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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 the
  72 <tt>llvm/utils/emacs</tt> and <tt>llvm/utils/vim</tt> directories 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 the 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 are 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 targeting (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
 108 TableGen definitions.  If a multiclass inherits from another multiclass,
 109 the definitions in the sub-multiclass become part of the current
 110 multiclass, as if they were declared in the current multiclass.</p>
 111
 112 </div>
 113
 114 <!-- ======================================================================= -->
 115 <div class="doc_subsection"><a name="example">An example record</a></div>
 116
 117 <div class="doc_text">
 118
 119 <p>With no other arguments, TableGen parses the specified file and prints out
 120 all of the classes, then all of the definitions.  This is a good way to see what
 121 the various definitions expand to fully.  Running this on the <tt>X86.td</tt>
 122 file prints this (at the time of this writing):</p>
 123
 124 <div class="doc_code">
 125 <pre>
 126 ...
 127 <b>def</b> ADD32rr {   <i>// Instruction X86Inst I</i>
 128   <b>string</b> Namespace = "X86";
 129   <b>dag</b> OutOperandList = (outs GR32:$dst);
 130   <b>dag</b> InOperandList = (ins GR32:$src1, GR32:$src2);
 131   <b>string</b> AsmString = "add{l}\t{$src2, $dst|$dst, $src2}";
 132   <b>list</b>&lt;dag&gt; Pattern = [(set GR32:$dst, (add GR32:$src1, GR32:$src2))];
 133   <b>list</b>&lt;Register&gt; Uses = [];
 134   <b>list</b>&lt;Register&gt; Defs = [EFLAGS];
 135   <b>list</b>&lt;Predicate&gt; Predicates = [];
 136   <b>int</b> CodeSize = 3;
 137   <b>int</b> AddedComplexity = 0;
 138   <b>bit</b> isReturn = 0;
 139   <b>bit</b> isBranch = 0;
 140   <b>bit</b> isIndirectBranch = 0;
 141   <b>bit</b> isBarrier = 0;
 142   <b>bit</b> isCall = 0;
 143   <b>bit</b> canFoldAsLoad = 0;
 144   <b>bit</b> mayLoad = 0;
 145   <b>bit</b> mayStore = 0;
 146   <b>bit</b> isImplicitDef = 0;
 147   <b>bit</b> isTwoAddress = 1;
 148   <b>bit</b> isConvertibleToThreeAddress = 1;
 149   <b>bit</b> isCommutable = 1;
 150   <b>bit</b> isTerminator = 0;
 151   <b>bit</b> isReMaterializable = 0;
 152   <b>bit</b> isPredicable = 0;
 153   <b>bit</b> hasDelaySlot = 0;
 154   <b>bit</b> usesCustomDAGSchedInserter = 0;
 155   <b>bit</b> hasCtrlDep = 0;
 156   <b>bit</b> isNotDuplicable = 0;
 157   <b>bit</b> hasSideEffects = 0;
 158   <b>bit</b> mayHaveSideEffects = 0;
 159   <b>bit</b> neverHasSideEffects = 0;
 160   InstrItinClass Itinerary = NoItinerary;
 161   <b>string</b> Constraints = "";
 162   <b>string</b> DisableEncoding = "";
 163   <b>bits</b>&lt;8&gt; Opcode = { 0, 0, 0, 0, 0, 0, 0, 1 };
 164   Format Form = MRMDestReg;
 165   <b>bits</b>&lt;6&gt; FormBits = { 0, 0, 0, 0, 1, 1 };
 166   ImmType ImmT = NoImm;
 167   <b>bits</b>&lt;3&gt; ImmTypeBits = { 0, 0, 0 };
 168   <b>bit</b> hasOpSizePrefix = 0;
 169   <b>bit</b> hasAdSizePrefix = 0;
 170   <b>bits</b>&lt;4&gt; Prefix = { 0, 0, 0, 0 };
 171   <b>bit</b> hasREX_WPrefix = 0;
 172   FPFormat FPForm = ?;
 173   <b>bits</b>&lt;3&gt; FPFormBits = { 0, 0, 0 };
 174 }
 175 ...
