diff --git a/scripts/decodetree.py b/scripts/decodetree.py
new file mode 100755
index 0000000000..22d2edce9d
--- /dev/null
+++ b/scripts/decodetree.py
@@ -0,0 +1,1062 @@
+#!/usr/bin/env python
+# Copyright (c) 2018 Linaro, Inc.
+#
+# This library is free software; you can redistribute it and/or
+# modify it under the terms of the GNU Lesser General Public
+# License as published by the Free Software Foundation; either
+# version 2 of the License, or (at your option) any later version.
+#
+# This library is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this library; if not, see <http://www.gnu.org/licenses/>.
+#
+
+#
+# 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 )+
+# 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.
+#
+# 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)
+#
+
+import io
+import os
+import re
+import sys
+import getopt
+import pdb
+
+insnwidth = 32
+insnmask = 0xffffffff
+fields = {}
+arguments = {}
+formats = {}
+patterns = []
+
+translate_prefix = 'trans'
+translate_scope = 'static '
+input_file = ''
+output_file = None
+output_fd = None
+insntype = 'uint32_t'
+
+re_ident = '[a-zA-Z][a-zA-Z0-9_]*'
+
+
+def error(lineno, *args):
+    """Print an error message from file:line and args and exit."""
+    global output_file
+    global output_fd
+
+    if lineno:
+        r = '{0}:{1}: error:'.format(input_file, lineno)
+    elif input_file:
+        r = '{0}: error:'.format(input_file)
+    else:
+        r = 'error:'
+    for a in args:
+        r += ' ' + str(a)
+    r += '\n'
+    sys.stderr.write(r)
+    if output_file and output_fd:
+        output_fd.close()
+        os.remove(output_file)
+    exit(1)
+
+
+def output(*args):
+    global output_fd
+    for a in args:
+        output_fd.write(a)
+
+
+if sys.version_info >= (3, 0):
+    re_fullmatch = re.fullmatch
+else:
+    def re_fullmatch(pat, str):
+        return re.match('^' + pat + '$', str)
+
+
+def output_autogen():
+    output('/* This file is autogenerated by scripts/decodetree.py.  */\n\n')
+
+
+def str_indent(c):
+    """Return a string with C spaces"""
+    return ' ' * c
+
+
+def str_fields(fields):
+    """Return a string uniquely identifing FIELDS"""
+    r = ''
+    for n in sorted(fields.keys()):
+        r += '_' + n
+    return r[1:]
+
+
+def str_match_bits(bits, mask):
+    """Return a string pretty-printing BITS/MASK"""
+    global insnwidth
+
+    i = 1 << (insnwidth - 1)
+    space = 0x01010100
+    r = ''
+    while i != 0:
+        if i & mask:
+            if i & bits:
+                r += '1'
+            else:
+                r += '0'
+        else:
+            r += '.'
+        if i & space:
+            r += ' '
+        i >>= 1
+    return r
+
+
+def is_pow2(x):
+    """Return true iff X is equal to a power of 2."""
+    return (x & (x - 1)) == 0
+
+
+def ctz(x):
+    """Return the number of times 2 factors into X."""
+    r = 0
+    while ((x >> r) & 1) == 0:
+        r += 1
+    return r
+
+
+def is_contiguous(bits):
+    shift = ctz(bits)
+    if is_pow2((bits >> shift) + 1):
+        return shift
+    else:
+        return -1
+
+
+def eq_fields_for_args(flds_a, flds_b):
+    if len(flds_a) != len(flds_b):
+        return False
+    for k, a in flds_a.items():
+        if k not in flds_b:
+            return False
+    return True
+
+
+def eq_fields_for_fmts(flds_a, flds_b):
+    if len(flds_a) != len(flds_b):
+        return False
+    for k, a in flds_a.items():
+        if k not in flds_b:
+            return False
+        b = flds_b[k]
+        if a.__class__ != b.__class__ or a != b:
+            return False
+    return True
+
+
+class Field:
+    """Class representing a simple instruction field"""
+    def __init__(self, sign, pos, len):
+        self.sign = sign
+        self.pos = pos
+        self.len = len
+        self.mask = ((1 << len) - 1) << pos
+
+    def __str__(self):
+        if self.sign:
+            s = 's'
+        else:
+            s = ''
+        return str(pos) + ':' + s + str(len)
+
+    def str_extract(self):
+        if self.sign:
+            extr = 'sextract32'
+        else:
+            extr = 'extract32'
+        return '{0}(insn, {1}, {2})'.format(extr, self.pos, self.len)
+
+    def __eq__(self, other):
+        return self.sign == other.sign and self.sign == other.sign
+
+    def __ne__(self, other):
+        return not self.__eq__(other)
+# end Field
+
+
+class MultiField:
+    """Class representing a compound instruction field"""
+    def __init__(self, subs, mask):
+        self.subs = subs
+        self.sign = subs[0].sign
+        self.mask = mask
+
+    def __str__(self):
+        return str(self.subs)
+
+    def str_extract(self):
+        ret = '0'
+        pos = 0
