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Recover TableGen/LangRef, make it official

Browse Source 
Making the new TableGen documentation official and marking the old file as
"Moved". Also, reverting the original LangRef as the normative formal
description of the language, while keeping the "new" LangRef as LangIntro
for the less inlcined to reading language grammars.

We should remove TableGenFundamentals.rst one day, but for now, just a
warning that it moved will have to do, while we make sure there are no more
links to it from elsewhere.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205289 91177308-0d34-0410-b5e6-96231b3b80d8
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Renato Golin
2014-04-01 09:51:49 +00:00
parent 1d75829ddc
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docs/TableGen/LangRef.rst
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@@ -2,6 +2,8 @@
TableGen Language Reference
===========================
.. sectionauthor:: Sean Silva <silvas@purdue.edu>
.. contents::
:local:
@@ -16,587 +18,369 @@ This document is meant to be a normative spec about the TableGen language
in and of itself (i.e. how to understand a given construct in terms of how
it affects the final set of records represented by the TableGen file). If
you are unsure if this document is really what you are looking for, please
read :doc:`the introduction <index>` first.
read :doc:`/TableGenFundamentals` first.
TableGen syntax
===============
Notation
========
TableGen doesn't care about the meaning of data (that is up to the backend to
define), but it does care about syntax, and it enforces a simple type system.
This section describes the syntax and the constructs allowed in a TableGen file.
The lexical and syntax notation used here is intended to imitate
`Python's`_. In particular, for lexical definitions, the productions
operate at the character level and there is no implied whitespace between
elements. The syntax definitions operate at the token level, so there is
implied whitespace between tokens.
TableGen primitives
-------------------
.. _`Python's`: http://docs.python.org/py3k/reference/introduction.html#notation
TableGen comments
^^^^^^^^^^^^^^^^^
Lexical Analysis
================
TableGen supports C++ style "``//``" comments, which run to the end of the
line, and it also supports **nestable** "``/* */``" comments.
TableGen supports BCPL (``// ...``) and nestable C-style (``/* ... */``)
comments.
.. _TableGen type:
The following is a listing of the basic punctuation tokens::
The TableGen type system
^^^^^^^^^^^^^^^^^^^^^^^^
- + [ ] { } ( ) < > : ; . = ? #
TableGen files are strongly typed, in a simple (but complete) type-system.
These types are used to perform automatic conversions, check for errors, and to
help interface designers constrain the input that they allow. Every `value
definition`_ is required to have an associated type.
Numeric literals take one of the following forms:
TableGen supports a mixture of very low-level types (such as ``bit``) and very
high-level types (such as ``dag``). This flexibility is what allows it to
describe a wide range of information conveniently and compactly. The TableGen
types are:
.. TableGen actually will lex some pretty strange sequences an interpret
them as numbers. What is shown here is an attempt to approximate what it
"should" accept.
``bit``
A 'bit' is a boolean value that can hold either 0 or 1.
.. productionlist::
TokInteger: `DecimalInteger` | `HexInteger` | `BinInteger`
DecimalInteger: ["+" | "-"] ("0"..."9")+
HexInteger: "0x" ("0"..."9" | "a"..."f" | "A"..."F")+
BinInteger: "0b" ("0" | "1")+
``int``
The 'int' type represents a simple 32-bit integer value, such as 5.
One aspect to note is that the :token:`DecimalInteger` token *includes* the
``+`` or ``-``, as opposed to having ``+`` and ``-`` be unary operators as
most languages do.
``string``
The 'string' type represents an ordered sequence of characters of arbitrary
length.
TableGen has identifier-like tokens:
``bits<n>``
A 'bits' type is an arbitrary, but fixed, size integer that is broken up
into individual bits. This type is useful because it can handle some bits
being defined while others are undefined.
.. productionlist::
ualpha: "a"..."z" | "A"..."Z" | "_"
TokIdentifier: ("0"..."9")* `ualpha` (`ualpha` | "0"..."9")*
TokVarName: "$" `ualpha` (`ualpha` | "0"..."9")*
``list<ty>``
This type represents a list whose elements are some other type. The
contained type is arbitrary: it can even be another list type.