 176 </pre>
 177 </div>
 178
 179 <p>This definition corresponds to a 32-bit register-register add instruction in
 180 the X86.  The string after the '<tt>def</tt>' string indicates the name of the
 181 record&mdash;"<tt>ADD32rr</tt>" in this case&mdash;and the comment at the end of
 182 the line indicates the superclasses of the definition.  The body of the record
 183 contains all of the data that TableGen assembled for the record, indicating that
 184 the instruction is part of the "X86" namespace, the pattern indicating how the
 185 the instruction should be emitted into the assembly file, that it is a
 186 two-address instruction, has a particular encoding, etc.  The contents and
 187 semantics of the information in the record is specific to the needs of the X86
 188 backend, and is only shown as an example.</p>
 189
 190 <p>As you can see, a lot of information is needed for every instruction
 191 supported by the code generator, and specifying it all manually would be
 192 unmaintainble, prone to bugs, and tiring to do in the first place.  Because we
 193 are using TableGen, all of the information was derived from the following
 194 definition:</p>
 195
 196 <div class="doc_code">
 197 <pre>
 198 let Defs = [EFLAGS],
 199     isCommutable = 1,                  <i>// X = ADD Y,Z --&gt; X = ADD Z,Y</i>
 200     isConvertibleToThreeAddress = 1 <b>in</b> <i>// Can transform into LEA.</i>
 201 def ADD32rr  : I&lt;0x01, MRMDestReg, (outs GR32:$dst),
 202                                    (ins GR32:$src1, GR32:$src2),
 203                  "add{l}\t{$src2, $dst|$dst, $src2}",
 204                  [(set GR32:$dst, (add GR32:$src1, GR32:$src2))]&gt;;
 205 </pre>
 206 </div>
 207
 208 <p>This definition makes use of the custom class <tt>I</tt> (extended from the
 209 custom class <tt>X86Inst</tt>), which is defined in the X86-specific TableGen
 210 file, to factor out the common features that instructions of its class share.  A
 211 key feature of TableGen is that it allows the end-user to define the
 212 abstractions they prefer to use when describing their information.</p>
 213
 214 </div>
 215
 216 <!-- ======================================================================= -->
 217 <div class="doc_subsection"><a name="running">Running TableGen</a></div>
 218
 219 <div class="doc_text">
 220
 221 <p>TableGen runs just like any other LLVM tool.  The first (optional) argument
 222 specifies the file to read.  If a filename is not specified, <tt>tblgen</tt>
 223 reads from standard input.</p>
 224
 225 <p>To be useful, one of the <a href="#backends">TableGen backends</a> must be
 226 used.  These backends are selectable on the command line (type '<tt>tblgen
 227 -help</tt>' for a list).  For example, to get a list of all of the definitions