+        for f in reversed(self.subs):
+            if pos == 0:
+                ret = f.str_extract()
+            else:
+                ret = 'deposit32({0}, {1}, {2}, {3})' \
+                      .format(ret, pos, 32 - pos, f.str_extract())
+            pos += f.len
+        return ret
+
+    def __ne__(self, other):
+        if len(self.subs) != len(other.subs):
+            return True
+        for a, b in zip(self.subs, other.subs):
+            if a.__class__ != b.__class__ or a != b:
+                return True
+        return False
+
+    def __eq__(self, other):
+        return not self.__ne__(other)
+# end MultiField
+
+
+class ConstField:
+    """Class representing an argument field with constant value"""
+    def __init__(self, value):
+        self.value = value
+        self.mask = 0
+        self.sign = value < 0
+
+    def __str__(self):
+        return str(self.value)
+
+    def str_extract(self):
+        return str(self.value)
+
+    def __cmp__(self, other):
+        return self.value - other.value
+# end ConstField
+
+
+class FunctionField:
+    """Class representing a field passed through an expander"""
+    def __init__(self, func, base):
+        self.mask = base.mask
+        self.sign = base.sign
+        self.base = base
+        self.func = func
+
+    def __str__(self):
+        return self.func + '(' + str(self.base) + ')'
+
+    def str_extract(self):
+        return self.func + '(' + self.base.str_extract() + ')'
+
+    def __eq__(self, other):
+        return self.func == other.func and self.base == other.base
+
+    def __ne__(self, other):
+        return not self.__eq__(other)
+# end FunctionField
+
+
+class Arguments:
+    """Class representing the extracted fields of a format"""
+    def __init__(self, nm, flds):
+        self.name = nm
+        self.fields = sorted(flds)
+
+    def __str__(self):
+        return self.name + ' ' + str(self.fields)
+
+    def struct_name(self):
+        return 'arg_' + self.name
+
+    def output_def(self):
+        output('typedef struct {\n')
+        for n in self.fields:
+            output('    int ', n, ';\n')
+        output('} ', self.struct_name(), ';\n\n')
+# end Arguments
+
+
+class General:
+    """Common code between instruction formats and instruction patterns"""
+    def __init__(self, name, lineno, base, fixb, fixm, udfm, fldm, flds):
+        self.name = name
+        self.lineno = lineno
+        self.base = base
+        self.fixedbits = fixb
+        self.fixedmask = fixm
+        self.undefmask = udfm
+        self.fieldmask = fldm
+        self.fields = flds
+
+    def __str__(self):
+        r = self.name
+        if self.base:
+            r = r + ' ' + self.base.name
+        else:
+            r = r + ' ' + str(self.fields)
+        r = r + ' ' + str_match_bits(self.fixedbits, self.fixedmask)
+        return r
+
+    def str1(self, i):
+        return str_indent(i) + self.__str__()
+# end General
+
+
+class Format(General):
+    """Class representing an instruction format"""
+
+    def extract_name(self):
+        return 'extract_' + self.name
+
+    def output_extract(self):
+        output('static void ', self.extract_name(), '(',
+               self.base.struct_name(), ' *a, ', insntype, ' insn)\n{\n')
+        for n, f in self.fields.items():
+            output('    a->', n, ' = ', f.str_extract(), ';\n')
+        output('}\n\n')
+# end Format
+
+
+class Pattern(General):
+    """Class representing an instruction pattern"""
+
+    def output_decl(self):
+        global translate_scope
+        global translate_prefix
+        output('typedef ', self.base.base.struct_name(),
+               ' arg_', self.name, ';\n')
+        output(translate_scope, 'void ', translate_prefix, '_', self.name,
+               '(DisasContext *ctx, arg_', self.name,
+               ' *a, ', insntype, ' insn);\n')
+
+    def output_code(self, i, extracted, outerbits, outermask):
+        global translate_prefix
+        ind = str_indent(i)
+        arg = self.base.base.name
+        output(ind, '/* line ', str(self.lineno), ' */\n')
+        if not extracted:
+            output(ind, self.base.extract_name(), '(&u.f_', arg, ', insn);\n')
+        for n, f in self.fields.items():
+            output(ind, 'u.f_', arg, '.', n, ' = ', f.str_extract(), ';\n')
+        output(ind, translate_prefix, '_', self.name,
+               '(ctx, &u.f_', arg, ', insn);\n')
+        output(ind, 'return true;\n')
+# end Pattern
+
+
+def parse_field(lineno, name, toks):
+    """Parse one instruction field from TOKS at LINENO"""
+    global fields
+    global re_ident
+    global insnwidth
+
+    # A "simple" field will have only one entry;
+    # a "multifield" will have several.
