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  10 <div class="doc_title">TableGen Fundamentals</div>
  11
  12 <div class="doc_text">
  13 <ul>
  14   <li><a href="#introduction">Introduction</a>
  15   <ol>
  16     <li><a href="#concepts">Basic concepts</a></li>
  17     <li><a href="#example">An example record</a></li>
  18     <li><a href="#running">Running TableGen</a></li>
  19   </ol></li>
  20   <li><a href="#syntax">TableGen syntax</a>
  21   <ol>
  22     <li><a href="#primitives">TableGen primitives</a>
  23     <ol>
  24       <li><a href="#comments">TableGen comments</a></li>
  25       <li><a href="#types">The TableGen type system</a></li>
  26       <li><a href="#values">TableGen values and expressions</a></li>
  27     </ol></li>
  28     <li><a href="#classesdefs">Classes and definitions</a>
  29     <ol>
  30       <li><a href="#valuedef">Value definitions</a></li>
  31       <li><a href="#recordlet">'let' expressions</a></li>
  32       <li><a href="#templateargs">Class template arguments</a></li>
  33       <li><a href="#multiclass">Multiclass definitions and instances</a></li>
  34     </ol></li>
  35     <li><a href="#filescope">File scope entities</a>
  36     <ol>
  37       <li><a href="#include">File inclusion</a></li>
  38       <li><a href="#globallet">'let' expressions</a></li>
  39     </ol></li>
  40   </ol></li>
  41   <li><a href="#backends">TableGen backends</a>
  42   <ol>
  43     <li><a href="#">todo</a></li>
  44   </ol></li>
  45 </ul>
  46 </div>
  47
  48 <div class="doc_author">
  49   <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
  50 </div>
  51
  52 <!-- *********************************************************************** -->
  53 <div class="doc_section"><a name="introduction">Introduction</a></div>
  54 <!-- *********************************************************************** -->
  55
  56 <div class="doc_text">
  57
  58 <p>TableGen's purpose is to help a human develop and maintain records of
  59 domain-specific information.  Because there may be a large number of these
  60 records, it is specifically designed to allow writing flexible descriptions and
  61 for common features of these records to be factored out.  This reduces the
  62 amount of duplication in the description, reduces the chance of error, and
  63 makes it easier to structure domain specific information.</p>
  64
  65 <p>The core part of TableGen <a href="#syntax">parses a file</a>, instantiates
  66 the declarations, and hands the result off to a domain-specific "<a
  67 href="#backends">TableGen backend</a>" for processing.  The current major user
  68 of TableGen is the <a href="CodeGenerator.html">LLVM code generator</a>.</p>
  69
  70 <p>Note that if you work on TableGen much, and use emacs or vim, that you can
  71 find an emacs "TableGen mode" and a vim language file in
  72 <tt>llvm/utils/emacs</tt> and <tt>llvm/utils/vim</tt> directory of your LLVM
  73 distribution, respectively.</p>
  74
  75 </div>
  76
  77 <!-- ======================================================================= -->
  78 <div class="doc_subsection"><a name="concepts">Basic concepts</a></div>
  79
  80 <div class="doc_text">
  81
  82 <p>TableGen files consist of two key parts: 'classes' and 'definitions', both
  83 of which are considered 'records'.</p>
  84
  85 <p><b>TableGen records</b> have a unique name, a list of values, and a list of
  86 superclasses.  The list of values is main data that TableGen builds for each
  87 record, it is this that holds the domain specific information for the
  88 application.  The interpretation of this data is left to a specific <a
  89 href="#backends">TableGen backend</a>, but the structure and format rules are
  90 taken care of and fixed by TableGen.</p>
  91
  92 <p><b>TableGen definitions</b> are the concrete form of 'records'.  These
  93 generally do not have any undefined values, and are marked with the
  94 '<tt>def</tt>' keyword.</p>
  95
  96 <p><b>TableGen classes</b> are abstract records that are used to build and
  97 describe other records.  These 'classes' allow the end-user to build
  98 abstractions for either the domain they are targetting (such as "Register",
  99 "RegisterClass", and "Instruction" in the LLVM code generator) or for the
 100 implementor to help factor out common properties of records (such as "FPInst",
 101 which is used to represent floating point instructions in the X86 backend).