 228 that subclass a particular type (which can be useful for building up an enum
 229 list of these records), use the <tt>-print-enums</tt> option:</p>
 230
 231 <div class="doc_code">
 232 <pre>
 233 $ tblgen X86.td -print-enums -class=Register
 234 AH, AL, AX, BH, BL, BP, BPL, BX, CH, CL, CX, DH, DI, DIL, DL, DX, EAX, EBP, EBX,
 235 ECX, EDI, EDX, EFLAGS, EIP, ESI, ESP, FP0, FP1, FP2, FP3, FP4, FP5, FP6, IP,
 236 MM0, MM1, MM2, MM3, MM4, MM5, MM6, MM7, R10, R10B, R10D, R10W, R11, R11B, R11D,
 237 R11W, R12, R12B, R12D, R12W, R13, R13B, R13D, R13W, R14, R14B, R14D, R14W, R15,
 238 R15B, R15D, R15W, R8, R8B, R8D, R8W, R9, R9B, R9D, R9W, RAX, RBP, RBX, RCX, RDI,
 239 RDX, RIP, RSI, RSP, SI, SIL, SP, SPL, ST0, ST1, ST2, ST3, ST4, ST5, ST6, ST7,
 240 XMM0, XMM1, XMM10, XMM11, XMM12, XMM13, XMM14, XMM15, XMM2, XMM3, XMM4, XMM5,
 241 XMM6, XMM7, XMM8, XMM9,
 242
 243 $ tblgen X86.td -print-enums -class=Instruction
 244 ABS_F, ABS_Fp32, ABS_Fp64, ABS_Fp80, ADC32mi, ADC32mi8, ADC32mr, ADC32ri,
 245 ADC32ri8, ADC32rm, ADC32rr, ADC64mi32, ADC64mi8, ADC64mr, ADC64ri32, ADC64ri8,
 246 ADC64rm, ADC64rr, ADD16mi, ADD16mi8, ADD16mr, ADD16ri, ADD16ri8, ADD16rm,
 247 ADD16rr, ADD32mi, ADD32mi8, ADD32mr, ADD32ri, ADD32ri8, ADD32rm, ADD32rr,
 248 ADD64mi32, ADD64mi8, ADD64mr, ADD64ri32, ...
 249 </pre>
 250 </div>
 251
 252 <p>The default backend prints out all of the records, as described <a
 253 href="#example">above</a>.</p>
 254
 255 <p>If you plan to use TableGen, you will most likely have to <a
 256 href="#backends">write a backend</a> that extracts the information specific to
 257 what you need and formats it in the appropriate way.</p>
 258
 259 </div>
 260
 261
 262 <!-- *********************************************************************** -->
 263 <div class="doc_section"><a name="syntax">TableGen syntax</a></div>
 264 <!-- *********************************************************************** -->
 265
 266 <div class="doc_text">
 267
 268 <p>TableGen doesn't care about the meaning of data (that is up to the backend to
 269 define), but it does care about syntax, and it enforces a simple type system.
 270 This section describes the syntax and the constructs allowed in a TableGen file.
 271 </p>
 272
 273 </div>
 274
 275 <!-- ======================================================================= -->
 276 <div class="doc_subsection"><a name="primitives">TableGen primitives</a></div>
 277
 278 <!-- -------------------------------------------------------------------------->
 279 <div class="doc_subsubsection"><a name="comments">TableGen comments</a></div>
 280
 281 <div class="doc_text">
 282
 283 <p>TableGen supports BCPL style "<tt>//</tt>" comments, which run to the end of
 284 the line, and it also supports <b>nestable</b> "<tt>/* */</tt>" comments.</p>
 285
 286 </div>
 287
 288 <!-- -------------------------------------------------------------------------->
 289 <div class="doc_subsubsection">
 290   <a name="types">The TableGen type system</a>
 291 </div>
 292
 293 <div class="doc_text">
 294
 295 <p>TableGen files are strongly typed, in a simple (but complete) type-system.
 296 These types are used to perform automatic conversions, check for errors, and to
 297 help interface designers constrain the input that they allow.  Every <a
 298 href="#valuedef">value definition</a> is required to have an associated type.
 299 </p>
 300
 301 <p>TableGen supports a mixture of very low-level types (such as <tt>bit</tt>)
 302 and very high-level types (such as <tt>dag</tt>).  This flexibility is what
 303 allows it to describe a wide range of information conveniently and compactly.
 304 The TableGen types are:</p>
 305
 306 <dl>
 307 <dt><tt><b>bit</b></tt></dt>
 308   <dd>A 'bit' is a boolean value that can hold either 0 or 1.</dd>
 309
 310 <dt><tt><b>int</b></tt></dt>
 311   <dd>The 'int' type represents a simple 32-bit integer value, such as 5.</dd>
 312
 313 <dt><tt><b>string</b></tt></dt>
 314   <dd>The 'string' type represents an ordered sequence of characters of
 315   arbitrary length.</dd>
 316
 317 <dt><tt><b>bits</b>&lt;n&gt;</tt></dt>