Note that unlike most languages, TableGen allows :token:`TokIdentifier` to
begin with a number. In case of ambiguity, a token will be interpreted as a
numeric literal rather than an identifier.
Class type
Specifying a class name in a type context means that the defined value must
be a subclass of the specified class. This is useful in conjunction with
the ``list`` type, for example, to constrain the elements of the list to a
common base class (e.g., a ``list<Register>`` can only contain definitions
derived from the "``Register``" class).
TableGen also has two string-like literals:
``dag``
This type represents a nestable directed graph of elements.
.. productionlist::
TokString: '"' <non-'"' characters and C-like escapes> '"'
TokCodeFragment: "[{" <shortest text not containing "}]"> "}]"
To date, these types have been sufficient for describing things that TableGen
has been used for, but it is straight-forward to extend this list if needed.
:token:`TokCodeFragment` is essentially a multiline string literal
delimited by ``[{`` and ``}]``.
.. _TableGen expressions:
.. note::
The current implementation accepts the following C-like escapes::
TableGen values and expressions
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
\\ \' \" \t \n
TableGen allows for a pretty reasonable number of different expression forms
when building up values. These forms allow the TableGen file to be written in a
natural syntax and flavor for the application. The current expression forms
supported include:
TableGen also has the following keywords::
``?``
uninitialized field
bit bits class code dag
def foreach defm field in
int let list multiclass string
``0b1001011``
binary integer value
TableGen also has "bang operators" which have a
wide variety of meanings:
``07654321``
octal integer value (indicated by a leading 0)
.. productionlist::
BangOperator: one of
:!eq !if !head !tail !con
:!add !shl !sra !srl
:!cast !empty !subst !foreach !strconcat
``7``
decimal integer value
Syntax
======
``0x7F``
hexadecimal integer value
TableGen has an ``include`` mechanism. It does not play a role in the
syntax per se, since it is lexically replaced with the contents of the
included file.
``"foo"``
string value
.. productionlist::
IncludeDirective: "include" `TokString`
``[{ ... }]``
usually called a "code fragment", but is just a multiline string literal
TableGen's top-level production consists of "objects".
``[ X, Y, Z ]<type>``
list value. <type> is the type of the list element and is usually optional.
In rare cases, TableGen is unable to deduce the element type in which case
the user must specify it explicitly.
.. productionlist::
TableGenFile: `Object`*
Object: `Class` | `Def` | `Defm` | `Let` | `MultiClass` | `Foreach`
``{ a, b, c }``
initializer for a "bits<3>" value
``class``\es
------------
``value``
value reference
.. productionlist::
Class: "class" `TokIdentifier` [`TemplateArgList`] `ObjectBody`
``value{17}``
access to one bit of a value
A ``class`` declaration creates a record which other records can inherit
from. A class can be parametrized by a list of "template arguments", whose
values can be used in the class body.
``value{15-17}``
access to multiple bits of a value
A given class can only be defined once. A ``class`` declaration is
considered to define the class if any of the following is true:
``DEF``
reference to a record definition
.. break ObjectBody into its consituents so that they are present here?
``CLASS<val list>``
reference to a new anonymous definition of CLASS with the specified template
arguments.
#. The :token:`TemplateArgList` is present.
#. The :token:`Body` in the :token:`ObjectBody` is present and is not empty.
#. The :token:`BaseClassList` in the :token:`ObjectBody` is present.
``X.Y``
reference to the subfield of a value
You can declare an empty class by giving and empty :token:`TemplateArgList`
and an empty :token:`ObjectBody`. This can serve as a restricted form of
forward declaration: note that records deriving from the forward-declared
class will inherit no fields from it since the record expansion is done
when the record is parsed.
``list[4-7,17,2-3]``
A slice of the 'list' list, including elements 4,5,6,7,17,2, and 3 from it.
Elements may be included multiple times.
.. productionlist::
TemplateArgList: "<" `Declaration` ("," `Declaration`)* ">"
``foreach <var> = [ <list> ] in { <body> }``
Declarations
------------
``foreach <var> = [ <list> ] in <def>``
Replicate <body> or <def>, replacing instances of <var> with each value
in <list>. <var> is scoped at the level of the ``foreach`` loop and must
not conflict with any other object introduced in <body> or <def>. Currently
only ``def``\s are expanded within <body>.