 102 TableGen keeps track of all of the classes that are used to build up a
 103 definition, so the backend can find all definitions of a particular class, such
 104 as "Instruction".</p>
 105
 106 <p><b>TableGen multiclasses</b> are groups of abstract records that are
 107 instantiated all at once.  Each instantiation can result in multiple TableGen
 108 definitions.</p>
 109
 110 </div>
 111
 112 <!-- ======================================================================= -->
 113 <div class="doc_subsection"><a name="example">An example record</a></div>
 114
 115 <div class="doc_text">
 116
 117 <p>With no other arguments, TableGen parses the specified file and prints out
 118 all of the classes, then all of the definitions.  This is a good way to see what
 119 the various definitions expand to fully.  Running this on the <tt>X86.td</tt>
 120 file prints this (at the time of this writing):</p>
 121
 122 <pre>
 123 ...
 124 <b>def</b> ADDrr8 {    <i>// Instruction X86Inst I2A8 Pattern</i>
 125   <b>string</b> Name = "add";
 126   <b>string</b> Namespace = "X86";
 127   <b>list</b>&lt;Register&gt; Uses = [];
 128   <b>list</b>&lt;Register&gt; Defs = [];
 129   <b>bit</b> isReturn = 0;
 130   <b>bit</b> isBranch = 0;
 131   <b>bit</b> isCall = 0;
 132   <b>bit</b> isTwoAddress = 1;
 133   <b>bit</b> isTerminator = 0;
 134   <b>dag</b> Pattern = (set R8, (plus R8, R8));
 135   <b>bits</b>&lt;8&gt; Opcode = { 0, 0, 0, 0, 0, 0, 0, 0 };
 136   Format Form = MRMDestReg;
 137   <b>bits</b>&lt;5&gt; FormBits = { 0, 0, 0, 1, 1 };
 138   ArgType Type = Arg8;
 139   <b>bits</b>&lt;3&gt; TypeBits = { 0, 0, 1 };
 140   <b>bit</b> hasOpSizePrefix = 0;
 141   <b>bit</b> printImplicitUses = 0;
 142   <b>bits</b>&lt;4&gt; Prefix = { 0, 0, 0, 0 };
 143   FPFormat FPForm = ?;
 144   <b>bits</b>&lt;3&gt; FPFormBits = { 0, 0, 0 };
 145 }
 146 ...
 147 </pre>
 148
 149 <p>This definition corresponds to an 8-bit register-register add instruction in
 150 the X86.  The string after the '<tt>def</tt>' string indicates the name of the
 151 record ("<tt>ADDrr8</tt>" in this case), and the comment at the end of the line
 152 indicates the superclasses of the definition.  The body of the record contains
 153 all of the data that TableGen assembled for the record, indicating that the
 154 instruction is part of the "X86" namespace, should be printed as "<tt>add</tt>"
 155 in the assembly file, it is a two-address instruction, has a particular
 156 encoding, etc.  The contents and semantics of the information in the record is
 157 specific to the needs of the X86 backend, and is only shown as an example.</p>
 158
 159 <p>As you can see, a lot of information is needed for every instruction
 160 supported by the code generator, and specifying it all manually would be
 161 unmaintainble, prone to bugs, and tiring to do in the first place.  Because we
 162 are using TableGen, all of the information was derived from the following
 163 definition:</p>
 164
 165 <pre>
 166 <b>def</b> ADDrr8   : I2A8&lt;"add", 0x00, MRMDestReg&gt;,
 167                Pattern&lt;(set R8, (plus R8, R8))&gt;;
 168 </pre>
 169
 170 <p>This definition makes use of the custom I2A8 (two address instruction with
 171 8-bit operand) class, which is defined in the X86-specific TableGen file to
 172 factor out the common features that instructions of its class share.  A key
 173 feature of TableGen is that it allows the end-user to define the abstractions
 174 they prefer to use when describing their information.</p>
 175
 176 </div>
 177
 178 <!-- ======================================================================= -->
 179 <div class="doc_subsection"><a name="running">Running TableGen</a></div>
 180
 181 <div class="doc_text">
 182
 183 <p>TableGen runs just like any other LLVM tool.  The first (optional) argument
 184 specifies the file to read.  If a filename is not specified, <tt>tblgen</tt>
 185 reads from standard input.</p>
 186
 187 <p>To be useful, one of the <a href="#backends">TableGen backends</a> must be
 188 used.  These backends are selectable on the command line (type '<tt>tblgen
 189 --help</tt>' for a list).  For example, to get a list of all of the definitions