+    subs = []
+    width = 0
+    func = None
+    for t in toks:
+        if re_fullmatch('!function=' + re_ident, t):
+            if func:
+                error(lineno, 'duplicate function')
+            func = t.split('=')
+            func = func[1]
+            continue
+
+        if re_fullmatch('[0-9]+:s[0-9]+', t):
+            # Signed field extract
+            subtoks = t.split(':s')
+            sign = True
+        elif re_fullmatch('[0-9]+:[0-9]+', t):
+            # Unsigned field extract
+            subtoks = t.split(':')
+            sign = False
+        else:
+            error(lineno, 'invalid field token "{0}"'.format(t))
+        po = int(subtoks[0])
+        le = int(subtoks[1])
+        if po + le > insnwidth:
+            error(lineno, 'field {0} too large'.format(t))
+        f = Field(sign, po, le)
+        subs.append(f)
+        width += le
+
+    if width > insnwidth:
+        error(lineno, 'field too large')
+    if len(subs) == 1:
+        f = subs[0]
+    else:
+        mask = 0
+        for s in subs:
+            if mask & s.mask:
+                error(lineno, 'field components overlap')
+            mask |= s.mask
+        f = MultiField(subs, mask)
+    if func:
+        f = FunctionField(func, f)
+
+    if name in fields:
+        error(lineno, 'duplicate field', name)
+    fields[name] = f
+# end parse_field
+
+
+def parse_arguments(lineno, name, toks):
+    """Parse one argument set from TOKS at LINENO"""
+    global arguments
+    global re_ident
+
+    flds = []
+    for t in toks:
+        if not re_fullmatch(re_ident, t):
+            error(lineno, 'invalid argument set token "{0}"'.format(t))
+        if t in flds:
+            error(lineno, 'duplicate argument "{0}"'.format(t))
+        flds.append(t)
+
+    if name in arguments:
+        error(lineno, 'duplicate argument set', name)
+    arguments[name] = Arguments(name, flds)
+# end parse_arguments
+
+
+def lookup_field(lineno, name):
+    global fields
+    if name in fields:
+        return fields[name]
+    error(lineno, 'undefined field', name)
+
+
+def add_field(lineno, flds, new_name, f):
+    if new_name in flds:
+        error(lineno, 'duplicate field', new_name)
+    flds[new_name] = f
+    return flds
+
+
+def add_field_byname(lineno, flds, new_name, old_name):
+    return add_field(lineno, flds, new_name, lookup_field(lineno, old_name))
+
+
+def infer_argument_set(flds):
+    global arguments
+
+    for arg in arguments.values():
+        if eq_fields_for_args(flds, arg.fields):
+            return arg
+
+    name = str(len(arguments))
+    arg = Arguments(name, flds.keys())
+    arguments[name] = arg
+    return arg
+
+
+def infer_format(arg, fieldmask, flds):
+    global arguments
+    global formats
+
+    const_flds = {}
+    var_flds = {}
+    for n, c in flds.items():
+        if c is ConstField:
+            const_flds[n] = c
+        else:
+            var_flds[n] = c
+
+    # Look for an existing format with the same argument set and fields
+    for fmt in formats.values():
+        if arg and fmt.base != arg:
+            continue
+        if fieldmask != fmt.fieldmask:
+            continue
+        if not eq_fields_for_fmts(flds, fmt.fields):
+            continue
+        return (fmt, const_flds)
+
+    name = 'Fmt_' + str(len(formats))
+    if not arg:
+        arg = infer_argument_set(flds)
+
+    fmt = Format(name, 0, arg, 0, 0, 0, fieldmask, var_flds)
+    formats[name] = fmt
+
+    return (fmt, const_flds)
+# end infer_format
+
+
+def parse_generic(lineno, is_format, name, toks):
+    """Parse one instruction format from TOKS at LINENO"""
+    global fields
+    global arguments
+    global formats
+    global patterns
+    global re_ident
+    global insnwidth
+    global insnmask
+
+    fixedmask = 0
+    fixedbits = 0
+    undefmask = 0
+    width = 0
+    flds = {}
+    arg = None
+    fmt = None
+    for t in toks:
+        # '&Foo' gives a format an explcit argument set.