 318   <dd>A 'bits' type is an arbitrary, but fixed, size integer that is broken up
 319   into individual bits.  This type is useful because it can handle some bits
 320   being defined while others are undefined.</dd>
 321
 322 <dt><tt><b>list</b>&lt;ty&gt;</tt></dt>
 323   <dd>This type represents a list whose elements are some other type.  The
 324   contained type is arbitrary: it can even be another list type.</dd>
 325
 326 <dt>Class type</dt>
 327   <dd>Specifying a class name in a type context means that the defined value
 328   must be a subclass of the specified class.  This is useful in conjunction with
 329   the <b><tt>list</tt></b> type, for example, to constrain the elements of the
 330   list to a common base class (e.g., a <tt><b>list</b>&lt;Register&gt;</tt> can
 331   only contain definitions derived from the "<tt>Register</tt>" class).</dd>
 332
 333 <dt><tt><b>dag</b></tt></dt>
 334   <dd>This type represents a nestable directed graph of elements.</dd>
 335
 336 <dt><tt><b>code</b></tt></dt>
 337   <dd>This represents a big hunk of text. NOTE: I don't remember why this is
 338   distinct from string!</dd>
 339 </dl>
 340
 341 <p>To date, these types have been sufficient for describing things that
 342 TableGen has been used for, but it is straight-forward to extend this list if
 343 needed.</p>
 344
 345 </div>
 346
 347 <!-- -------------------------------------------------------------------------->
 348 <div class="doc_subsubsection">
 349   <a name="values">TableGen values and expressions</a>
 350 </div>
 351
 352 <div class="doc_text">
 353
 354 <p>TableGen allows for a pretty reasonable number of different expression forms
 355 when building up values.  These forms allow the TableGen file to be written in a
 356 natural syntax and flavor for the application.  The current expression forms
 357 supported include:</p>
 358
 359 <dl>
 360 <dt><tt>?</tt></dt>
 361   <dd>uninitialized field</dd>
 362 <dt><tt>0b1001011</tt></dt>
 363   <dd>binary integer value</dd>
 364 <dt><tt>07654321</tt></dt>
 365   <dd>octal integer value (indicated by a leading 0)</dd>
 366 <dt><tt>7</tt></dt>
 367   <dd>decimal integer value</dd>
 368 <dt><tt>0x7F</tt></dt>
 369   <dd>hexadecimal integer value</dd>
 370 <dt><tt>"foo"</tt></dt>
 371   <dd>string value</dd>
 372 <dt><tt>[{ ... }]</tt></dt>
 373   <dd>code fragment</dd>
 374 <dt><tt>[ X, Y, Z ]&lt;type&gt;</tt></dt>
 375   <dd>list value.  &lt;type&gt; is the type of the list
 376 element and is usually optional.  In rare cases,
 377 TableGen is unable to deduce the element type in
 378 which case the user must specify it explicitly.</dd>
 379 <dt><tt>{ a, b, c }</tt></dt>
 380   <dd>initializer for a "bits&lt;3&gt;" value</dd>
 381 <dt><tt>value</tt></dt>
 382   <dd>value reference</dd>
 383 <dt><tt>value{17}</tt></dt>
 384   <dd>access to one bit of a value</dd>
 385 <dt><tt>value{15-17}</tt></dt>
 386   <dd>access to multiple bits of a value</dd>
 387 <dt><tt>DEF</tt></dt>
 388   <dd>reference to a record definition</dd>
 389 <dt><tt>CLASS&lt;val list&gt;</tt></dt>
 390   <dd>reference to a new anonymous definition of CLASS with the specified
 391       template arguments.</dd>
 392 <dt><tt>X.Y</tt></dt>
 393   <dd>reference to the subfield of a value</dd>
 394 <dt><tt>list[4-7,17,2-3]</tt></dt>
 395   <dd>A slice of the 'list' list, including elements 4,5,6,7,17,2, and 3 from
 396   it.  Elements may be included multiple times.</dd>
 397 <dt><tt>(DEF a, b)</tt></dt>
 398   <dd>a dag value.  The first element is required to be a record definition, the
 399   remaining elements in the list may be arbitrary other values, including nested
 400   `<tt>dag</tt>' values.</dd>
 401 <dt><tt>(implicit a)</tt></dt>
 402   <dd>an implicitly defined physical register.  This tells the dag instruction
 403   selection emitter the input pattern's extra definitions matches implicit
 404   physical register definitions.</dd>
 405 <dt><tt>(parallel (a), (b))</tt></dt>
 406   <dd>a list of dags specifying parallel operations which map to the same
 407   instruction.</dd>
