that if you work on TableGen much, and use emacs or vim, that you can
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find an emacs "TableGen mode" and a vim language file in
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<tt>llvm/utils/emacs</tt> and <tt>llvm/utils/vim</tt> directory of your LLVM
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distribution, respectively.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection"><a name="running">Basic concepts</a></div>
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<div class="doc_text">
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<p>TableGen files consist of two key parts: 'classes' and 'definitions', both
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of which are considered 'records'.</p>
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<p><b>TableGen records</b> have a unique name, a list of values, and a list of
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superclasses. The list of values is main data that TableGen builds for each
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record, it is this that holds the domain specific information for the
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application. The interpretation of this data is left to a specific <a
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href="#backends">TableGen backend</a>, but the structure and format rules are
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taken care of and fixed by TableGen.</p>
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<p><b>TableGen definitions</b> are the concrete form of 'records'. These
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generally do not have any undefined values, and are marked with the
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'<tt>def</tt>' keyword.</p>
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<p><b>TableGen classes</b> are abstract records that are used to build and
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describe other records. These 'classes' allow the end-user to build
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abstractions for either the domain they are targetting (such as "Register",
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"RegisterClass", and "Instruction" in the LLVM code generator) or for the
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implementor to help factor out common properties of records (such as "FPInst",
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which is used to represent floating point instructions in the X86 backend).
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TableGen keeps track of all of the classes that are used to build up a
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definition, so the backend can find all definitions of a particular class, such
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as "Instruction".</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection"><a name="example">An example record</a></div>
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<div class="doc_text">
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<p>With no other arguments, TableGen parses the specified file and prints out
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all of the classes, then all of the definitions. This is a good way to see what
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the various definitions expand to fully. Running this on the <tt>X86.td</tt>
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file prints this (at the time of this writing):</p>
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<pre>
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...
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<b>def</b> ADDrr8 { <i>// Instruction X86Inst I2A8 Pattern</i>
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<b>string</b> Name = "add";
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<b>string</b> Namespace = "X86";
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<b>list</b><Register> Uses = [];
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<b>list</b><Register> Defs = [];
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<b>bit</b> isReturn = 0;
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<b>bit</b> isBranch = 0;
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<b>bit</b> isCall = 0;
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<b>bit</b> isTwoAddress = 1;
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<b>bit</b> isTerminator = 0;
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<b>dag</b> Pattern = (set R8, (plus R8, R8));
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<b>bits</b><8> Opcode = { 0, 0, 0, 0, 0, 0, 0, 0 };
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Format Form = MRMDestReg;
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<b>bits</b><5> FormBits = { 0, 0, 0, 1, 1 };
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ArgType Type = Arg8;
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<b>bits</b><3> TypeBits = { 0, 0, 1 };
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<b>bit</b> hasOpSizePrefix = 0;
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<b>bit</b> printImplicitUses = 0;
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<b>bits</b><4> Prefix = { 0, 0, 0, 0 };
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FPFormat FPForm = ?;
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<b>bits</b><3> FPFormBits = { 0, 0, 0 };
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}
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...