+        if t[0] == '&':
+            tt = t[1:]
+            if arg:
+                error(lineno, 'multiple argument sets')
+            if tt in arguments:
+                arg = arguments[tt]
+            else:
+                error(lineno, 'undefined argument set', t)
+            continue
+
+        # '@Foo' gives a pattern an explicit format.
+        if t[0] == '@':
+            tt = t[1:]
+            if fmt:
+                error(lineno, 'multiple formats')
+            if tt in formats:
+                fmt = formats[tt]
+            else:
+                error(lineno, 'undefined format', t)
+            continue
+
+        # '%Foo' imports a field.
+        if t[0] == '%':
+            tt = t[1:]
+            flds = add_field_byname(lineno, flds, tt, tt)
+            continue
+
+        # 'Foo=%Bar' imports a field with a different name.
+        if re_fullmatch(re_ident + '=%' + re_ident, t):
+            (fname, iname) = t.split('=%')
+            flds = add_field_byname(lineno, flds, fname, iname)
+            continue
+
+        # 'Foo=number' sets an argument field to a constant value
+        if re_fullmatch(re_ident + '=[0-9]+', t):
+            (fname, value) = t.split('=')
+            value = int(value)
+            flds = add_field(lineno, flds, fname, ConstField(value))
+            continue
+
+        # Pattern of 0s, 1s, dots and dashes indicate required zeros,
+        # required ones, or dont-cares.
+        if re_fullmatch('[01.-]+', t):
+            shift = len(t)
+            fms = t.replace('0', '1')
+            fms = fms.replace('.', '0')
+            fms = fms.replace('-', '0')
+            fbs = t.replace('.', '0')
+            fbs = fbs.replace('-', '0')
+            ubm = t.replace('1', '0')
+            ubm = ubm.replace('.', '0')
+            ubm = ubm.replace('-', '1')
+            fms = int(fms, 2)
+            fbs = int(fbs, 2)
+            ubm = int(ubm, 2)
+            fixedbits = (fixedbits << shift) | fbs
+            fixedmask = (fixedmask << shift) | fms
+            undefmask = (undefmask << shift) | ubm
+        # Otherwise, fieldname:fieldwidth
+        elif re_fullmatch(re_ident + ':s?[0-9]+', t):
+            (fname, flen) = t.split(':')
+            sign = False
+            if flen[0] == 's':
+                sign = True
+                flen = flen[1:]
+            shift = int(flen, 10)
+            f = Field(sign, insnwidth - width - shift, shift)
+            flds = add_field(lineno, flds, fname, f)
+            fixedbits <<= shift
+            fixedmask <<= shift
+            undefmask <<= shift
+        else:
+            error(lineno, 'invalid token "{0}"'.format(t))
+        width += shift
+
+    # We should have filled in all of the bits of the instruction.
+    if not (is_format and width == 0) and width != insnwidth:
+        error(lineno, 'definition has {0} bits'.format(width))
+
+    # Do not check for fields overlaping fields; one valid usage
+    # is to be able to duplicate fields via import.
+    fieldmask = 0
+    for f in flds.values():
+        fieldmask |= f.mask
+
+    # Fix up what we've parsed to match either a format or a pattern.
+    if is_format:
+        # Formats cannot reference formats.
+        if fmt:
+            error(lineno, 'format referencing format')
+        # If an argument set is given, then there should be no fields
+        # without a place to store it.
+        if arg:
+            for f in flds.keys():
+                if f not in arg.fields:
+                    error(lineno, 'field {0} not in argument set {1}'
+                                  .format(f, arg.name))
+        else:
+            arg = infer_argument_set(flds)
+        if name in formats:
+            error(lineno, 'duplicate format name', name)
+        fmt = Format(name, lineno, arg, fixedbits, fixedmask,
+                     undefmask, fieldmask, flds)
+        formats[name] = fmt
+    else:
+        # Patterns can reference a format ...
+        if fmt:
+            # ... but not an argument simultaneously
+            if arg:
+                error(lineno, 'pattern specifies both format and argument set')
+            if fixedmask & fmt.fixedmask:
+                error(lineno, 'pattern fixed bits overlap format fixed bits')
+            fieldmask |= fmt.fieldmask
+            fixedbits |= fmt.fixedbits
+            fixedmask |= fmt.fixedmask
+            undefmask |= fmt.undefmask
+        else:
+            (fmt, flds) = infer_format(arg, fieldmask, flds)
+        arg = fmt.base
+        for f in flds.keys():
+            if f not in arg.fields:
+                error(lineno, 'field {0} not in argument set {1}'
+                              .format(f, arg.name))
+            if f in fmt.fields.keys():
+                error(lineno, 'field {0} set by format and pattern'.format(f))
+        for f in arg.fields:
+            if f not in flds.keys() and f not in fmt.fields.keys():
+                error(lineno, 'field {0} not initialized'.format(f))
+        pat = Pattern(name, lineno, fmt, fixedbits, fixedmask,
+                      undefmask, fieldmask, flds)
+        patterns.append(pat)
+
+    # Validate the masks that we have assembled.