 408 <dt><tt>!strconcat(a, b)</tt></dt>
 409   <dd>A string value that is the result of concatenating the 'a' and 'b'
 410   strings.</dd>
 411 <dt><tt>!cast&lt;type&gt;(a)</tt></dt>
 412   <dd>A symbol of type <em>type</em> obtained by looking up the string 'a' in
 413 the symbol table.  If the type of 'a' does not match <em>type</em>, TableGen
 414 aborts with an error. !cast&lt;string&gt; is a special case in that the argument must
 415 be an object defined by a 'def' construct.</dd>
 416 <dt><tt>!nameconcat&lt;type&gt;(a, b)</tt></dt>
 417   <dd>Shorthand for !cast&lt;type&gt;(!strconcat(a, b))</dd>
 418 <dt><tt>!subst(a, b, c)</tt></dt>
 419   <dd>If 'a' and 'b' are of string type or are symbol references, substitute
 420 'b' for 'a' in 'c.'  This operation is analogous to $(subst) in GNU make.</dd>
 421 <dt><tt>!foreach(a, b, c)</tt></dt>
 422   <dd>For each member 'b' of dag or list 'a' apply operator 'c.'  'b' is a
 423 dummy variable that should be declared as a member variable of an instantiated
 424 class.  This operation is analogous to $(foreach) in GNU make.</dd>
 425 <dt><tt>!car(a)</tt></dt>
 426   <dd>The first element of list 'a.'</dd>
 427 <dt><tt>!cdr(a)</tt></dt>
 428   <dd>The 2nd-N elements of list 'a.'</dd>
 429 <dt><tt>!null(a)</tt></dt>
 430   <dd>An integer {0,1} indicating whether list 'a' is empty.</dd>
 431 <dt><tt>!if(a,b,c)</tt></dt>
 432   <dd>'b' if the result of integer operator 'a' is nonzero, 'c' otherwise.</dd>
 433 </dl>
 434
 435 <p>Note that all of the values have rules specifying how they convert to values
 436 for different types.  These rules allow you to assign a value like "<tt>7</tt>"
 437 to a "<tt>bits&lt;4&gt;</tt>" value, for example.</p>
 438
 439 </div>
 440
 441 <!-- ======================================================================= -->
 442 <div class="doc_subsection">
 443   <a name="classesdefs">Classes and definitions</a>
 444 </div>
 445
 446 <div class="doc_text">
 447
 448 <p>As mentioned in the <a href="#concepts">intro</a>, classes and definitions
 449 (collectively known as 'records') in TableGen are the main high-level unit of
 450 information that TableGen collects.  Records are defined with a <tt>def</tt> or
 451 <tt>class</tt> keyword, the record name, and an optional list of "<a
 452 href="#templateargs">template arguments</a>".  If the record has superclasses,
 453 they are specified as a comma separated list that starts with a colon character
 454 ("<tt>:</tt>").  If <a href="#valuedef">value definitions</a> or <a
 455 href="#recordlet">let expressions</a> are needed for the class, they are
 456 enclosed in curly braces ("<tt>{}</tt>"); otherwise, the record ends with a
 457 semicolon.</p>
 458
 459 <p>Here is a simple TableGen file:</p>
 460
 461 <div class="doc_code">
 462 <pre>
 463 <b>class</b> C { <b>bit</b> V = 1; }
 464 <b>def</b> X : C;
 465 <b>def</b> Y : C {
 466   <b>string</b> Greeting = "hello";
 467 }
 468 </pre>
 469 </div>
 470
 471 <p>This example defines two definitions, <tt>X</tt> and <tt>Y</tt>, both of
 472 which derive from the <tt>C</tt> class.  Because of this, they both get the
 473 <tt>V</tt> bit value.  The <tt>Y</tt> definition also gets the Greeting member
 474 as well.</p>
 475
 476 <p>In general, classes are useful for collecting together the commonality
 477 between a group of records and isolating it in a single place.  Also, classes
 478 permit the specification of default values for their subclasses, allowing the
 479 subclasses to override them as they wish.</p>
 480
 481 </div>
 482
 483 <!---------------------------------------------------------------------------->
 484 <div class="doc_subsubsection">
 485   <a name="valuedef">Value definitions</a>
 486 </div>
 487
 488 <div class="doc_text">
 489
 490 <p>Value definitions define named entries in records.  A value must be defined
 491 before it can be referred to as the operand for another value definition or
 492 before the value is reset with a <a href="#recordlet">let expression</a>.  A
 493 value is defined by specifying a <a href="#types">TableGen type</a> and a name.