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</pre>
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<p>This definition corresponds to an 8-bit register-register add instruction in
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the X86. The string after the '<tt>def</tt>' string indicates the name of the
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record ("<tt>ADDrr8</tt>" in this case), and the comment at the end of the line
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indicates the superclasses of the definition. The body of the record contains
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all of the data that TableGen assembled for the record, indicating that the
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instruction is part of the "X86" namespace, should be printed as "<tt>add</tt>"
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in the assembly file, it is a two-address instruction, has a particular
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encoding, etc. The contents and semantics of the information in the record is
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specific to the needs of the X86 backend, and is only shown as an example.</p>
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<p>As you can see, a lot of information is needed for every instruction
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supported by the code generator, and specifying it all manually would be
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unmaintainble, prone to bugs, and tiring to do in the first place. Because we
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are using TableGen, all of the information was derived from the following
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definition:</p>
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<pre>
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<b>def</b> ADDrr8 : I2A8<"add", 0x00, MRMDestReg>,
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Pattern<(set R8, (plus R8, R8))>;
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</pre>
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<p>This definition makes use of the custom I2A8 (two address instruction with
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8-bit operand) class, which is defined in the X86-specific TableGen file to
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factor out the common features that instructions of its class share. A key
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feature of TableGen is that it allows the end-user to define the abstractions
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they prefer to use when describing their information.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection"><a name="running">Running TableGen</a></div>
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<div class="doc_text">
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<p>TableGen runs just like any other LLVM tool. The first (optional) argument
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specifies the file to read. If a filename is not specified, <tt>tblgen</tt>
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reads from standard input.</p>
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<p>To be useful, one of the <a href="#backends">TableGen backends</a> must be
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used. These backends are selectable on the command line (type '<tt>tblgen
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--help</tt>' for a list). For example, to get a list of all of the definitions
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that subclass a particular type (which can be useful for building up an enum
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list of these records), use the <tt>--print-enums</tt> option:</p>
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<pre>
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$ tblgen X86.td -print-enums -class=Register
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AH, AL, AX, BH, BL, BP, BX, CH, CL, CX, DH, DI, DL, DX,
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EAX, EBP, EBX, ECX, EDI, EDX, ESI, ESP, FP0, FP1, FP2, FP3, FP4, FP5, FP6,
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SI, SP, ST0, ST1, ST2, ST3, ST4, ST5, ST6, ST7,
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$ tblgen X86.td -print-enums -class=Instruction
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ADCrr32, ADDri16, ADDri16b, ADDri32, ADDri32b, ADDri8, ADDrr16, ADDrr32,
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ADDrr8, ADJCALLSTACKDOWN, ADJCALLSTACKUP, ANDri16, ANDri16b, ANDri32, ANDri32b,
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ANDri8, ANDrr16, ANDrr32, ANDrr8, BSWAPr32, CALLm32, CALLpcrel32, ...
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</pre>
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<p>The default backend prints out all of the records, as described <a
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href="#example">above</a>.</p>
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<p>If you plan to use TableGen for some purpose, you will most likely have to
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<a href="#backends">write a backend</a> that extracts the information specific
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to what you need and formats it in the appropriate way.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"><a name="syntax">TableGen syntax</a></div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>TableGen doesn't care about the meaning of data (that is up to the backend
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to define), but it does care about syntax, and it enforces a simple type system.
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This section describes the syntax and the constructs allowed in a TableGen file.
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</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection"><a name="primitives">TableGen primitives</a></div>
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<!-- -------------------------------------------------------------------------->
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<div class="doc_subsubsection"><a name="comments">TableGen comments</a></div>
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<div class="doc_text">
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<p>TableGen supports BCPL style "<tt>//</tt>" comments, which run to the end of
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the line, and it also supports <b>nestable</b> "<tt>/* */</tt>" comments.</p>
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</div>
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<!-- -------------------------------------------------------------------------->
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<div class="doc_subsubsection">
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<a name="types">The TableGen type system</a>
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</div>
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<div class="doc_text">
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<p>TableGen files are strongly typed, in a simple (but complete) type-system.
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These types are used to perform automatic conversions, check for errors, and to
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help interface designers constrain the input that they allow. Every <a
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href="#valuedef">value definition</a> is required to have an associated type.
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</p>
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<p>TableGen supports a mixture of very low-level types (such as <tt>bit</tt>)
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and very high-level types (such as <tt>dag</tt>). This flexibility is what
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allows it to describe a wide range of information conveniently and compactly.