 190 that subclass a particular type (which can be useful for building up an enum
 191 list of these records), use the <tt>--print-enums</tt> option:</p>
 192
 193 <pre>
 194 $ tblgen X86.td -print-enums -class=Register
 195 AH, AL, AX, BH, BL, BP, BX, CH, CL, CX, DH, DI, DL, DX,
 196 EAX, EBP, EBX, ECX, EDI, EDX, ESI, ESP, FP0, FP1, FP2, FP3, FP4, FP5, FP6,
 197 SI, SP, ST0, ST1, ST2, ST3, ST4, ST5, ST6, ST7,
 198
 199 $ tblgen X86.td -print-enums -class=Instruction
 200 ADCrr32, ADDri16, ADDri16b, ADDri32, ADDri32b, ADDri8, ADDrr16, ADDrr32,
 201 ADDrr8, ADJCALLSTACKDOWN, ADJCALLSTACKUP, ANDri16, ANDri16b, ANDri32, ANDri32b,
 202 ANDri8, ANDrr16, ANDrr32, ANDrr8, BSWAPr32, CALLm32, CALLpcrel32, ...
 203 </pre>
 204
 205 <p>The default backend prints out all of the records, as described <a
 206 href="#example">above</a>.</p>
 207
 208 <p>If you plan to use TableGen for some purpose, you will most likely have to
 209 <a href="#backends">write a backend</a> that extracts the information specific
 210 to what you need and formats it in the appropriate way.</p>
 211
 212 </div>
 213
 214
 215 <!-- *********************************************************************** -->
 216 <div class="doc_section"><a name="syntax">TableGen syntax</a></div>
 217 <!-- *********************************************************************** -->
 218
 219 <div class="doc_text">
 220 <p>TableGen doesn't care about the meaning of data (that is up to the backend
 221 to define), but it does care about syntax, and it enforces a simple type system.
 222 This section describes the syntax and the constructs allowed in a TableGen file.
 223 </p>
 224 </div>
 225
 226 <!-- ======================================================================= -->
 227 <div class="doc_subsection"><a name="primitives">TableGen primitives</a></div>
 228
 229 <!-- -------------------------------------------------------------------------->
 230 <div class="doc_subsubsection"><a name="comments">TableGen comments</a></div>
 231
 232 <div class="doc_text">
 233 <p>TableGen supports BCPL style "<tt>//</tt>" comments, which run to the end of
 234 the line, and it also supports <b>nestable</b> "<tt>/* */</tt>" comments.</p>
 235 </div>
 236
 237 <!-- -------------------------------------------------------------------------->
 238 <div class="doc_subsubsection">
 239   <a name="types">The TableGen type system</a>
 240 </div>
 241
 242 <div class="doc_text">
 243 <p>TableGen files are strongly typed, in a simple (but complete) type-system.
 244 These types are used to perform automatic conversions, check for errors, and to
 245 help interface designers constrain the input that they allow.  Every <a
 246 href="#valuedef">value definition</a> is required to have an associated type.
 247 </p>
 248
 249 <p>TableGen supports a mixture of very low-level types (such as <tt>bit</tt>)
 250 and very high-level types (such as <tt>dag</tt>).  This flexibility is what
 251 allows it to describe a wide range of information conveniently and compactly.
