decodetree: Multi-cleanup

Includes multiple changes by Richard Henderson as follows:

- Use proper varargs to print the arguments. (2fd51b19c9)
- Rename MultiPattern to IncMultiPattern (040145c4f8)
- Split out MultiPattern from IncMultiPattern (df63044d02)
- Allow group covering the entire insn space (b44b3449a0)
- Move semantic propagation into classes (08561fc128)
- Implement non-overlapping groups (067e8b0f45)
- Drop check for less than 2 patterns in a group (fe079aa13d)

qemu/scripts/decodetree.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # Copyright (c) 2018 Linaro Limited
 #
 # This library is free software; you can redistribute it and/or
@@ -17,139 +17,7 @@
 
 #
 # Generate a decoding tree from a specification file.
-#
+# See the syntax and semantics in docs/devel/decodetree.rst.
-# 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)
 #
 
 import os
@@ -163,7 +31,6 @@ variablewidth = False
 fields = {}
 arguments = {}
 formats = {}
-patterns = []
 allpatterns = []
 anyextern = False
 
@@ -183,24 +50,26 @@ def error_with_file(file, lineno, *args):
     global output_file
     global output_fd
 
+    prefix = ''
+    if file:
+        prefix += '{0}:'.format(file)
     if lineno:
-        r = '{0}:{1}: error:'.format(file, lineno)
+        prefix += '{0}:'.format(lineno)
-    elif input_file:
+    if prefix:
-        r = '{0}: error:'.format(file)
+        prefix += ' '
-    else:
+    print(prefix, end='error: ', file=sys.stderr)
-        r = 'error:'
+    print(*args, file=sys.stderr)
-    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)
+# end error_with_file
 
 
 def error(lineno, *args):
-    error_with_file(input_file, lineno, args)
+    error_with_file(input_file, lineno, *args)
+# end error
 
 
 def output(*args):
@@ -209,13 +78,6 @@ def output(*args):
         output_fd.write(a)
 
 
-if sys.version_info >= (3, 4):
-    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')
 
@@ -261,6 +123,7 @@ def is_pow2(x):
 
 def ctz(x):
     """Return the number of times 2 factors into X."""
+    assert x != 0
     r = 0
     while ((x >> r) & 1) == 0:
         r += 1
@@ -268,6 +131,8 @@ def ctz(x):
 
 
 def is_contiguous(bits):
+    if bits == 0:
+        return -1
     shift = ctz(bits)
     if is_pow2((bits >> shift) + 1):
         return shift
@@ -505,32 +370,99 @@ class Pattern(General):
             output(ind, 'u.f_', arg, '.', n, ' = ', f.str_extract(), ';\n')
         output(ind, 'if (', translate_prefix, '_', self.name,
                '(ctx, &u.f_', arg, ')) return true;\n')
+
+    # Normal patterns do not have children.
+    def build_tree(self):
+        return
+    def prop_masks(self):
+        return
+    def prop_format(self):
+        return
+    def prop_width(self):
+        return
+
 # end Pattern
 
 
 class MultiPattern(General):
-    """Class representing an overlapping set of instruction patterns"""
+    """Class representing a set of instruction patterns"""
 
-    def __init__(self, lineno, pats, fixb, fixm, udfm, w):
+    def __init__(self, lineno):
         self.file = input_file
         self.lineno = lineno
-        self.pats = pats
+        self.pats = []
         self.base = None
-        self.fixedbits = fixb
+        self.fixedbits = 0
-        self.fixedmask = fixm
+        self.fixedmask = 0
-        self.undefmask = udfm
+        self.undefmask = 0
-        self.width = w
+        self.width = None
 
     def __str__(self):
-        r = "{"
+        r = 'group'
-        for p in self.pats:
+        if self.fixedbits is not None:
-           r = r + ' ' + str(p)
+            r += ' ' + str_match_bits(self.fixedbits, self.fixedmask)
-        return r + "}"
+        return r
 
     def output_decl(self):
         for p in self.pats:
             p.output_decl()
 