.. Omitting mention of arcane "field" prefix to discourage its use.
``foreach <var> = 0-15 in ...``
The declaration syntax is pretty much what you would expect as a C++
programmer.
``foreach <var> = {0-15,32-47} in ...``
Loop over ranges of integers. The braces are required for multiple ranges.
.. productionlist::
Declaration: `Type` `TokIdentifier` ["=" `Value`]
``(DEF a, b)``
a dag value. The first element is required to be a record definition, the
remaining elements in the list may be arbitrary other values, including
nested ```dag``' values.
It assigns the value to the identifer.
``!strconcat(a, b)``
A string value that is the result of concatenating the 'a' and 'b' strings.
Types
-----
``str1#str2``
"#" (paste) is a shorthand for !strconcat. It may concatenate things that
are not quoted strings, in which case an implicit !cast<string> is done on
the operand of the paste.
.. productionlist::
Type: "string" | "code" | "bit" | "int" | "dag"
:| "bits" "<" `TokInteger` ">"
:| "list" "<" `Type` ">"
:| `ClassID`
ClassID: `TokIdentifier`
``!cast<type>(a)``
A symbol of type *type* obtained by looking up the string 'a' in the symbol
table. If the type of 'a' does not match *type*, TableGen aborts with an
error. !cast<string> is a special case in that the argument must be an
object defined by a 'def' construct.
Both ``string`` and ``code`` correspond to the string type; the difference
is purely to indicate programmer intention.
``!subst(a, b, c)``
If 'a' and 'b' are of string type or are symbol references, substitute 'b'
for 'a' in 'c.' This operation is analogous to $(subst) in GNU make.
The :token:`ClassID` must identify a class that has been previously
declared or defined.
``!foreach(a, b, c)``
For each member 'b' of dag or list 'a' apply operator 'c.' 'b' is a dummy
variable that should be declared as a member variable of an instantiated
class. This operation is analogous to $(foreach) in GNU make.
Values
------
``!head(a)``
The first element of list 'a.'
.. productionlist::
Value: `SimpleValue` `ValueSuffix`*
ValueSuffix: "{" `RangeList` "}"
:| "[" `RangeList` "]"
:| "." `TokIdentifier`
RangeList: `RangePiece` ("," `RangePiece`)*
RangePiece: `TokInteger`
:| `TokInteger` "-" `TokInteger`
:| `TokInteger` `TokInteger`
``!tail(a)``
The 2nd-N elements of list 'a.'
The peculiar last form of :token:`RangePiece` is due to the fact that the
"``-``" is included in the :token:`TokInteger`, hence ``1-5`` gets lexed as
two consecutive :token:`TokInteger`'s, with values ``1`` and ``-5``,
instead of "1", "-", and "5".
The :token:`RangeList` can be thought of as specifying "list slice" in some
contexts.
``!empty(a)``
An integer {0,1} indicating whether list 'a' is empty.
``!if(a,b,c)``
'b' if the result of 'int' or 'bit' operator 'a' is nonzero, 'c' otherwise.
:token:`SimpleValue` has a number of forms:
``!eq(a,b)``
'bit 1' if string a is equal to string b, 0 otherwise. This only operates
on string, int and bit objects. Use !cast<string> to compare other types of
objects.
Note that all of the values have rules specifying how they convert to values
for different types. These rules allow you to assign a value like "``7``"
to a "``bits<4>``" value, for example.
.. productionlist::
SimpleValue: `TokIdentifier`
Classes and definitions
-----------------------
The value will be the variable referenced by the identifier. It can be one
of:
As mentioned in the :doc:`introduction <index>`, classes and definitions (collectively known as
'records') in TableGen are the main high-level unit of information that TableGen
collects. Records are defined with a ``def`` or ``class`` keyword, the record
name, and an optional list of "`template arguments`_". If the record has
superclasses, they are specified as a comma separated list that starts with a
colon character ("``:``"). If `value definitions`_ or `let expressions`_ are
needed for the class, they are enclosed in curly braces ("``{}``"); otherwise,
the record ends with a semicolon.