 252 The TableGen types are:</p>
 253
 254 <ul>
 255 <li>"<tt><b>bit</b></tt>" - A 'bit' is a boolean value that can hold either 0 or
 256 1.</li>
 257
 258 <li>"<tt><b>int</b></tt>" - The 'int' type represents a simple 32-bit integer
 259 value, such as 5.</li>
 260
 261 <li>"<tt><b>string</b></tt>" - The 'string' type represents an ordered sequence
 262 of characters of arbitrary length.</li>
 263
 264 <li>"<tt><b>bits</b>&lt;n&gt;</tt>" - A 'bits' type is an arbitrary, but fixed,
 265 size integer that is broken up into individual bits.  This type is useful
 266 because it can handle some bits being defined while others are undefined.</li>
 267
 268 <li>"<tt><b>list</b>&lt;ty&gt;</tt>" - This type represents a list whose
 269 elements are some other type.  The contained type is arbitrary: it can even be
 270 another list type.</li>
 271
 272 <li>Class type - Specifying a class name in a type context means that the
 273 defined value must be a subclass of the specified class.  This is useful in
 274 conjunction with the "list" type, for example, to constrain the elements of the
 275 list to a common base class (e.g., a <tt><b>list</b>&lt;Register&gt;</tt> can
 276 only contain definitions derived from the "<tt>Register</tt>" class).</li>
 277
 278 <li>"<tt><b>code</b></tt>" - This represents a big hunk of text.  NOTE: I don't
 279 remember why this is distinct from string!</li>
 280
 281 <li>"<tt><b>dag</b></tt>" - This type represents a nestable directed graph of
 282 elements.</li>
 283 </ul>
 284
 285 <p>To date, these types have been sufficient for describing things that
 286 TableGen has been used for, but it is straight-forward to extend this list if
 287 needed.</p>
 288
 289 </div>
 290
 291 <!-- -------------------------------------------------------------------------->
 292 <div class="doc_subsubsection">
 293   <a name="values">TableGen values and expressions</a>
 294 </div>
 295
 296 <div class="doc_text">
 297
 298 <p>TableGen allows for a pretty reasonable number of different expression forms
 299 when building up values.  These forms allow the TableGen file to be written in a
 300 natural syntax and flavor for the application.  The current expression forms
 301 supported include:</p>
 302
 303 <ul>
 304 <li><tt>?</tt> - uninitialized field</li>
 305 <li><tt>0b1001011</tt> - binary integer value</li>
 306 <li><tt>07654321</tt> - octal integer value (indicated by a leading 0)</li>
 307 <li><tt>7</tt> - decimal integer value</li>
 308 <li><tt>0x7F</tt> - hexadecimal integer value</li>
 309 <li><tt>"foo"</tt> - string value</li>
 310 <li><tt>[{ ... }]</tt> - code fragment</li>
 311 <li><tt>[ X, Y, Z ]</tt> - list value.</li>
 312 <li><tt>{ a, b, c }</tt> - initializer for a "bits&lt;3&gt;" value</li>
 313 <li><tt>value</tt> - value reference</li>
 314 <li><tt>value{17}</tt> - access to one bit of a value</li>
 315 <li><tt>value{15-17}</tt> - access to multiple bits of a value</li>
 316 <li><tt>DEF</tt> - reference to a record definition</li>
 317 <li><tt>CLASS&lt;val list&gt;</tt> - reference to a new anonymous definition of
 318         CLASS with the specified template arguments.</li>
 319 <li><tt>X.Y</tt> - reference to the subfield of a value</li>
 320 <li><tt>list[4-7,17,2-3]</tt> - A slice of the 'list' list, including elements
 321 4,5,6,7,17,2, and 3 from it.  Elements may be included multiple times.</li>
 322 <li><tt>(DEF a, b)</tt> - a dag value.  The first element is required to be a
 323 record definition, the remaining elements in the list may be arbitrary other
 324 values, including nested `<tt>dag</tt>' values.</li>
 325 <li><tt>!strconcat(a, b)</tt> - A string value that is the result of
 326  concatenating the 'a' and 'b' strings.</li>
 327 </ul>
 328
 329 <p>Note that all of the values have rules specifying how they convert to values
 330 for different types.  These rules allow you to assign a value like "7" to a
 331 "bits&lt;4&gt;" value, for example.</p>
 332
 333 </div>
 334
 335 <!-- ======================================================================= -->
 336 <div class="doc_subsection">
 337   <a name="classesdefs">Classes and definitions</a>
 338 </div>
 339
 340 <div class="doc_text">
 341
 342 <p>As mentioned in the <a href="#concepts">intro</a>, classes and definitions
 343 (collectively known as 'records') in TableGen are the main high-level unit of
 344 information that TableGen collects.  Records are defined with a <tt>def</tt> or
 345 <tt>class</tt> keyword, the record name, and an optional list of "<a
 346 href="#templateargs">template arguments</a>".  If the record has superclasses,
 347 they are specified as a comma separated list that starts with a colon character
 348 (":").  If <a href="#valuedef">value definitions</a> or <a href="#recordlet">let
 349 expressions</a> are needed for the class, they are enclosed in curly braces
 350 ("{}"); otherwise, the record ends with a semicolon.  Here is a simple TableGen
 351 file:</p>
 352
 353 <pre>
 354 <b>class</b> C { <b>bit</b> V = 1; }
 355 <b>def</b> X : C;
 356 <b>def</b> Y : C {
 357   <b>string</b> Greeting = "hello";
 358 }
 359 </pre>
 360
 361 <p>This example defines two definitions, <tt>X</tt> and <tt>Y</tt>, both of
 362 which derive from the <tt>C</tt> class.  Because of this, they both get the
 363 <tt>V</tt> bit value.  The <tt>Y</tt> definition also gets the Greeting member
 364 as well.</p>
 365
 366 <p>In general, classes are useful for collecting together the commonality
 367 between a group of records and isolating it in a single place.  Also, classes
 368 permit the specification of default values for their subclasses, allowing the
 369 subclasses to override them as they wish.</p>
 370
 371 </div>
 372
 373 <!---------------------------------------------------------------------------->
 374 <div class="doc_subsubsection">
 375   <a name="valuedef">Value definitions</a>
 376 </div>
 377
 378 <div class="doc_text">
 379 <p>Value definitions define named entries in records.  A value must be defined
 380 before it can be referred to as the operand for another value definition or
 381 before the value is reset with a <a href="#recordlet">let expression</a>.  A
 382 value is defined by specifying a <a href="#types">TableGen type</a> and a name.