+    def prop_masks(self):
+        global insnmask
+
+        fixedmask = insnmask
+        undefmask = insnmask
+
+        # Collect fixedmask/undefmask for all of the children.
+        for p in self.pats:
+            p.prop_masks()
+            fixedmask &= p.fixedmask
+            undefmask &= p.undefmask
+
+        # Widen fixedmask until all fixedbits match
+        repeat = True
+        fixedbits = 0
+        while repeat and fixedmask != 0:
+            fixedbits = None
+            for p in self.pats:
+                thisbits = p.fixedbits & fixedmask
+                if fixedbits is None:
+                    fixedbits = thisbits
+                elif fixedbits != thisbits:
+                    fixedmask &= ~(fixedbits ^ thisbits)
+                    break
+            else:
+                repeat = False
+
+        self.fixedbits = fixedbits
+        self.fixedmask = fixedmask
+        self.undefmask = undefmask
+
+    def build_tree(self):
+        for p in self.pats:
+            p.build_tree()
+
+    def prop_format(self):
+        for p in self.pats:
+            p.build_tree()
+
+    def prop_width(self):
+        width = None
+        for p in self.pats:
+            p.prop_width()
+            if width is None:
+                width = p.width
+            elif width != p.width:
+                error_with_file(self.file, self.lineno,
+                                'width mismatch in patterns within braces')
+        self.width = width
+
+# end MultiPattern
+
+
+class IncMultiPattern(MultiPattern):
+    """Class representing an overlapping set of instruction patterns"""
+
     def output_code(self, i, extracted, outerbits, outermask):
         global translate_prefix
         ind = str_indent(i)
@@ -547,7 +479,154 @@ class MultiPattern(General):
                 output(ind, '}\n')
             else:
                 p.output_code(i, extracted, p.fixedbits, p.fixedmask)
-#end MultiPattern
+#end IncMultiPattern
+
+
+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(),
+                   '(ctx, &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, '    return false;\n')
+        output(ind, '}\n')
+# end Tree
+
+
+class ExcMultiPattern(MultiPattern):
+    """Class representing a non-overlapping set of instruction patterns"""
+
+    def output_code(self, i, extracted, outerbits, outermask):
+        # Defer everything to our decomposed Tree node
+        self.tree.output_code(i, extracted, outerbits, outermask)
+
+    @staticmethod
+    def __build_tree(pats, outerbits, outermask):
+        # Find the intersection of all remaining fixedmask.
+        innermask = ~outermask & insnmask
+        for i in pats:
+            innermask &= i.fixedmask
+
+        if innermask == 0:
+            # Edge condition: One pattern covers the entire insnmask
+            if len(pats) == 1:
+                t = Tree(outermask, innermask)
+                t.subs.append((0, pats[0]))
+                return t
+
+            text = 'overlapping patterns:'
+            for p in pats:
+                text += '\n' + p.file + ':' + str(p.lineno) + ': ' + str(p)
+            error_with_file(pats[0].file, pats[0].lineno, text)
+
+        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 = ExcMultiPattern.__build_tree(l, b | outerbits, fullmask)
+            t.subs.append((b, s))
+
+        return t
+
+    def build_tree(self):
+        super().prop_format()
+        self.tree = self.__build_tree(self.pats, self.fixedbits,
+                                      self.fixedmask)
+
+    @staticmethod
+    def __prop_format(tree):
+        """Propagate Format objects into the decode tree"""
+
+        # Depth first search.
+        for (b, s) in tree.subs:
+            if isinstance(s, Tree):
+                ExcMultiPattern.__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
+
+    def prop_format(self):
+        super().prop_format()
+        self.__prop_format(self.tree)
+
+# end ExcMultiPattern
 
 
 def parse_field(lineno, name, toks):
@@ -562,18 +641,18 @@ def parse_field(lineno, name, toks):
     width = 0
     func = None
     for t in toks:
-        if re_fullmatch('!function=' + re_ident, t):
+        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):
+        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):
+        elif re.fullmatch('[0-9]+:[0-9]+', t):
             # Unsigned field extract
             subtoks = t.split(':')
             sign = False
@@ -622,11 +701,11 @@ def parse_arguments(lineno, name, toks):
     flds = []
     extern = False
     for t in toks:
-        if re_fullmatch('!extern', t):
+        if re.fullmatch('!extern', t):
             extern = True
             anyextern = True
             continue
-        if not re_fullmatch(re_ident, t):
+        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))
@@ -706,18 +785,19 @@ def infer_format(arg, fieldmask, flds, width):
 # end infer_format
 
 
-def parse_generic(lineno, is_format, name, toks):
+def parse_generic(lineno, parent_pat, name, toks):
     """Parse one instruction format from TOKS at LINENO"""
     global fields
     global arguments
     global formats
-    global patterns
     global allpatterns
     global re_ident
     global insnwidth
     global insnmask
     global variablewidth
 