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The TableGen types are:</p>
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<ul>
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<li>"<tt><b>bit</b></tt>" - A 'bit' is a boolean value that can hold either 0 or
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1.</li>
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<li>"<tt><b>int</b></tt>" - The 'int' type represents a simple 32-bit integer
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value, such as 5.</li>
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<li>"<tt><b>string</b></tt>" - The 'string' type represents an ordered sequence
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of characters of arbitrary length.</li>
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<li>"<tt><b>bits</b><n></tt>" - A 'bits' type is an arbitrary, but fixed,
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size integer that is broken up into individual bits. This type is useful
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because it can handle some bits being defined while others are undefined.</li>
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<li>"<tt><b>list</b><ty></tt>" - This type represents a list whose
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elements are some other type. The contained type is arbitrary: it can even be
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another list type.</li>
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<li>Class type - Specifying a class name in a type context means that the
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defined value must be a subclass of the specified class. This is useful in
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conjunction with the "list" type, for example, to constrain the elements of the
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list to a common base class (e.g., a <tt><b>list</b><Register></tt> can
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only contain definitions derived from the "<tt>Register</tt>" class).</li>
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<li>"<tt><b>code</b></tt>" - This represents a big hunk of text. NOTE: I don't
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remember why this is distinct from string!</li>
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<li>"<tt><b>dag</b></tt>" - This type represents a nestable directed graph of
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elements.</li>
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</ul>
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<p>To date, these types have been sufficient for describing things that
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TableGen has been used for, but it is straight-forward to extend this list if
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needed.</p>
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</div>
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<!-- -------------------------------------------------------------------------->
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<div class="doc_subsubsection">
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<a name="values">TableGen values and expressions</a>
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</div>
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<div class="doc_text">
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<p>TableGen allows for a pretty reasonable number of different expression forms
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when building up values. These forms allow the TableGen file to be written in a
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natural syntax and flavor for the application. The current expression forms
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supported include:</p>
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<ul>
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<li><tt>?</tt> - uninitialized field</li>
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<li><tt>0b1001011</tt> - binary integer value</li>
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<li><tt>07654321</tt> - octal integer value (indicated by a leading 0)</li>
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<li><tt>7</tt> - decimal integer value</li>
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<li><tt>0x7F</tt> - hexadecimal integer value</li>
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<li><tt>"foo"</tt> - string value</li>
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<li><tt>[{ ... }]</tt> - code fragment</li>
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<li><tt>[ X, Y, Z ]</tt> - list value.</li>
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<li><tt>{ a, b, c }</tt> - initializer for a "bits<3>" value</li>
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<li><tt>value</tt> - value reference</li>
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<li><tt>value{17}</tt> - access to one bit of a value</li>
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<li><tt>value{15-17}</tt> - access to multiple bits of a value</li>
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<li><tt>DEF</tt> - reference to a record definition</li>
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<li><tt>CLASS<val list></tt> - reference to a new anonymous definition of
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CLASS with the specified template arguments.</li>
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<li><tt>X.Y</tt> - reference to the subfield of a value</li>
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<li><tt>list[4-7,17,2-3]</tt> - A slice of the 'list' list, including elements
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4,5,6,7,17,2, and 3 from it. Elements may be included multiple times.</li>
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<li><tt>(DEF a, b)</tt> - a dag value. The first element is required to be a
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record definition, the remaining elements in the list may be arbitrary other
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values, including nested `<tt>dag</tt>' values.</li>
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</ul>
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<p>Note that all of the values have rules specifying how they convert to values
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for different types. These rules allow you to assign a value like "7" to a
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"bits<4>" value, for example.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="classesdefs">Classes and definitions</a>
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</div>
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<div class="doc_text">
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<p>As mentioned in the <a href="#concepts">intro</a>, classes and definitions
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(collectively known as 'records') in TableGen are the main high-level unit of
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information that TableGen collects. Records are defined with a <tt>def</tt> or
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<tt>class</tt> keyword, the record name, and an optional list of "<a
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href="#templateargs">template arguments</a>". If the record has superclasses,
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they are specified as a comma separated list that starts with a colon character
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(":"). If <a href="#valuedef">value definitions</a> or <a href="#recordlet">let
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expressions</a> are needed for the class, they are enclosed in curly braces
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("{}"); otherwise, the record ends with a semicolon. Here is a simple TableGen
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file:</p>
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<pre>
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<b>class</b> C { <b>bit</b> V = 1; }
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<b>def</b> X : C;
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<b>def</b> Y : C {
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<b>string</b> Greeting = "hello";
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}
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</pre>
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<p>This example defines two definitions, <tt>X</tt> and <tt>Y</tt>, both of
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which derive from the <tt>C</tt> class. Because of this, they both get the
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<tt>V</tt> bit value. The <tt>Y</tt> definition also gets the Greeting member
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as well.</p>
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<p>In general, classes are useful for collecting together the commonality
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between a group of records and isolating it in a single place. Also, classes
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permit the specification of default values for their subclasses, allowing the
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subclasses to override them as they wish.</p>
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</div>
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<!---------------------------------------------------------------------------->
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<div class="doc_subsubsection">
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<a name="valuedef">Value definitions</a>
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</div>
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<div class="doc_text">
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<p>Value definitions define named entries in records. A value must be defined
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before it can be referred to as the operand for another value definition or
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before the value is reset with a <a href="#recordlet">let expression</a>. A
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value is defined by specifying a <a href="#types">TableGen type</a> and a name.