 383 If an initial value is available, it may be specified after the type with an
 384 equal sign.  Value definitions require terminating semicolons.</p>
 385 </div>
 386
 387 <!-- -------------------------------------------------------------------------->
 388 <div class="doc_subsubsection">
 389   <a name="recordlet">'let' expressions</a>
 390 </div>
 391
 392 <div class="doc_text">
 393 <p>A record-level let expression is used to change the value of a value
 394 definition in a record.  This is primarily useful when a superclass defines a
 395 value that a derived class or definition wants to override.  Let expressions
 396 consist of the '<tt>let</tt>' keyword followed by a value name, an equal sign
 397 ("="), and a new value.  For example, a new class could be added to the example
 398 above, redefining the <tt>V</tt> field for all of its subclasses:</p>
 399
 400 <pre>
 401 <b>class</b> D : C { let V = 0; }
 402 <b>def</b> Z : D;
 403 </pre>
 404
 405 <p>In this case, the <tt>Z</tt> definition will have a zero value for its "V"
 406 value, despite the fact that it derives (indirectly) from the <tt>C</tt> class,
 407 because the <tt>D</tt> class overrode its value.</p>
 408
 409 </div>
 410
 411 <!-- -------------------------------------------------------------------------->
 412 <div class="doc_subsubsection">
 413   <a name="templateargs">Class template arguments</a>
 414 </div>
 415
 416 <div class="doc_text">
 417 <p>TableGen permits the definition of parameterized classes as well as normal
 418 concrete classes.  Parameterized TableGen classes specify a list of variable
 419 bindings (which may optionally have defaults) that are bound when used.  Here is
 420 a simple example:</p>
 421
 422 <pre>
 423 <b>class</b> FPFormat&lt;<b>bits</b>&lt;3&gt; val&gt; {
 424   <b>bits</b>&lt;3&gt; Value = val;
 425 }
 426 <b>def</b> NotFP      : FPFormat&lt;0&gt;;
 427 <b>def</b> ZeroArgFP  : FPFormat&lt;1&gt;;
 428 <b>def</b> OneArgFP   : FPFormat&lt;2&gt;;
 429 <b>def</b> OneArgFPRW : FPFormat&lt;3&gt;;
 430 <b>def</b> TwoArgFP   : FPFormat&lt;4&gt;;
 431 <b>def</b> SpecialFP  : FPFormat&lt;5&gt;;
 432 </pre>
 433
 434 <p>In this case, template arguments are used as a space efficient way to specify
 435 a list of "enumeration values", each with a "Value" field set to the specified
 436 integer.</p>
 437
 438 <p>The more esoteric forms of <a href="#values">TableGen expressions</a> are
 439 useful in conjunction with template arguments.  As an example:</p>
 440
 441 <pre>
 442 <b>class</b> ModRefVal&lt;<b>bits</b>&lt;2&gt; val&gt; {
 443   <b>bits</b>&lt;2&gt; Value = val;
 444 }
 445
 446 <b>def</b> None   : ModRefVal&lt;0&gt;;
 447 <b>def</b> Mod    : ModRefVal&lt;1&gt;;
 448 <b>def</b> Ref    : ModRefVal&lt;2&gt;;
 449 <b>def</b> ModRef : ModRefVal&lt;3&gt;;
 450
 451 <b>class</b> Value&lt;ModRefVal MR&gt; {
 452   <i>// decode some information into a more convenient format, while providing
 453   // a nice interface to the user of the "Value" class.</i>
 454   <b>bit</b> isMod = MR.Value{0};
 455   <b>bit</b> isRef = MR.Value{1};
 456
 457   <i>// other stuff...</i>
 458 }
 459
 460 <i>// Example uses</i>
 461 <b>def</b> bork : Value&lt;Mod&gt;;
 462 <b>def</b> zork : Value&lt;Ref&gt;;
 463 <b>def</b> hork : Value&lt;ModRef&gt;;
 464 </pre>
 465
 466 <p>This is obviously a contrived example, but it shows how template arguments
 467 can be used to decouple the interface provided to the user of the class from the
