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>

1 files changed, 1 insertions, 133 deletions

diff --git a/scripts/decodetree.py b/scripts/decodetree.py
index e342d278b8..33e32ee87f 100755
--- a/scripts/decodetree.py
+++ b/scripts/decodetree.py
@@ -17,139 +17,7 @@
 
 #
 # Generate a decoding tree from a specification file.
-#
-# The tree is built from instruction "patterns".  A pattern may represent
-# a single architectural instruction or a group of same, depending on what
-# is convenient for further processing.
-#
-# Each pattern has "fixedbits" & "fixedmask", the combination of which
-# describes the condition under which the pattern is matched:
-#
-#   (insn & fixedmask) == fixedbits
-#
-# Each pattern may have "fields", which are extracted from the insn and
-# passed along to the translator.  Examples of such are registers,
-# immediates, and sub-opcodes.
-#
-# In support of patterns, one may declare fields, argument sets, and
-# formats, each of which may be re-used to simplify further definitions.
-#
-# *** Field syntax:
-#
-# field_def     := '%' identifier ( unnamed_field )+ ( !function=identifier )?
-# unnamed_field := number ':' ( 's' ) number
-#
-# For unnamed_field, the first number is the least-significant bit position of
-# the field and the second number is the length of the field.  If the 's' is
-# present, the field is considered signed.  If multiple unnamed_fields are
-# present, they are concatenated.  In this way one can define disjoint fields.
-#
-# If !function is specified, the concatenated result is passed through the
-# named function, taking and returning an integral value.
-#
-# FIXME: the fields of the structure into which this result will be stored
-# is restricted to "int".  Which means that we cannot expand 64-bit items.
-#
-# Field examples:
-#
-#   %disp   0:s16          -- sextract(i, 0, 16)
-#   %imm9   16:6 10:3      -- extract(i, 16, 6) << 3 | extract(i, 10, 3)
-#   %disp12 0:s1 1:1 2:10  -- sextract(i, 0, 1) << 11
-#                             | extract(i, 1, 1) << 10
-#                             | extract(i, 2, 10)
-#   %shimm8 5:s8 13:1 !function=expand_shimm8
-#                          -- expand_shimm8(sextract(i, 5, 8) << 1
-#                                           | extract(i, 13, 1))
-#
-# *** Argument set syntax:
-#
-# args_def    := '&' identifier ( args_elt )+ ( !extern )?
-# args_elt    := identifier
-#
-# Each args_elt defines an argument within the argument set.
-# Each argument set will be rendered as a C structure "arg_$name"
-# with each of the fields being one of the member arguments.
-#
-# If !extern is specified, the backing structure is assumed to
-# have been already declared, typically via a second decoder.
-#
-# Argument set examples:
-#
-#   &reg3       ra rb rc
-#   &loadstore  reg base offset
-#
-# *** Format syntax:
-#
-# fmt_def      := '@' identifier ( fmt_elt )+
-# fmt_elt      := fixedbit_elt | field_elt | field_ref | args_ref
-# fixedbit_elt := [01.-]+
-# field_elt    := identifier ':' 's'? number
-# field_ref    := '%' identifier | identifier '=' '%' identifier
-# args_ref     := '&' identifier
-#
-# Defining a format is a handy way to avoid replicating groups of fields
-# across many instruction patterns.
-#
-# A fixedbit_elt describes a contiguous sequence of bits that must
-# be 1, 0, [.-] for don't care.  The difference between '.' and '-'
-# is that '.' means that the bit will be covered with a field or a
-# final [01] from the pattern, and '-' means that the bit is really
-# ignored by the cpu and will not be specified.
-#
-# A field_elt describes a simple field only given a width; the position of
-# the field is implied by its position with respect to other fixedbit_elt
-# and field_elt.
-#
-# If any fixedbit_elt or field_elt appear then all bits must be defined.
-# Padding with a fixedbit_elt of all '.' is an easy way to accomplish that.
-#
-# A field_ref incorporates a field by reference.  This is the only way to
-# add a complex field to a format.  A field may be renamed in the process
-# via assignment to another identifier.  This is intended to allow the
-# same argument set be used with disjoint named fields.
-#
-# A single args_ref may specify an argument set to use for the format.
-# The set of fields in the format must be a subset of the arguments in
-# the argument set.  If an argument set is not specified, one will be
-# inferred from the set of fields.
-#
-# It is recommended, but not required, that all field_ref and args_ref
-# appear at the end of the line, not interleaving with fixedbit_elf or
-# field_elt.
-#
-# Format examples:
-#
-#   @opr    ...... ra:5 rb:5 ... 0 ....... rc:5
-#   @opi    ...... ra:5 lit:8    1 ....... rc:5
-#
-# *** Pattern syntax:
-#
-# pat_def      := identifier ( pat_elt )+
-# pat_elt      := fixedbit_elt | field_elt | field_ref
-#               | args_ref | fmt_ref | const_elt
-# fmt_ref      := '@' identifier
-# const_elt    := identifier '=' number
-#
-# The fixedbit_elt and field_elt specifiers are unchanged from formats.
-# A pattern that does not specify a named format will have one inferred
-# from a referenced argument set (if present) and the set of fields.
-#
-# A const_elt allows a argument to be set to a constant value.  This may
-# come in handy when fields overlap between patterns and one has to
-# include the values in the fixedbit_elt instead.
-#
-# The decoder will call a translator function for each pattern matched.
-#
-# Pattern examples:
-#
-#   addl_r   010000 ..... ..... .... 0000000 ..... @opr
-#   addl_i   010000 ..... ..... .... 0000000 ..... @opi
-#
-# which will, in part, invoke
-#
-#   trans_addl_r(ctx, &arg_opr, insn)
-# and
-#   trans_addl_i(ctx, &arg_opi, insn)
+# See the syntax and semantics in docs/devel/decodetree.rst.
 #
 
 import os