decodetree: Move documentation to docs/devel/decodetree.rst
One great big block comment isn't the best way to document the syntax of a language. Reviewed-by: Bastian Koppelmann <kbastian@mail.uni-paderborn.de> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
This commit is contained in:
Richard Henderson
parent
55fd189205
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3fdbf5d679
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@ -118,6 +118,7 @@ F: exec.c
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F: accel/tcg/
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F: accel/stubs/tcg-stub.c
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F: scripts/decodetree.py
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F: docs/devel/decodetree.rst
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F: include/exec/cpu*.h
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F: include/exec/exec-all.h
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F: include/exec/helper*.h
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@ -0,0 +1,156 @@
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========================
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Decodetree Specification
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========================
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A *decodetree* is built from instruction *patterns*. A pattern may
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represent a single architectural instruction or a group of same, depending
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on what is convenient for further processing.
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Each pattern has both *fixedbits* and *fixedmask*, the combination of which
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describes the condition under which the pattern is matched::
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(insn & fixedmask) == fixedbits
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Each pattern may have *fields*, which are extracted from the insn and
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passed along to the translator. Examples of such are registers,
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immediates, and sub-opcodes.
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In support of patterns, one may declare *fields*, *argument sets*, and
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*formats*, each of which may be re-used to simplify further definitions.
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Fields
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======
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Syntax::
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field_def := '%' identifier ( unnamed_field )+ ( !function=identifier )?
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unnamed_field := number ':' ( 's' ) number
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For *unnamed_field*, the first number is the least-significant bit position
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of the field and the second number is the length of the field. If the 's' is
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present, the field is considered signed. If multiple ``unnamed_fields`` are
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present, they are concatenated. In this way one can define disjoint fields.
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If ``!function`` is specified, the concatenated result is passed through the
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named function, taking and returning an integral value.
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FIXME: the fields of the structure into which this result will be stored
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is restricted to ``int``. Which means that we cannot expand 64-bit items.
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Field examples:
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+---------------------------+---------------------------------------------+
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| Input | Generated code |
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+===========================+=============================================+
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| %disp 0:s16 | sextract(i, 0, 16) |
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+---------------------------+---------------------------------------------+
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| %imm9 16:6 10:3 | extract(i, 16, 6) << 3 | extract(i, 10, 3) |
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+---------------------------+---------------------------------------------+
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| %disp12 0:s1 1:1 2:10 | sextract(i, 0, 1) << 11 | |
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| | extract(i, 1, 1) << 10 | |
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| | extract(i, 2, 10) |
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+---------------------------+---------------------------------------------+
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| %shimm8 5:s8 13:1 | expand_shimm8(sextract(i, 5, 8) << 1 | |
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| !function=expand_shimm8 | extract(i, 13, 1)) |
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+---------------------------+---------------------------------------------+
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Argument Sets
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=============
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Syntax::
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args_def := '&' identifier ( args_elt )+ ( !extern )?
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args_elt := identifier
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Each *args_elt* defines an argument within the argument set.
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Each argument set will be rendered as a C structure "arg_$name"
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with each of the fields being one of the member arguments.
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If ``!extern`` is specified, the backing structure is assumed
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to have been already declared, typically via a second decoder.
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Argument set examples::
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®3 ra rb rc
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&loadstore reg base offset
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Formats
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=======
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Syntax::
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fmt_def := '@' identifier ( fmt_elt )+
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fmt_elt := fixedbit_elt | field_elt | field_ref | args_ref
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fixedbit_elt := [01.-]+
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field_elt := identifier ':' 's'? number
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field_ref := '%' identifier | identifier '=' '%' identifier
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args_ref := '&' identifier
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Defining a format is a handy way to avoid replicating groups of fields
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across many instruction patterns.
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A *fixedbit_elt* describes a contiguous sequence of bits that must
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be 1, 0, or don't care. The difference between '.' and '-'
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is that '.' means that the bit will be covered with a field or a
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final 0 or 1 from the pattern, and '-' means that the bit is really
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ignored by the cpu and will not be specified.