+    if fieldmask & fixedmask:
+        error(lineno, 'fieldmask overlaps fixedmask (0x{0:08x} & 0x{1:08x})'
+                      .format(fieldmask, fixedmask))
+    if fieldmask & undefmask:
+        error(lineno, 'fieldmask overlaps undefmask (0x{0:08x} & 0x{1:08x})'
+                      .format(fieldmask, undefmask))
+    if fixedmask & undefmask:
+        error(lineno, 'fixedmask overlaps undefmask (0x{0:08x} & 0x{1:08x})'
+                      .format(fixedmask, undefmask))
+    if not is_format:
+        allbits = fieldmask | fixedmask | undefmask
+        if allbits != insnmask:
+            error(lineno, 'bits left unspecified (0x{0:08x})'
+                          .format(allbits ^ insnmask))
+# end parse_general
+
+
+def parse_file(f):
+    """Parse all of the patterns within a file"""
+
+    # Read all of the lines of the file.  Concatenate lines
+    # ending in backslash; discard empty lines and comments.
+    toks = []
+    lineno = 0
+    for line in f:
+        lineno += 1
+
+        # Discard comments
+        end = line.find('#')
+        if end >= 0:
+            line = line[:end]
+
+        t = line.split()
+        if len(toks) != 0:
+            # Next line after continuation
+            toks.extend(t)
+        elif len(t) == 0:
+            # Empty line
+            continue
+        else:
+            toks = t
+
+        # Continuation?
+        if toks[-1] == '\\':
+            toks.pop()
+            continue
+
+        if len(toks) < 2:
+            error(lineno, 'short line')
+
+        name = toks[0]
+        del toks[0]
+
+        # Determine the type of object needing to be parsed.
+        if name[0] == '%':
+            parse_field(lineno, name[1:], toks)
+        elif name[0] == '&':
+            parse_arguments(lineno, name[1:], toks)
+        elif name[0] == '@':
+            parse_generic(lineno, True, name[1:], toks)
+        else:
+            parse_generic(lineno, False, name, toks)
+        toks = []
+# end parse_file
+
+
+class Tree:
+    """Class representing a node in a decode tree"""
+
+    def __init__(self, fm, tm):
+        self.fixedmask = fm
+        self.thismask = tm
+        self.subs = []
+        self.base = None
+
+    def str1(self, i):
+        ind = str_indent(i)
+        r = '{0}{1:08x}'.format(ind, self.fixedmask)
+        if self.format:
+            r += ' ' + self.format.name
+        r += ' [\n'
+        for (b, s) in self.subs:
+            r += '{0}  {1:08x}:\n'.format(ind, b)
+            r += s.str1(i + 4) + '\n'
+        r += ind + ']'
+        return r
+
+    def __str__(self):
+        return self.str1(0)
+
+    def output_code(self, i, extracted, outerbits, outermask):
+        ind = str_indent(i)
+
+        # If we identified all nodes below have the same format,
+        # extract the fields now.
+        if not extracted and self.base:
+            output(ind, self.base.extract_name(),
+                   '(&u.f_', self.base.base.name, ', insn);\n')
+            extracted = True
+
+        # Attempt to aid the compiler in producing compact switch statements.
+        # If the bits in the mask are contiguous, extract them.
+        sh = is_contiguous(self.thismask)
+        if sh > 0:
+            # Propagate SH down into the local functions.
+            def str_switch(b, sh=sh):
+                return '(insn >> {0}) & 0x{1:x}'.format(sh, b >> sh)
+
+            def str_case(b, sh=sh):
+                return '0x{0:x}'.format(b >> sh)
+        else:
+            def str_switch(b):
+                return 'insn & 0x{0:08x}'.format(b)
+
+            def str_case(b):
+                return '0x{0:08x}'.format(b)
+
+        output(ind, 'switch (', str_switch(self.thismask), ') {\n')
+        for b, s in sorted(self.subs):
+            assert (self.thismask & ~s.fixedmask) == 0
+            innermask = outermask | self.thismask
+            innerbits = outerbits | b
+            output(ind, 'case ', str_case(b), ':\n')
+            output(ind, '    /* ',
+                   str_match_bits(innerbits, innermask), ' */\n')
+            s.output_code(i + 4, extracted, innerbits, innermask)
+        output(ind, '}\n')
+        output(ind, 'return false;\n')
+# end Tree
+
+
+def build_tree(pats, outerbits, outermask):
+    # Find the intersection of all remaining fixedmask.