.. The code for this is exceptionally abstruse. These examples are a
best-effort attempt.
Here is a simple TableGen file:
* name of a ``def``, such as the use of ``Bar`` in::
.. code-block:: llvm
def Bar : SomeClass {
int X = 5;
}
class C { bit V = 1; }
def X : C;
def Y : C {
string Greeting = "hello";
}
This example defines two definitions, ``X`` and ``Y``, both of which derive from
the ``C`` class. Because of this, they both get the ``V`` bit value. The ``Y``
definition also gets the Greeting member as well.
In general, classes are useful for collecting together the commonality between a
group of records and isolating it in a single place. Also, classes permit the
specification of default values for their subclasses, allowing the subclasses to
override them as they wish.
def Foo {
SomeClass Baz = Bar;
}
.. _value definition:
.. _value definitions:
Value definitions
^^^^^^^^^^^^^^^^^
Value definitions define named entries in records. A value must be defined
before it can be referred to as the operand for another value definition or
before the value is reset with a `let expression`_. A value is defined by
specifying a `TableGen type`_ and a name. If an initial value is available, it
may be specified after the type with an equal sign. Value definitions require
terminating semicolons.
.. _let expression:
.. _let expressions:
.. _"let" expressions within a record:
'let' expressions
^^^^^^^^^^^^^^^^^
A record-level let expression is used to change the value of a value definition
in a record. This is primarily useful when a superclass defines a value that a
derived class or definition wants to override. Let expressions consist of the
'``let``' keyword followed by a value name, an equal sign ("``=``"), and a new
value. For example, a new class could be added to the example above, redefining
the ``V`` field for all of its subclasses:
.. code-block:: llvm
class D : C { let V = 0; }
def Z : D;
In this case, the ``Z`` definition will have a zero value for its ``V`` value,
despite the fact that it derives (indirectly) from the ``C`` class, because the
``D`` class overrode its value.
.. _template arguments:
Class template arguments
^^^^^^^^^^^^^^^^^^^^^^^^
TableGen permits the definition of parameterized classes as well as normal
concrete classes. Parameterized TableGen classes specify a list of variable
bindings (which may optionally have defaults) that are bound when used. Here is
a simple example:
.. code-block:: llvm
class FPFormat<bits<3> val> {
bits<3> Value = val;
}
def NotFP : FPFormat<0>;
def ZeroArgFP : FPFormat<1>;
def OneArgFP : FPFormat<2>;
def OneArgFPRW : FPFormat<3>;
def TwoArgFP : FPFormat<4>;
def CompareFP : FPFormat<5>;
def CondMovFP : FPFormat<6>;
def SpecialFP : FPFormat<7>;
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.
The more esoteric forms of `TableGen expressions`_ are useful in conjunction
with template arguments. As an example:
.. code-block:: llvm
class ModRefVal<bits<2> val> {
bits<2> Value = val;
}
def None : ModRefVal<0>;
def Mod : ModRefVal<1>;
def Ref : ModRefVal<2>;
def ModRef : ModRefVal<3>;
class Value<ModRefVal MR> {
// Decode some information into a more convenient format, while providing
// a nice interface to the user of the "Value" class.
bit isMod = MR.Value{0};
bit isRef = MR.Value{1};
// other stuff...
}
// Example uses
def bork : Value<Mod>;
def zork : Value<Ref>;
def hork : Value<ModRef>;
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 ``llvm-tblgen`` on the example prints the following definitions:
.. code-block:: llvm
def bork { // Value
bit isMod = 1;
bit isRef = 0;
}
def hork { // Value
bit isMod = 1;
bit isRef = 1;
}
def zork { // Value
bit isMod = 0;
bit isRef = 1;
}
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.
Multiclass definitions and instances
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
While classes with template arguments are a good way to factor commonality
between two instances of a definition, multiclasses allow a convenient notation
for defining multiple definitions at once (instances of implicitly constructed
classes). For example, consider an 3-address instruction set whose instructions
come in two forms: "``reg = reg op reg``" and "``reg = reg op imm``"
(e.g. SPARC). In this case, you'd like to specify in one place that this
commonality exists, then in a separate place indicate what all the ops are.