 494 If an initial value is available, it may be specified after the type with an
 495 equal sign.  Value definitions require terminating semicolons.</p>
 496
 497 </div>
 498
 499 <!-- -------------------------------------------------------------------------->
 500 <div class="doc_subsubsection">
 501   <a name="recordlet">'let' expressions</a>
 502 </div>
 503
 504 <div class="doc_text">
 505
 506 <p>A record-level let expression is used to change the value of a value
 507 definition in a record.  This is primarily useful when a superclass defines a
 508 value that a derived class or definition wants to override.  Let expressions
 509 consist of the '<tt>let</tt>' keyword followed by a value name, an equal sign
 510 ("<tt>=</tt>"), and a new value.  For example, a new class could be added to the
 511 example above, redefining the <tt>V</tt> field for all of its subclasses:</p>
 512
 513 <div class="doc_code">
 514 <pre>
 515 <b>class</b> D : C { let V = 0; }
 516 <b>def</b> Z : D;
 517 </pre>
 518 </div>
 519
 520 <p>In this case, the <tt>Z</tt> definition will have a zero value for its "V"
 521 value, despite the fact that it derives (indirectly) from the <tt>C</tt> class,
 522 because the <tt>D</tt> class overrode its value.</p>
 523
 524 </div>
 525
 526 <!-- -------------------------------------------------------------------------->
 527 <div class="doc_subsubsection">
 528   <a name="templateargs">Class template arguments</a>
 529 </div>
 530
 531 <div class="doc_text">
 532
 533 <p>TableGen permits the definition of parameterized classes as well as normal
 534 concrete classes.  Parameterized TableGen classes specify a list of variable
 535 bindings (which may optionally have defaults) that are bound when used.  Here is
 536 a simple example:</p>
 537
 538 <div class="doc_code">
 539 <pre>
 540 <b>class</b> FPFormat&lt;<b>bits</b>&lt;3&gt; val&gt; {
 541   <b>bits</b>&lt;3&gt; Value = val;
 542 }
 543 <b>def</b> NotFP      : FPFormat&lt;0&gt;;
 544 <b>def</b> ZeroArgFP  : FPFormat&lt;1&gt;;
 545 <b>def</b> OneArgFP   : FPFormat&lt;2&gt;;
 546 <b>def</b> OneArgFPRW : FPFormat&lt;3&gt;;
 547 <b>def</b> TwoArgFP   : FPFormat&lt;4&gt;;
 548 <b>def</b> CompareFP  : FPFormat&lt;5&gt;;
 549 <b>def</b> CondMovFP  : FPFormat&lt;6&gt;;
 550 <b>def</b> SpecialFP  : FPFormat&lt;7&gt;;
 551 </pre>
 552 </div>
 553
 554 <p>In this case, template arguments are used as a space efficient way to specify
 555 a list of "enumeration values", each with a "<tt>Value</tt>" field set to the
 556 specified integer.</p>
 557
 558 <p>The more esoteric forms of <a href="#values">TableGen expressions</a> are
 559 useful in conjunction with template arguments.  As an example:</p>
 560
 561 <div class="doc_code">
 562 <pre>
 563 <b>class</b> ModRefVal&lt;<b>bits</b>&lt;2&gt; val&gt; {
 564   <b>bits</b>&lt;2&gt; Value = val;
 565 }
 566
 567 <b>def</b> None   : ModRefVal&lt;0&gt;;
 568 <b>def</b> Mod    : ModRefVal&lt;1&gt;;
 569 <b>def</b> Ref    : ModRefVal&lt;2&gt;;
 570 <b>def</b> ModRef : ModRefVal&lt;3&gt;;
 571
 572 <b>class</b> Value&lt;ModRefVal MR&gt; {
 573   <i>// Decode some information into a more convenient format, while providing
 574   // a nice interface to the user of the "Value" class.</i>
 575   <b>bit</b> isMod = MR.Value{0};
 576   <b>bit</b> isRef = MR.Value{1};
 577
 578   <i>// other stuff...</i>
 579 }
 580
 581 <i>// Example uses</i>
 582 <b>def</b> bork : Value&lt;Mod&gt;;
 583 <b>def</b> zork : Value&lt;Ref&gt;;
 584 <b>def</b> hork : Value&lt;ModRef&gt;;
 585 </pre>
 586 </div>
 587
 588 <p>This is obviously a contrived example, but it shows how template arguments
 589 can be used to decouple the interface provided to the user of the class from the
 590 actual internal data representation expected by the class.  In this case,
 591 running <tt>tblgen</tt> on the example prints the following definitions:</p>
 592
 593 <div class="doc_code">
 594 <pre>
 595 <b>def</b> bork {      <i>// Value</i>
 596   <b>bit</b> isMod = 1;
 597   <b>bit</b> isRef = 0;
 598 }
 599 <b>def</b> hork {      <i>// Value</i>
 600   <b>bit</b> isMod = 1;
 601   <b>bit</b> isRef = 1;
 602 }
 603 <b>def</b> zork {      <i>// Value</i>
 604   <b>bit</b> isMod = 0;
 605   <b>bit</b> isRef = 1;
 606 }
 607 </pre>
 608 </div>
 609
 610 <p> This shows that TableGen was able to dig into the argument and extract a
 611 piece of information that was requested by the designer of the "Value" class.