+    innermask = ~outermask
+    for i in pats:
+        innermask &= i.fixedmask
+
+    if innermask == 0:
+        pnames = []
+        for p in pats:
+            pnames.append(p.name + ':' + str(p.lineno))
+        error(pats[0].lineno, 'overlapping patterns:', pnames)
+
+    fullmask = outermask | innermask
+
+    # Sort each element of pats into the bin selected by the mask.
+    bins = {}
+    for i in pats:
+        fb = i.fixedbits & innermask
+        if fb in bins:
+            bins[fb].append(i)
+        else:
+            bins[fb] = [i]
+
+    # We must recurse if any bin has more than one element or if
+    # the single element in the bin has not been fully matched.
+    t = Tree(fullmask, innermask)
+
+    for b, l in bins.items():
+        s = l[0]
+        if len(l) > 1 or s.fixedmask & ~fullmask != 0:
+            s = build_tree(l, b | outerbits, fullmask)
+        t.subs.append((b, s))
+
+    return t
+# end build_tree
+
+
+def prop_format(tree):
+    """Propagate Format objects into the decode tree"""
+
+    # Depth first search.
+    for (b, s) in tree.subs:
+        if isinstance(s, Tree):
+            prop_format(s)
+
+    # If all entries in SUBS have the same format, then
+    # propagate that into the tree.
+    f = None
+    for (b, s) in tree.subs:
+        if f is None:
+            f = s.base
+            if f is None:
+                return
+        if f is not s.base:
+            return
+    tree.base = f
+# end prop_format
+
+
+def main():
+    global arguments
+    global formats
+    global patterns
+    global translate_scope
+    global translate_prefix
+    global output_fd
+    global output_file
+    global input_file
+    global insnwidth
+    global insntype
+
+    decode_function = 'decode'
+    decode_scope = 'static '
+
+    long_opts = ['decode=', 'translate=', 'output=', 'insnwidth=']
+    try:
+        (opts, args) = getopt.getopt(sys.argv[1:], 'o:w:', long_opts)
+    except getopt.GetoptError as err:
+        error(0, err)
+    for o, a in opts:
+        if o in ('-o', '--output'):
+            output_file = a
+        elif o == '--decode':
+            decode_function = a
+            decode_scope = ''
+        elif o == '--translate':
+            translate_prefix = a
+            translate_scope = ''
+        elif o in ('-w', '--insnwidth'):
+            insnwidth = int(a)
+            if insnwidth == 16:
+                insntype = 'uint16_t'
+                insnmask = 0xffff
+            elif insnwidth != 32:
+                error(0, 'cannot handle insns of width', insnwidth)
+        else:
+            assert False, 'unhandled option'
+
+    if len(args) < 1:
+        error(0, 'missing input file')
+    input_file = args[0]
+    f = open(input_file, 'r')
+    parse_file(f)
+    f.close()
+
+    t = build_tree(patterns, 0, 0)
+    prop_format(t)
+
+    if output_file:
+        output_fd = open(output_file, 'w')
+    else:
+        output_fd = sys.stdout
+
+    output_autogen()
+    for n in sorted(arguments.keys()):
+        f = arguments[n]
+        f.output_def()
+
+    # A single translate function can be invoked for different patterns.
+    # Make sure that the argument sets are the same, and declare the
+    # function only once.