Here is an example TableGen fragment that shows this idea:
.. code-block:: llvm
def ops;
def GPR;
def Imm;
class inst<int opc, string asmstr, dag operandlist>;
multiclass ri_inst<int opc, string asmstr> {
def _rr : inst<opc, !strconcat(asmstr, " $dst, $src1, $src2"),
(ops GPR:$dst, GPR:$src1, GPR:$src2)>;
def _ri : inst<opc, !strconcat(asmstr, " $dst, $src1, $src2"),
(ops GPR:$dst, GPR:$src1, Imm:$src2)>;
}
// Instantiations of the ri_inst multiclass.
defm ADD : ri_inst<0b111, "add">;
defm SUB : ri_inst<0b101, "sub">;
defm MUL : ri_inst<0b100, "mul">;
...
The name of the resultant definitions has the multidef fragment names appended
to them, so this defines ``ADD_rr``, ``ADD_ri``, ``SUB_rr``, etc. A defm may
inherit from multiple multiclasses, instantiating definitions from each
multiclass. Using a multiclass this way is exactly equivalent to instantiating
the classes multiple times yourself, e.g. by writing:
.. code-block:: llvm
def ops;
def GPR;
def Imm;
class inst<int opc, string asmstr, dag operandlist>;
class rrinst<int opc, string asmstr>
: inst<opc, !strconcat(asmstr, " $dst, $src1, $src2"),
(ops GPR:$dst, GPR:$src1, GPR:$src2)>;
class riinst<int opc, string asmstr>
: inst<opc, !strconcat(asmstr, " $dst, $src1, $src2"),
(ops GPR:$dst, GPR:$src1, Imm:$src2)>;
// Instantiations of the ri_inst multiclass.
def ADD_rr : rrinst<0b111, "add">;
def ADD_ri : riinst<0b111, "add">;
def SUB_rr : rrinst<0b101, "sub">;
def SUB_ri : riinst<0b101, "sub">;
def MUL_rr : rrinst<0b100, "mul">;
def MUL_ri : riinst<0b100, "mul">;
...
A ``defm`` can also be used inside a multiclass providing several levels of
multiclass instantiations.
.. code-block:: llvm
class Instruction<bits<4> opc, string Name> {
bits<4> opcode = opc;
string name = Name;
}
multiclass basic_r<bits<4> opc> {
def rr : Instruction<opc, "rr">;
def rm : Instruction<opc, "rm">;
}
multiclass basic_s<bits<4> opc> {
defm SS : basic_r<opc>;
defm SD : basic_r<opc>;
def X : Instruction<opc, "x">;
}
multiclass basic_p<bits<4> opc> {
defm PS : basic_r<opc>;
defm PD : basic_r<opc>;
def Y : Instruction<opc, "y">;
}
defm ADD : basic_s<0xf>, basic_p<0xf>;
...
// Results
def ADDPDrm { ...
def ADDPDrr { ...
def ADDPSrm { ...
def ADDPSrr { ...
def ADDSDrm { ...
def ADDSDrr { ...
def ADDY { ...
def ADDX { ...
``defm`` declarations can inherit from classes too, the rule to follow is that
the class list must start after the last multiclass, and there must be at least
one multiclass before them.
.. code-block:: llvm
class XD { bits<4> Prefix = 11; }
class XS { bits<4> Prefix = 12; }
class I<bits<4> op> {
bits<4> opcode = op;
}
multiclass R {
def rr : I<4>;
def rm : I<2>;
}
multiclass Y {
defm SS : R, XD;
defm SD : R, XS;
}
defm Instr : Y;
// Results
def InstrSDrm {
bits<4> opcode = { 0, 0, 1, 0 };
bits<4> Prefix = { 1, 1, 0, 0 };
}
...
def InstrSSrr {
bits<4> opcode = { 0, 1, 0, 0 };
bits<4> Prefix = { 1, 0, 1, 1 };
}
File scope entities
-------------------
File inclusion
^^^^^^^^^^^^^^
TableGen supports the '``include``' 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 '``include``' keyword.