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If an initial value is available, it may be specified after the type with an
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equal sign. Value definitions require terminating semicolons.</p>
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</div>
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<!-- -------------------------------------------------------------------------->
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<div class="doc_subsubsection">
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<a name="recordlet">'let' expressions</a>
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</div>
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<div class="doc_text">
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<p>A record-level let expression is used to change the value of a value
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definition in a record. This is primarily useful when a superclass defines a
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value that a derived class or definition wants to override. Let expressions
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consist of the '<tt>let</tt>' keyword followed by a value name, an equal sign
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("="), and a new value. For example, a new class could be added to the example
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above, redefining the <tt>V</tt> field for all of its subclasses:</p>
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<pre>
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<b>class</b> D : C { let V = 0; }
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<b>def</b> Z : D;
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</pre>
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<p>In this case, the <tt>Z</tt> definition will have a zero value for its "V"
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value, despite the fact that it derives (indirectly) from the <tt>C</tt> class,
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because the <tt>D</tt> class overrode its value.</p>
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</div>
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<!-- -------------------------------------------------------------------------->
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<div class="doc_subsubsection">
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<a name="templateargs">Class template arguments</a>
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</div>
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<div class="doc_text">
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<p>TableGen permits the definition of parameterized classes as well as normal
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concrete classes. Parameterized TableGen classes specify a list of variable
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bindings (which may optionally have defaults) that are bound when used. Here is
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a simple example:</p>
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<pre>
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<b>class</b> FPFormat<<b>bits</b><3> val> {
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<b>bits</b><3> Value = val;
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}
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<b>def</b> NotFP : FPFormat<0>;
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<b>def</b> ZeroArgFP : FPFormat<1>;
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<b>def</b> OneArgFP : FPFormat<2>;
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<b>def</b> OneArgFPRW : FPFormat<3>;
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<b>def</b> TwoArgFP : FPFormat<4>;
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<b>def</b> SpecialFP : FPFormat<5>;
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</pre>
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<p>In this case, template arguments are used as a space efficient way to specify
|
|
a list of "enumeration values", each with a "Value" field set to the specified
|
|
integer.</p>
|
|
|
|
<p>The more esoteric forms of <a href="#values">TableGen expressions</a> are
|
|
useful in conjunction with template arguments. As an example:</p>
|
|
|
|
<pre>
|
|
<b>class</b> ModRefVal<<b>bits</b><2> val> {
|
|
<b>bits</b><2> Value = val;
|
|
}
|
|
|
|
<b>def</b> None : ModRefVal<0>;
|
|
<b>def</b> Mod : ModRefVal<1>;
|
|
<b>def</b> Ref : ModRefVal<2>;
|
|
<b>def</b> ModRef : ModRefVal<3>;
|
|
|
|
<b>class</b> Value<ModRefVal MR> {
|
|
<i>// decode some information into a more convenient format, while providing
|
|
// a nice interface to the user of the "Value" class.</i>
|
|
<b>bit</b> isMod = MR.Value{0};
|
|
<b>bit</b> isRef = MR.Value{1};
|
|
|
|
<i>// other stuff...</i>
|
|
}
|
|
|
|
<i>// Example uses</i>
|
|
<b>def</b> bork : Value<Mod>;
|
|
<b>def</b> zork : Value<Ref>;
|
|
<b>def</b> hork : Value<ModRef>;
|
|
</pre>
|
|
|
|
<p>This is obviously a contrived example, but it shows how template arguments
|
|
can be used to decouple the interface provided to the user of the class from the
|
|
actual internal data representation expected by the class. In this case,
|
|
running <tt>tblgen</tt> on the example prints the following definitions:</p>
|
|
|
|
<pre>
|
|
<b>def</b> bork { <i>// Value</i>
|
|
bit isMod = 1;
|
|
bit isRef = 0;
|
|
}
|
|
<b>def</b> hork { <i>// Value</i>
|
|
bit isMod = 1;
|
|
bit isRef = 1;
|
|
}
|
|
<b>def</b> zork { <i>// Value</i>
|
|
bit isMod = 0;
|
|
bit isRef = 1;
|
|
}
|
|
</pre>
|
|
|
|
<p> This shows that TableGen was able to dig into the argument and extract a
|
|
piece of information that was requested by the designer of the "Value" class.