+    is_format = parent_pat is None
+
     fixedmask = 0
     fixedbits = 0
     undefmask = 0
@@ -755,13 +835,13 @@ def parse_generic(lineno, is_format, name, toks):
             continue
 
         # 'Foo=%Bar' imports a field with a different name.
-        if re_fullmatch(re_ident + '=%' + re_ident, t):
+        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):
+        if re.fullmatch(re_ident + '=[+-]?[0-9]+', t):
             (fname, value) = t.split('=')
             value = int(value)
             flds = add_field(lineno, flds, fname, ConstField(value))
@@ -769,7 +849,7 @@ def parse_generic(lineno, is_format, name, toks):
 
         # Pattern of 0s, 1s, dots and dashes indicate required zeros,
         # required ones, or dont-cares.
-        if re_fullmatch('[01.-]+', t):
+        if re.fullmatch('[01.-]+', t):
             shift = len(t)
             fms = t.replace('0', '1')
             fms = fms.replace('.', '0')
@@ -786,7 +866,7 @@ def parse_generic(lineno, is_format, name, toks):
             fixedmask = (fixedmask << shift) | fms
             undefmask = (undefmask << shift) | ubm
         # Otherwise, fieldname:fieldwidth
-        elif re_fullmatch(re_ident + ':s?[0-9]+', t):
+        elif re.fullmatch(re_ident + ':s?[0-9]+', t):
             (fname, flen) = t.split(':')
             sign = False
             if flen[0] == 's':
@@ -868,7 +948,7 @@ def parse_generic(lineno, is_format, name, toks):
                 error(lineno, 'field {0} not initialized'.format(f))
         pat = Pattern(name, lineno, fmt, fixedbits, fixedmask,
                       undefmask, fieldmask, flds, width)
-        patterns.append(pat)
+        parent_pat.pats.append(pat)
         allpatterns.append(pat)
 
     # Validate the masks that we have assembled.
@@ -889,63 +969,15 @@ def parse_generic(lineno, is_format, name, toks):
 # end parse_general
 
 
-def build_multi_pattern(lineno, pats):
+def parse_file(f, parent_pat):
-    """Validate the Patterns going into a MultiPattern."""
-    global patterns
-    global insnmask
-
-    if len(pats) < 2:
-        error(lineno, 'less than two patterns within braces')
-
-    fixedmask = insnmask
-    undefmask = insnmask
-
-    # Collect fixed/undefmask for all of the children.
-    # Move the defining lineno back to that of the first child.
-    for p in pats:
-        fixedmask &= p.fixedmask
-        undefmask &= p.undefmask
-        if p.lineno < lineno:
-            lineno = p.lineno
-
-    width = None
-    for p in pats:
-        if width is None:
-            width = p.width
-        elif width != p.width:
-            error(lineno, 'width mismatch in patterns within braces')
-
-    repeat = True
-    while repeat:
-        if fixedmask == 0:
-            error(lineno, 'no overlap in patterns within braces')
-        fixedbits = None
-        for p in pats:
-            thisbits = p.fixedbits & fixedmask
-            if fixedbits is None:
-                fixedbits = thisbits
-            elif fixedbits != thisbits:
-                fixedmask &= ~(fixedbits ^ thisbits)
-                break
-        else:
-            repeat = False
-
-    mp = MultiPattern(lineno, pats, fixedbits, fixedmask, undefmask, width)
-    patterns.append(mp)
-# end build_multi_pattern
-
-
-def parse_file(f):
     """Parse all of the patterns within a file"""
 
-    global patterns
-
     # Read all of the lines of the file.  Concatenate lines
     # ending in backslash; discard empty lines and comments.
     toks = []
     lineno = 0
     nesting = 0
-    saved_pats = []
+    nesting_pats = []
 
     for line in f:
         lineno += 1
@@ -989,17 +1021,23 @@ def parse_file(f):
         del toks[0]
 