Example:
.. code-block:: llvm
include "foo.td"
'let' expressions
^^^^^^^^^^^^^^^^^
"Let" expressions at file scope are similar to `"let" expressions within a
record`_, 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.
File-scope "let" expressions take a comma-separated list of bindings to apply,
and one or more records to bind the values in. Here are some examples:
.. code-block:: llvm
let isTerminator = 1, isReturn = 1, isBarrier = 1, hasCtrlDep = 1 in
def RET : I<0xC3, RawFrm, (outs), (ins), "ret", [(X86retflag 0)]>;
let isCall = 1 in
// All calls clobber the non-callee saved registers...
let Defs = [EAX, ECX, EDX, FP0, FP1, FP2, FP3, FP4, FP5, FP6, ST0,
MM0, MM1, MM2, MM3, MM4, MM5, MM6, MM7,
XMM0, XMM1, XMM2, XMM3, XMM4, XMM5, XMM6, XMM7, EFLAGS] in {
def CALLpcrel32 : Ii32<0xE8, RawFrm, (outs), (ins i32imm:$dst,variable_ops),
"call\t${dst:call}", []>;
def CALL32r : I<0xFF, MRM2r, (outs), (ins GR32:$dst, variable_ops),
"call\t{*}$dst", [(X86call GR32:$dst)]>;
def CALL32m : I<0xFF, MRM2m, (outs), (ins i32mem:$dst, variable_ops),
"call\t{*}$dst", []>;
}
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.
It's also possible to use "let" expressions inside multiclasses, providing more
ways to factor out commonality from the records, specially if using several
levels of multiclass instantiations. This also avoids the need of using "let"
expressions within subsequent records inside a multiclass.
* value local to a ``def``, such as the use of ``Bar`` in::
.. code-block:: llvm
multiclass basic_r<bits<4> opc> {
let Predicates = [HasSSE2] in {
def rr : Instruction<opc, "rr">;
def rm : Instruction<opc, "rm">;
}
let Predicates = [HasSSE3] in
def rx : Instruction<opc, "rx">;
}
def Foo {
int Bar = 5;
int Baz = Bar;
}
multiclass basic_ss<bits<4> opc> {
let IsDouble = 0 in
defm SS : basic_r<opc>;
* a template arg of a ``class``, such as the use of ``Bar`` in::
let IsDouble = 1 in
defm SD : basic_r<opc>;
}
class Foo<int Bar> {
int Baz = Bar;
}
defm ADD : basic_ss<0xf>;
* value local to a ``multiclass``, such as the use of ``Bar`` in::
Looping
^^^^^^^
multiclass Foo {
int Bar = 5;
int Baz = Bar;
}
TableGen supports the '``foreach``' block, which textually replicates the loop
body, substituting iterator values for iterator references in the body.
Example:
* a template arg to a ``multiclass``, such as the use of ``Bar`` in::
.. code-block:: llvm
multiclass Foo<int Bar> {
int Baz = Bar;
}
foreach i = [0, 1, 2, 3] in {
def R#i : Register<...>;
def F#i : Register<...>;
}
.. productionlist::
SimpleValue: `TokInteger`
This will create objects ``R0``, ``R1``, ``R2`` and ``R3``. ``foreach`` blocks
may be nested. If there is only one item in the body the braces may be
elided:
This represents the numeric value of the integer.
.. code-block:: llvm
.. productionlist::
SimpleValue: `TokString`+
foreach i = [0, 1, 2, 3] in
def R#i : Register<...>;
Multiple adjacent string literals are concatenated like in C/C++. The value
is the concatenation of the strings.
Code Generator backend info
===========================
.. productionlist::
SimpleValue: `TokCodeFragment`
Expressions used by code generator to describe instructions and isel patterns:
The value is the string value of the code fragment.
``(implicit a)``
an implicitly defined physical register. This tells the dag instruction
selection emitter the input pattern's extra definitions matches implicit
physical register definitions.
.. productionlist::
SimpleValue: "?"
``?`` represents an "unset" initializer.