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A *field_elt* describes a simple field only given a width; the position of
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the field is implied by its position with respect to other *fixedbit_elt*
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and *field_elt*.
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If any *fixedbit_elt* or *field_elt* appear, then all bits must be defined.
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Padding with a *fixedbit_elt* of all '.' is an easy way to accomplish that.
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A *field_ref* incorporates a field by reference. This is the only way to
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add a complex field to a format. A field may be renamed in the process
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via assignment to another identifier. This is intended to allow the
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same argument set be used with disjoint named fields.
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A single *args_ref* may specify an argument set to use for the format.
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The set of fields in the format must be a subset of the arguments in
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the argument set. If an argument set is not specified, one will be
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inferred from the set of fields.
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It is recommended, but not required, that all *field_ref* and *args_ref*
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appear at the end of the line, not interleaving with *fixedbit_elf* or
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*field_elt*.
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Format examples::
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@opr ...... ra:5 rb:5 ... 0 ....... rc:5
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@opi ...... ra:5 lit:8 1 ....... rc:5
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Patterns
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========
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Syntax::
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pat_def := identifier ( pat_elt )+
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pat_elt := fixedbit_elt | field_elt | field_ref | args_ref | fmt_ref | const_elt
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fmt_ref := '@' identifier
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const_elt := identifier '=' number
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The *fixedbit_elt* and *field_elt* specifiers are unchanged from formats.
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A pattern that does not specify a named format will have one inferred
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from a referenced argument set (if present) and the set of fields.
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A *const_elt* allows a argument to be set to a constant value. This may
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come in handy when fields overlap between patterns and one has to
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include the values in the *fixedbit_elt* instead.
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The decoder will call a translator function for each pattern matched.
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Pattern examples::
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addl_r 010000 ..... ..... .... 0000000 ..... @opr
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addl_i 010000 ..... ..... .... 0000000 ..... @opi
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which will, in part, invoke::
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trans_addl_r(ctx, &arg_opr, insn)
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and::
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trans_addl_i(ctx, &arg_opi, insn)
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@ -19,4 +19,4 @@ Contents:
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migration
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stable-process
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testing
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decodetree
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@ -17,139 +17,7 @@
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#
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# Generate a decoding tree from a specification file.
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#
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# The tree is built from instruction "patterns". A pattern may represent
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# a single architectural instruction or a group of same, depending on what
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# is convenient for further processing.
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#
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# Each pattern has "fixedbits" & "fixedmask", the combination of which
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# describes the condition under which the pattern is matched:
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#
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# (insn & fixedmask) == fixedbits
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#
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# Each pattern may have "fields", which are extracted from the insn and
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# passed along to the translator. Examples of such are registers,
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# immediates, and sub-opcodes.
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#
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# In support of patterns, one may declare fields, argument sets, and
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# formats, each of which may be re-used to simplify further definitions.
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#
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# *** Field syntax:
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#
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# field_def := '%' identifier ( unnamed_field )+ ( !function=identifier )?
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# unnamed_field := number ':' ( 's' ) number
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#
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# For unnamed_field, the first number is the least-significant bit position of
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# the field and the second number is the length of the field. If the 's' is
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# present, the field is considered signed. If multiple unnamed_fields are
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# present, they are concatenated. In this way one can define disjoint fields.
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#
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# If !function is specified, the concatenated result is passed through the
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# named function, taking and returning an integral value.
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#
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# FIXME: the fields of the structure into which this result will be stored
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# is restricted to "int". Which means that we cannot expand 64-bit items.