 468 actual internal data representation expected by the class.  In this case,
 469 running <tt>tblgen</tt> on the example prints the following definitions:</p>
 470
 471 <pre>
 472 <b>def</b> bork {      <i>// Value</i>
 473   <b>bit</b> isMod = 1;
 474   <b>bit</b> isRef = 0;
 475 }
 476 <b>def</b> hork {      <i>// Value</i>
 477   <b>bit</b> isMod = 1;
 478   <b>bit</b> isRef = 1;
 479 }
 480 <b>def</b> zork {      <i>// Value</i>
 481   <b>bit</b> isMod = 0;
 482   <b>bit</b> isRef = 1;
 483 }
 484 </pre>
 485
 486 <p> This shows that TableGen was able to dig into the argument and extract a
 487 piece of information that was requested by the designer of the "Value" class.
 488 For more realistic examples, please see existing users of TableGen, such as the
 489 X86 backend.</p>
 490
 491 </div>
 492
 493 <!-- -------------------------------------------------------------------------->
 494 <div class="doc_subsubsection">
 495   <a name="multiclass">Multiclass definitions and instances</a>
 496 </div>
 497
 498 <div class="doc_text">
 499
 500 <p>
 501 While classes with template arguments are a good way to factor commonality
 502 between two instances of a definition, multiclasses allow a convenient notation
 503 for defining multiple definitions at once (instances of implicitly constructed
 504 classes).  For example, consider an 3-address instruction set whose instructions
 505 come in two forms: "reg = reg op reg" and "reg = reg op imm" (e.g. SPARC). In
 506 this case, you'd like to specify in one place that this commonality exists, then
 507 in a separate place indicate what all the ops are.
 508 </p>
 509
 510 <p>
 511 Here is an example TableGen fragment that shows this idea:
 512 </p>
 513
 514 <pre>
 515 <b>def</b> ops;
 516 <b>def</b> GPR;
 517 <b>def</b> Imm;
 518 <b>class</b> inst&lt;<b>int</b> opc, <b>string</b> asmstr, <b>dag</b> operandlist&gt;;
 519
 520 <b>multiclass</b> ri_inst&lt;<b>int</b> opc, <b>string</b> asmstr&gt; {
 521   def _rr : inst&lt;opc, !strconcat(asmstr, " $dst, $src1, $src2"),
 522                  (ops GPR:$dst, GPR:$src1, GPR:$src2)&gt;;
 523   def _ri : inst&lt;opc, !strconcat(asmstr, " $dst, $src1, $src2"),
 524                  (ops GPR:$dst, GPR:$src1, Imm:$src2)&gt;;
 525 }
 526
 527 // Instantiations of the ri_inst multiclass.
 528 <b>defm</b> ADD : ri_inst&lt;0b111, "add"&gt;;
 529 <b>defm</b> SUB : ri_inst&lt;0b101, "sub"&gt;;
 530 <b>defm</b> MUL : ri_inst&lt;0b100, "mul"&gt;;
 531 ...
 532 </pre>
 533
 534 <p>The name of the resultant definitions has the multidef fragment names
 535    appended to them, so this defines ADD_rr, ADD_ri, SUB_rr, etc.  Using a
 536    multiclass this way is exactly equivalent to instantiating the
 537    classes multiple times yourself, e.g. by writing:</p>
 538
 539 <pre>
 540 <b>def</b> ops;
 541 <b>def</b> GPR;
 542 <b>def</b> Imm;
 543 <b>class</b> inst&lt;<b>int</b> opc, <b>string</b> asmstr, <b>dag</b> operandlist&gt;;
 544
 545 <b>class</b> rrinst&lt;<b>int</b> opc, <b>string</b> asmstr&gt;
 546   : inst&lt;opc, !strconcat(asmstr, " $dst, $src1, $src2"),
 547          (ops GPR:$dst, GPR:$src1, GPR:$src2)&gt;;
 548
 549 <b>class</b> riinst&lt;<b>int</b> opc, <b>string</b> asmstr&gt;
 550   : inst&lt;opc, !strconcat(asmstr, " $dst, $src1, $src2"),
 551          (ops GPR:$dst, GPR:$src1, Imm:$src2)&gt;;
 552
 553 // Instantiations of the ri_inst multiclass.