+    out_pats = {}
+    for i in patterns:
+        if i.name in out_pats:
+            p = out_pats[i.name]
+            if i.base.base != p.base.base:
+                error(0, i.name, ' has conflicting argument sets')
+        else:
+            i.output_decl()
+            out_pats[i.name] = i
+    output('\n')
+
+    for n in sorted(formats.keys()):
+        f = formats[n]
+        f.output_extract()
+
+    output(decode_scope, 'bool ', decode_function,
+           '(DisasContext *ctx, ', insntype, ' insn)\n{\n')
+
+    i4 = str_indent(4)
+    output(i4, 'union {\n')
+    for n in sorted(arguments.keys()):
+        f = arguments[n]
+        output(i4, i4, f.struct_name(), ' f_', f.name, ';\n')
+    output(i4, '} u;\n\n')
+
+    t.output_code(4, False, 0, 0)
+
+    output('}\n')
+
+    if output_file:
+        output_fd.close()
+# end main
+
+
+if __name__ == '__main__':
+    main()
diff --git a/tests/Makefile.include b/tests/Makefile.include
index a1bcbffe12..577eb573a2 100644
--- a/tests/Makefile.include
+++ b/tests/Makefile.include
@@ -932,6 +932,13 @@ $(patsubst %, check-%, $(check-qapi-schema-y)): check-%.json: $(SRC_PATH)/%.json
 check-tests/qapi-schema/doc-good.texi: tests/qapi-schema/doc-good.test.texi
 	@diff -q $(SRC_PATH)/tests/qapi-schema/doc-good.texi $<
 
+.PHONY: check-decodetree
+check-decodetree:
+	$(call quiet-command, \
+	  cd $(SRC_PATH)/tests/decode && \
+          ./check.sh "$(PYTHON)" "$(SRC_PATH)/scripts/decodetree.py", \
+          TEST, decodetree.py)
+
 # Consolidated targets
 
 .PHONY: check-qapi-schema check-qtest check-unit check check-clean
@@ -940,7 +947,7 @@ check-qtest: $(patsubst %,check-qtest-%, $(QTEST_TARGETS))
 check-unit: $(patsubst %,check-%, $(check-unit-y))
 check-speed: $(patsubst %,check-%, $(check-speed-y))
 check-block: $(patsubst %,check-%, $(check-block-y))
-check: check-qapi-schema check-unit check-qtest
+check: check-qapi-schema check-unit check-qtest check-decodetree
 check-clean:
 	$(MAKE) -C tests/tcg clean
 	rm -rf $(check-unit-y) tests/*.o $(QEMU_IOTESTS_HELPERS-y)
diff --git a/tests/decode/check.sh b/tests/decode/check.sh
new file mode 100755
index 0000000000..79a06c37cd
--- /dev/null
+++ b/tests/decode/check.sh
@@ -0,0 +1,18 @@
+#!/bin/sh
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+PYTHON=$1
+DECODETREE=$2
+E=0
+
+# All of these tests should produce errors
+for i in err_*.decode; do
+    if $PYTHON $DECODETREE $i > /dev/null 2> /dev/null; then
+        # Pass, aka failed to fail.
+        echo FAIL: $i 1>&2
+        E=1
+    fi
+done
+
+exit $E
diff --git a/tests/decode/err_argset1.decode b/tests/decode/err_argset1.decode
new file mode 100644
index 0000000000..fcaebcce76
--- /dev/null
+++ b/tests/decode/err_argset1.decode
@@ -0,0 +1,5 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose duplicate member names
+&args	a a
diff --git a/tests/decode/err_argset2.decode b/tests/decode/err_argset2.decode
new file mode 100644
index 0000000000..256b2f9b12
--- /dev/null
+++ b/tests/decode/err_argset2.decode
@@ -0,0 +1,5 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose invalid member names
+&args	a b c d0 0e
diff --git a/tests/decode/err_field1.decode b/tests/decode/err_field1.decode
new file mode 100644
index 0000000000..e07a5a73e0
--- /dev/null
+++ b/tests/decode/err_field1.decode
@@ -0,0 +1,5 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose invalid field syntax
+%field	asdf
diff --git a/tests/decode/err_field2.decode b/tests/decode/err_field2.decode
new file mode 100644
index 0000000000..7664a3ebe9
--- /dev/null
+++ b/tests/decode/err_field2.decode
@@ -0,0 +1,5 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose invalid field width.
+%field	0:33
diff --git a/tests/decode/err_field3.decode b/tests/decode/err_field3.decode
new file mode 100644
index 0000000000..87e680f103
--- /dev/null
+++ b/tests/decode/err_field3.decode
@@ -0,0 +1,5 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose invalid field position.
+%field	31:2
diff --git a/tests/decode/err_field4.decode b/tests/decode/err_field4.decode
new file mode 100644
index 0000000000..888ce4729b
--- /dev/null
+++ b/tests/decode/err_field4.decode
@@ -0,0 +1,6 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose duplicate field name.
+%field	0:1
+%field	0:1
diff --git a/tests/decode/err_field5.decode b/tests/decode/err_field5.decode
new file mode 100644
index 0000000000..b0c62af866
--- /dev/null
+++ b/tests/decode/err_field5.decode
@@ -0,0 +1,5 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose duplicate function specifier.
+%field	0:1	!function=a !function=a
diff --git a/tests/decode/err_init1.decode b/tests/decode/err_init1.decode
new file mode 100644
index 0000000000..da855bd00a
--- /dev/null
+++ b/tests/decode/err_init1.decode
@@ -0,0 +1,6 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose uninitialized member in pattern.