         # End nesting?
-        if name == '}':
+        if name == '}' or name == ']':
-            if nesting == 0:
-                error(start_lineno, 'mismatched close brace')
             if len(toks) != 0:
                 error(start_lineno, 'extra tokens after close brace')
+
+            # Make sure { } and [ ] nest properly.
+            if (name == '}') != isinstance(parent_pat, IncMultiPattern):
+                error(lineno, 'mismatched close brace')
+
+            try:
+                parent_pat = nesting_pats.pop()
+            except:
+                error(lineno, 'extra close brace')
+
             nesting -= 2
             if indent != nesting:
-                error(start_lineno, 'indentation ', indent, ' != ', nesting)
+                error(lineno, 'indentation ', indent, ' != ', nesting)
-            pats = patterns
+
-            patterns = saved_pats.pop()
-            build_multi_pattern(lineno, pats)
             toks = []
             continue
 
@@ -1008,11 +1046,18 @@ def parse_file(f):
             error(start_lineno, 'indentation ', indent, ' != ', nesting)
 
         # Start nesting?
-        if name == '{':
+        if name == '{' or name == '[':
             if len(toks) != 0:
                 error(start_lineno, 'extra tokens after open brace')
-            saved_pats.append(patterns)
+
-            patterns = []
+            if name == '{':
+                nested_pat = IncMultiPattern(start_lineno)
+            else:
+                nested_pat = ExcMultiPattern(start_lineno)
+            parent_pat.pats.append(nested_pat)
+            nesting_pats.append(parent_pat)
+            parent_pat = nested_pat
+
             nesting += 2
             toks = []
             continue
@@ -1023,115 +1068,16 @@ def parse_file(f):
         elif name[0] == '&':
             parse_arguments(start_lineno, name[1:], toks)
         elif name[0] == '@':
-            parse_generic(start_lineno, True, name[1:], toks)
+            parse_generic(start_lineno, None, name[1:], toks)
         else:
-            parse_generic(start_lineno, False, name, toks)
+            parse_generic(start_lineno, parent_pat, name, toks)
         toks = []
+
+    if nesting != 0:
+        error(lineno, 'missing close brace')
 # 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(),
-                   '(ctx, &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, '    return false;\n')
-        output(ind, '}\n')
-# end Tree
-
-
-def build_tree(pats, outerbits, outermask):
-    # Find the intersection of all remaining fixedmask.
-    innermask = ~outermask & insnmask
-    for i in pats:
-        innermask &= i.fixedmask
-
-    if innermask == 0:
-        text = 'overlapping patterns:'
-        for p in pats:
-            text += '\n' + p.file + ':' + str(p.lineno) + ': ' + str(p)
-        error_with_file(pats[0].file, pats[0].lineno, text)
-
-    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
-
-
 class SizeTree:
     """Class representing a node in a size decode tree"""
 
@@ -1273,28 +1219,6 @@ def build_size_tree(pats, width, outerbits, outermask):
 # end build_size_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 prop_size(tree):
     """Propagate minimum widths up the decode size tree"""
 
@@ -1315,7 +1239,6 @@ def prop_size(tree):
 def main():
     global arguments
     global formats
-    global patterns
     global allpatterns
     global translate_scope
     global translate_prefix
@@ -1362,18 +1285,29 @@ def main():
 
     if len(args) < 1:
         error(0, 'missing input file')
+
+    toppat = ExcMultiPattern(0)
+
     for filename in args:
         input_file = filename
         f = open(filename, 'r')
-        parse_file(f)
+        parse_file(f, toppat)
         f.close()
 
-    if variablewidth:
+    # We do not want to compute masks for toppat, because those masks
-        stree = build_size_tree(patterns, 8, 0, 0)
+    # are used as a starting point for build_tree.  For toppat, we must
-        prop_size(stree)
+    # insist that decode begins from naught.
+    for i in toppat.pats:
+        i.prop_masks()
 
-    dtree = build_tree(patterns, 0, 0)
+    toppat.build_tree()
-    prop_format(dtree)
+    toppat.prop_format()
+
+    if variablewidth:
+        for i in toppat.pats:
+            i.prop_width()
+        stree = build_size_tree(toppat.pats, 8, 0, 0)
+        prop_size(stree)
 
     if output_file:
         output_fd = open(output_file, 'w')
@@ -1425,13 +1359,14 @@ def main():
            '(DisasContext *ctx, ', insntype, ' insn)\n{\n')
 
     i4 = str_indent(4)
+
     if len(allpatterns) != 0:
         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')
-        dtree.output_code(4, False, 0, 0)
+        toppat.output_code(4, False, 0, 0)
 
     output(i4, 'return false;\n')
     output('}\n')