 612 For more realistic examples, please see existing users of TableGen, such as the
 613 X86 backend.</p>
 614
 615 </div>
 616
 617 <!-- -------------------------------------------------------------------------->
 618 <div class="doc_subsubsection">
 619   <a name="multiclass">Multiclass definitions and instances</a>
 620 </div>
 621
 622 <div class="doc_text">
 623
 624 <p>
 625 While classes with template arguments are a good way to factor commonality
 626 between two instances of a definition, multiclasses allow a convenient notation
 627 for defining multiple definitions at once (instances of implicitly constructed
 628 classes).  For example, consider an 3-address instruction set whose instructions
 629 come in two forms: "<tt>reg = reg op reg</tt>" and "<tt>reg = reg op imm</tt>"
 630 (e.g. SPARC). In this case, you'd like to specify in one place that this
 631 commonality exists, then in a separate place indicate what all the ops are.
 632 </p>
 633
 634 <p>
 635 Here is an example TableGen fragment that shows this idea:
 636 </p>
 637
 638 <div class="doc_code">
 639 <pre>
 640 <b>def</b> ops;
 641 <b>def</b> GPR;
 642 <b>def</b> Imm;
 643 <b>class</b> inst&lt;<b>int</b> opc, <b>string</b> asmstr, <b>dag</b> operandlist&gt;;
 644
 645 <b>multiclass</b> ri_inst&lt;<b>int</b> opc, <b>string</b> asmstr&gt; {
 646   def _rr : inst&lt;opc, !strconcat(asmstr, " $dst, $src1, $src2"),
 647                  (ops GPR:$dst, GPR:$src1, GPR:$src2)&gt;;
 648   def _ri : inst&lt;opc, !strconcat(asmstr, " $dst, $src1, $src2"),
 649                  (ops GPR:$dst, GPR:$src1, Imm:$src2)&gt;;
 650 }
 651
 652 <i>// Instantiations of the ri_inst multiclass.</i>
 653 <b>defm</b> ADD : ri_inst&lt;0b111, "add"&gt;;
 654 <b>defm</b> SUB : ri_inst&lt;0b101, "sub"&gt;;
 655 <b>defm</b> MUL : ri_inst&lt;0b100, "mul"&gt;;
 656 ...
 657 </pre>
 658 </div>
 659
 660 <p>The name of the resultant definitions has the multidef fragment names
 661    appended to them, so this defines <tt>ADD_rr</tt>, <tt>ADD_ri</tt>,
 662    <tt>SUB_rr</tt>, etc.  A defm may inherit from multiple multiclasses,
 663    instantiating definitions from each multiclass.  Using a multiclass
 664    this way is exactly equivalent to instantiating the classes multiple
 665    times yourself, e.g. by writing:</p>
 666
 667 <div class="doc_code">
 668 <pre>
 669 <b>def</b> ops;
 670 <b>def</b> GPR;
 671 <b>def</b> Imm;
 672 <b>class</b> inst&lt;<b>int</b> opc, <b>string</b> asmstr, <b>dag</b> operandlist&gt;;
 673
 674 <b>class</b> rrinst&lt;<b>int</b> opc, <b>string</b> asmstr&gt;
 675   : inst&lt;opc, !strconcat(asmstr, " $dst, $src1, $src2"),
 676          (ops GPR:$dst, GPR:$src1, GPR:$src2)&gt;;
 677
 678 <b>class</b> riinst&lt;<b>int</b> opc, <b>string</b> asmstr&gt;
 679   : inst&lt;opc, !strconcat(asmstr, " $dst, $src1, $src2"),
 680          (ops GPR:$dst, GPR:$src1, Imm:$src2)&gt;;
 681
 682 <i>// Instantiations of the ri_inst multiclass.</i>
 683 <b>def</b> ADD_rr : rrinst&lt;0b111, "add"&gt;;
 684 <b>def</b> ADD_ri : riinst&lt;0b111, "add"&gt;;
 685 <b>def</b> SUB_rr : rrinst&lt;0b101, "sub"&gt;;
 686 <b>def</b> SUB_ri : riinst&lt;0b101, "sub"&gt;;
 687 <b>def</b> MUL_rr : rrinst&lt;0b100, "mul"&gt;;
 688 <b>def</b> MUL_ri : riinst&lt;0b100, "mul"&gt;;
 689 ...