+&args	a b
+insn	00000000 00000000 00000000 b:8	&args
diff --git a/tests/decode/err_init2.decode b/tests/decode/err_init2.decode
new file mode 100644
index 0000000000..b58de30098
--- /dev/null
+++ b/tests/decode/err_init2.decode
@@ -0,0 +1,6 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose member initialized twice in pattern.
+&args	a b
+insn	00000000 00000000 a:8 b:8	&args a=1
diff --git a/tests/decode/err_init3.decode b/tests/decode/err_init3.decode
new file mode 100644
index 0000000000..96790abfdc
--- /dev/null
+++ b/tests/decode/err_init3.decode
@@ -0,0 +1,7 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose member initialized twice in pattern + format.
+&args	a
+@format	........ ........ a:16	&args
+insn	00000000 00000000 a:16	@format
diff --git a/tests/decode/err_init4.decode b/tests/decode/err_init4.decode
new file mode 100644
index 0000000000..4336d4632f
--- /dev/null
+++ b/tests/decode/err_init4.decode
@@ -0,0 +1,7 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose uninitialized member in pattern + format.
+&args	a b
+@format	........ ........ a:16			&args
+insn	00000000 00000000 ........ ........	@format
diff --git a/tests/decode/err_overlap1.decode b/tests/decode/err_overlap1.decode
new file mode 100644
index 0000000000..1ae63472dc
--- /dev/null
+++ b/tests/decode/err_overlap1.decode
@@ -0,0 +1,6 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose field overlapping fixedbits.
+%field	0:1
+insn	00000000 00000000 00000000 00000000 	%field
diff --git a/tests/decode/err_overlap2.decode b/tests/decode/err_overlap2.decode
new file mode 100644
index 0000000000..1d6d7a3930
--- /dev/null
+++ b/tests/decode/err_overlap2.decode
@@ -0,0 +1,6 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose field overlapping fixedbits w/format.
+@format		........ ........ ........ ....... fld:1
+insn		00000000 00000000 00000000 00000000		@format
diff --git a/tests/decode/err_overlap3.decode b/tests/decode/err_overlap3.decode
new file mode 100644
index 0000000000..3ab0c4850a
--- /dev/null
+++ b/tests/decode/err_overlap3.decode
@@ -0,0 +1,6 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose field overlapping unspecified bits.
+%field	0:1
+insn	00000000 00000000 00000000 --------	%field
diff --git a/tests/decode/err_overlap4.decode b/tests/decode/err_overlap4.decode
new file mode 100644
index 0000000000..53c5399d39
--- /dev/null
+++ b/tests/decode/err_overlap4.decode
@@ -0,0 +1,6 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose fixed bits overlapping unspecified bits.
+@format		........ ........ ........ .......-
+insn		00000000 00000000 00000000 00000000	@format
diff --git a/tests/decode/err_overlap5.decode b/tests/decode/err_overlap5.decode
new file mode 100644
index 0000000000..df0e31fffb
--- /dev/null
+++ b/tests/decode/err_overlap5.decode
@@ -0,0 +1,5 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose overlapping sub-fields.
+%field	3:5 0:5
diff --git a/tests/decode/err_overlap6.decode b/tests/decode/err_overlap6.decode
new file mode 100644
index 0000000000..cc69fc8fdd
--- /dev/null
+++ b/tests/decode/err_overlap6.decode
@@ -0,0 +1,6 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose overlapping fixed bits w/format.
+@format		........ ........ ........ .......1
+insn		00000000 00000000 00000000 00000000	@format
diff --git a/tests/decode/err_overlap7.decode b/tests/decode/err_overlap7.decode
new file mode 100644
index 0000000000..6f555295ab
--- /dev/null
+++ b/tests/decode/err_overlap7.decode
@@ -0,0 +1,6 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose overlapping patterns.
+insn1		00000000 00000000 00000000 00000000
+insn2		00000000 00000000 00000000 00000000
diff --git a/tests/decode/err_overlap8.decode b/tests/decode/err_overlap8.decode
new file mode 100644
index 0000000000..df4bae8f1c
--- /dev/null
+++ b/tests/decode/err_overlap8.decode
@@ -0,0 +1,5 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose not specified bit (. vs -).
+insn	00000000 00000000 00000000 0000000.
diff --git a/tests/decode/err_overlap9.decode b/tests/decode/err_overlap9.decode
new file mode 100644
index 0000000000..58b6ac121a
--- /dev/null
+++ b/tests/decode/err_overlap9.decode
@@ -0,0 +1,6 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose not specified bit (. vs -) w/format.
+@format		........ a:8      ........ b:7 .
+insn		00000000 ........ 00000000 ........	@format