.. productionlist::
SimpleValue: "{" `ValueList` "}"
ValueList: [`ValueListNE`]
ValueListNE: `Value` ("," `Value`)*
This represents a sequence of bits, as would be used to initialize a
``bits<n>`` field (where ``n`` is the number of bits).
.. productionlist::
SimpleValue: `ClassID` "<" `ValueListNE` ">"
This generates a new anonymous record definition (as would be created by an
unnamed ``def`` inheriting from the given class with the given template
arguments) and the value is the value of that record definition.
.. productionlist::
SimpleValue: "[" `ValueList` "]" ["<" `Type` ">"]
A list initializer. The optional :token:`Type` can be used to indicate a
specific element type, otherwise the element type will be deduced from the
given values.
.. The initial `DagArg` of the dag must start with an identifier or
!cast, but this is more of an implementation detail and so for now just
leave it out.
.. productionlist::
SimpleValue: "(" `DagArg` `DagArgList` ")"
DagArgList: `DagArg` ("," `DagArg`)*
DagArg: `Value` [":" `TokVarName`] | `TokVarName`
The initial :token:`DagArg` is called the "operator" of the dag.
.. productionlist::
SimpleValue: `BangOperator` ["<" `Type` ">"] "(" `ValueListNE` ")"
Bodies
------
.. productionlist::
ObjectBody: `BaseClassList` `Body`
BaseClassList: [":" `BaseClassListNE`]
BaseClassListNE: `SubClassRef` ("," `SubClassRef`)*
SubClassRef: (`ClassID` | `MultiClassID`) ["<" `ValueList` ">"]
DefmID: `TokIdentifier`
The version with the :token:`MultiClassID` is only valid in the
:token:`BaseClassList` of a ``defm``.
The :token:`MultiClassID` should be the name of a ``multiclass``.
.. put this somewhere else
It is after parsing the base class list that the "let stack" is applied.
.. productionlist::
Body: ";" | "{" BodyList "}"
BodyList: BodyItem*
BodyItem: `Declaration` ";"
:| "let" `TokIdentifier` [`RangeList`] "=" `Value` ";"
The ``let`` form allows overriding the value of an inherited field.
``def``
-------
.. TODO::
There can be pastes in the names here, like ``#NAME#``. Look into that
and document it (it boils down to ParseIDValue with IDParseMode ==
ParseNameMode). ParseObjectName calls into the general ParseValue, with
the only different from "arbitrary expression parsing" being IDParseMode
== Mode.
.. productionlist::
Def: "def" `TokIdentifier` `ObjectBody`
Defines a record whose name is given by the :token:`TokIdentifier`. The
fields of the record are inherited from the base classes and defined in the
body.
Special handling occurs if this ``def`` appears inside a ``multiclass`` or
a ``foreach``.
``defm``
--------
.. productionlist::
Defm: "defm" `TokIdentifier` ":" `BaseClassListNE` ";"
Note that in the :token:`BaseClassList`, all of the ``multiclass``'s must
precede any ``class``'s that appear.
``foreach``
-----------
.. productionlist::
Foreach: "foreach" `Declaration` "in" "{" `Object`* "}"
:| "foreach" `Declaration` "in" `Object`
The value assigned to the variable in the declaration is iterated over and
the object or object list is reevaluated with the variable set at each
iterated value.
Top-Level ``let``
-----------------
.. productionlist::
Let: "let" `LetList` "in" "{" `Object`* "}"
:| "let" `LetList` "in" `Object`
LetList: `LetItem` ("," `LetItem`)*
LetItem: `TokIdentifier` [`RangeList`] "=" `Value`
This is effectively equivalent to ``let`` inside the body of a record
except that it applies to multiple records at a time. The bindings are
applied at the end of parsing the base classes of a record.
``multiclass``
--------------
.. productionlist::
MultiClass: "multiclass" `TokIdentifier` [`TemplateArgList`]
: [":" `BaseMultiClassList`] "{" `MultiClassObject`+ "}"
BaseMultiClassList: `MultiClassID` ("," `MultiClassID`)*
MultiClassID: `TokIdentifier`
MultiClassObject: `Def` | `Defm` | `Let` | `Foreach`