 554 <b>def</b> ADD_rr : rrinst&lt;0b111, "add"&gt;;
 555 <b>def</b> ADD_ri : riinst&lt;0b111, "add"&gt;;
 556 <b>def</b> SUB_rr : rrinst&lt;0b101, "sub"&gt;;
 557 <b>def</b> SUB_ri : riinst&lt;0b101, "sub"&gt;;
 558 <b>def</b> MUL_rr : rrinst&lt;0b100, "mul"&gt;;
 559 <b>def</b> MUL_ri : riinst&lt;0b100, "mul"&gt;;
 560 ...
 561 </pre>
 562
 563 </div>
 564
 565 <!-- ======================================================================= -->
 566 <div class="doc_subsection">
 567   <a name="filescope">File scope entities</a>
 568 </div>
 569
 570 <!-- -------------------------------------------------------------------------->
 571 <div class="doc_subsubsection">
 572   <a name="include">File inclusion</a>
 573 </div>
 574
 575 <div class="doc_text">
 576 <p>TableGen supports the '<tt>include</tt>' token, which textually substitutes
 577 the specified file in place of the include directive.  The filename should be
 578 specified as a double quoted string immediately after the '<tt>include</tt>'
 579 keyword.  Example:</p>
 580
 581 <pre>
 582 <b>include</b> "foo.td"
 583 </pre>
 584
 585 </div>
 586
 587 <!-- -------------------------------------------------------------------------->
 588 <div class="doc_subsubsection">
 589   <a name="globallet">'let' expressions</a>
 590 </div>
 591
 592 <div class="doc_text">
 593 <p> "let" expressions at file scope are similar to <a href="#recordlet">"let"
 594 expressions within a record</a>, except they can specify a value binding for
 595 multiple records at a time, and may be useful in certain other cases.
 596 File-scope let expressions are really just another way that TableGen allows the
 597 end-user to factor out commonality from the records.</p>
 598
 599 <p>File-scope "let" expressions take a comma-separated list of bindings to
 600 apply, and one of more records to bind the values in.  Here are some
 601 examples:</p>
 602
 603 <pre>
 604 <b>let</b> isTerminator = 1, isReturn = 1 <b>in</b>
 605   <b>def</b> RET : X86Inst&lt;"ret", 0xC3, RawFrm, NoArg&gt;;
 606
 607 <b>let</b> isCall = 1 <b>in</b>
 608   <i>// All calls clobber the non-callee saved registers...</i>
 609   <b>let</b> Defs = [EAX, ECX, EDX, FP0, FP1, FP2, FP3, FP4, FP5, FP6] in {
 610     <b>def</b> CALLpcrel32 : X86Inst&lt;"call", 0xE8, RawFrm, NoArg&gt;;
 611     <b>def</b> CALLr32     : X86Inst&lt;"call", 0xFF, MRMS2r, Arg32&gt;;
 612     <b>def</b> CALLm32     : X86Inst&lt;"call", 0xFF, MRMS2m, Arg32&gt;;
 613   }
 614 </pre>
 615
 616 <p>File-scope "let" expressions are often useful when a couple of definitions
 617 need to be added to several records, and the records do not otherwise need to be
 618 opened, as in the case with the CALL* instructions above.</p>
 619 </div>
 620
 621 <!-- *********************************************************************** -->
 622 <div class="doc_section"><a name="backends">TableGen backends</a></div>
 623 <!-- *********************************************************************** -->
 624
 625 <div class="doc_text">
 626 <p>How they work, how to write one.  This section should not contain details
 627 about any particular backend, except maybe -print-enums as an example.  This
 628 should highlight the APIs in <tt>TableGen/Record.h</tt>.</p>
 629 </div>
 630
 631 <!-- *********************************************************************** -->
 632
 633 <hr>
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 639
 640   <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
 641   <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
 642   Last modified: $Date$
 643 </address>
 644
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