|
|
For more realistic examples, please see existing users of TableGen, such as the
|
|
X86 backend.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="filescope">File scope entities</a>
|
|
</div>
|
|
|
|
<!-- -------------------------------------------------------------------------->
|
|
<div class="doc_subsubsection">
|
|
<a name="include">File inclusion</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<p>TableGen supports the '<tt>include</tt>' token, which textually substitutes
|
|
the specified file in place of the include directive. The filename should be
|
|
specified as a double quoted string immediately after the '<tt>include</tt>'
|
|
keyword. Example:</p>
|
|
|
|
<pre>
|
|
<b>include</b> "foo.td"
|
|
</pre>
|
|
|
|
</div>
|
|
|
|
<!-- -------------------------------------------------------------------------->
|
|
<div class="doc_subsubsection">
|
|
<a name="globallet">'let' expressions</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<p> "let" expressions at file scope are similar to <a href="#recordlet">"let"
|
|
expressions within a record</a>, except they can specify a value binding for
|
|
multiple records at a time, and may be useful in certain other cases.
|
|
File-scope let expressions are really just another way that TableGen allows the
|
|
end-user to factor out commonality from the records.</p>
|
|
|
|
<p>File-scope "let" expressions take a comma-separated list of bindings to
|
|
apply, and one of more records to bind the values in. Here are some
|
|
examples:</p>
|
|
|
|
<pre>
|
|
<b>let</b> isTerminator = 1, isReturn = 1 <b>in</b>
|
|
<b>def</b> RET : X86Inst<"ret", 0xC3, RawFrm, NoArg>;
|
|
|
|
<b>let</b> isCall = 1 <b>in</b>
|
|
<i>// All calls clobber the non-callee saved registers...</i>
|
|
<b>let</b> Defs = [EAX, ECX, EDX, FP0, FP1, FP2, FP3, FP4, FP5, FP6] in {
|
|
<b>def</b> CALLpcrel32 : X86Inst<"call", 0xE8, RawFrm, NoArg>;
|
|
<b>def</b> CALLr32 : X86Inst<"call", 0xFF, MRMS2r, Arg32>;
|
|
<b>def</b> CALLm32 : X86Inst<"call", 0xFF, MRMS2m, Arg32>;
|
|
}
|
|
</pre>
|
|
|
|
<p>File-scope "let" expressions are often useful when a couple of definitions
|
|
need to be added to several records, and the records do not otherwise need to be
|
|
opened, as in the case with the CALL* instructions above.</p>
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section"><a name="backends">TableGen backends</a></div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
<p>How they work, how to write one. This section should not contain details
|
|
about any particular backend, except maybe -print-enums as an example. This
|
|
should highlight the APIs in <tt>TableGen/Record.h</tt>.</p>
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
<hr>
|
|
<address>
|
|
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|
|
<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
|
|
<a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
|
|
Last modified: $Date$
|
|
</address>
|
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