 690 </pre>
 691 </div>
 692
 693 </div>
 694
 695 <!-- ======================================================================= -->
 696 <div class="doc_subsection">
 697   <a name="filescope">File scope entities</a>
 698 </div>
 699
 700 <!-- -------------------------------------------------------------------------->
 701 <div class="doc_subsubsection">
 702   <a name="include">File inclusion</a>
 703 </div>
 704
 705 <div class="doc_text">
 706 <p>TableGen supports the '<tt>include</tt>' token, which textually substitutes
 707 the specified file in place of the include directive.  The filename should be
 708 specified as a double quoted string immediately after the '<tt>include</tt>'
 709 keyword.  Example:</p>
 710
 711 <div class="doc_code">
 712 <pre>
 713 <b>include</b> "foo.td"
 714 </pre>
 715 </div>
 716
 717 </div>
 718
 719 <!-- -------------------------------------------------------------------------->
 720 <div class="doc_subsubsection">
 721   <a name="globallet">'let' expressions</a>
 722 </div>
 723
 724 <div class="doc_text">
 725
 726 <p>"Let" expressions at file scope are similar to <a href="#recordlet">"let"
 727 expressions within a record</a>, except they can specify a value binding for
 728 multiple records at a time, and may be useful in certain other cases.
 729 File-scope let expressions are really just another way that TableGen allows the
 730 end-user to factor out commonality from the records.</p>
 731
 732 <p>File-scope "let" expressions take a comma-separated list of bindings to
 733 apply, and one or more records to bind the values in.  Here are some
 734 examples:</p>
 735
 736 <div class="doc_code">
 737 <pre>
 738 <b>let</b> isTerminator = 1, isReturn = 1, isBarrier = 1, hasCtrlDep = 1 <b>in</b>
 739   <b>def</b> RET : I&lt;0xC3, RawFrm, (outs), (ins), "ret", [(X86retflag 0)]&gt;;
 740
 741 <b>let</b> isCall = 1 <b>in</b>
 742   <i>// All calls clobber the non-callee saved registers...</i>
 743   <b>let</b> Defs = [EAX, ECX, EDX, FP0, FP1, FP2, FP3, FP4, FP5, FP6, ST0,
 744               MM0, MM1, MM2, MM3, MM4, MM5, MM6, MM7,
 745               XMM0, XMM1, XMM2, XMM3, XMM4, XMM5, XMM6, XMM7, EFLAGS] <b>in</b> {
 746     <b>def</b> CALLpcrel32 : Ii32&lt;0xE8, RawFrm, (outs), (ins i32imm:$dst,variable_ops),
 747                            "call\t${dst:call}", []&gt;;
 748     <b>def</b> CALL32r     : I&lt;0xFF, MRM2r, (outs), (ins GR32:$dst, variable_ops),
 749                         "call\t{*}$dst", [(X86call GR32:$dst)]&gt;;
 750     <b>def</b> CALL32m     : I&lt;0xFF, MRM2m, (outs), (ins i32mem:$dst, variable_ops),
 751                         "call\t{*}$dst", []&gt;;
 752   }
 753 </pre>
 754 </div>
 755
 756 <p>File-scope "let" expressions are often useful when a couple of definitions
 757 need to be added to several records, and the records do not otherwise need to be
 758 opened, as in the case with the <tt>CALL*</tt> instructions above.</p>
 759
 760 </div>
 761
 762 <!-- *********************************************************************** -->
 763 <div class="doc_section"><a name="backends">TableGen backends</a></div>
 764 <!-- *********************************************************************** -->
 765
 766 <div class="doc_text">
 767
 768 <p>TODO: How they work, how to write one.  This section should not contain
 769 details about any particular backend, except maybe -print-enums as an example.
 770 This should highlight the APIs in <tt>TableGen/Record.h</tt>.</p>
 771
 772 </div>
 773
 774 <!-- *********************************************************************** -->
 775
 776 <hr>
 777 <address>
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 782
 783   <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
 784   <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
 785   Last modified: $Date$
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