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#
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# Field examples:
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#
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# %disp 0:s16 -- sextract(i, 0, 16)
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# %imm9 16:6 10:3 -- extract(i, 16, 6) << 3 | extract(i, 10, 3)
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# %disp12 0:s1 1:1 2:10 -- sextract(i, 0, 1) << 11
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# | extract(i, 1, 1) << 10
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# | extract(i, 2, 10)
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# %shimm8 5:s8 13:1 !function=expand_shimm8
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# -- expand_shimm8(sextract(i, 5, 8) << 1
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# | extract(i, 13, 1))
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#
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# *** Argument set syntax:
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#
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# args_def := '&' identifier ( args_elt )+ ( !extern )?
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# args_elt := identifier
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#
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# Each args_elt defines an argument within the argument set.
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# Each argument set will be rendered as a C structure "arg_$name"
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# with each of the fields being one of the member arguments.
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#
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# If !extern is specified, the backing structure is assumed to
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# have been already declared, typically via a second decoder.
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#
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# Argument set examples:
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#
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# ®3 ra rb rc
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# &loadstore reg base offset
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#
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# *** Format syntax:
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#
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# fmt_def := '@' identifier ( fmt_elt )+
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# fmt_elt := fixedbit_elt | field_elt | field_ref | args_ref
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# fixedbit_elt := [01.-]+
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# field_elt := identifier ':' 's'? number
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# field_ref := '%' identifier | identifier '=' '%' identifier
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# args_ref := '&' identifier
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#
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# Defining a format is a handy way to avoid replicating groups of fields
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# across many instruction patterns.
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#
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# A fixedbit_elt describes a contiguous sequence of bits that must
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# be 1, 0, [.-] for don't care. The difference between '.' and '-'
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# is that '.' means that the bit will be covered with a field or a
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# final [01] from the pattern, and '-' means that the bit is really
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# ignored by the cpu and will not be specified.
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#
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# A field_elt describes a simple field only given a width; the position of
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# the field is implied by its position with respect to other fixedbit_elt
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# and field_elt.
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#
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# If any fixedbit_elt or field_elt appear then all bits must be defined.
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# Padding with a fixedbit_elt of all '.' is an easy way to accomplish that.
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#
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# A field_ref incorporates a field by reference. This is the only way to
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# add a complex field to a format. A field may be renamed in the process
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# via assignment to another identifier. This is intended to allow the
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# same argument set be used with disjoint named fields.
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#
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# A single args_ref may specify an argument set to use for the format.
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# The set of fields in the format must be a subset of the arguments in
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# the argument set. If an argument set is not specified, one will be
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# inferred from the set of fields.
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#
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# It is recommended, but not required, that all field_ref and args_ref
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# appear at the end of the line, not interleaving with fixedbit_elf or
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# field_elt.
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#
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# Format examples:
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#
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# @opr ...... ra:5 rb:5 ... 0 ....... rc:5
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# @opi ...... ra:5 lit:8 1 ....... rc:5
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#
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# *** Pattern syntax:
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#
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# pat_def := identifier ( pat_elt )+
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# pat_elt := fixedbit_elt | field_elt | field_ref
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# | args_ref | fmt_ref | const_elt
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# fmt_ref := '@' identifier
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# const_elt := identifier '=' number
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#
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# The fixedbit_elt and field_elt specifiers are unchanged from formats.
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# A pattern that does not specify a named format will have one inferred
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# from a referenced argument set (if present) and the set of fields.
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#
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# A const_elt allows a argument to be set to a constant value. This may
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# come in handy when fields overlap between patterns and one has to
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# include the values in the fixedbit_elt instead.
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#
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# The decoder will call a translator function for each pattern matched.
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#
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# Pattern examples:
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#
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# addl_r 010000 ..... ..... .... 0000000 ..... @opr
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# addl_i 010000 ..... ..... .... 0000000 ..... @opi
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#
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# which will, in part, invoke
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#
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# trans_addl_r(ctx, &arg_opr, insn)
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# and
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# trans_addl_i(ctx, &arg_opi, insn)
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# See the syntax and semantics in docs/devel/decodetree.rst.
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#
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import os
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