scripts/decodetree: Implement named field support

Implement support for named fields, i.e.  where one field is defined
in terms of another, rather than directly in terms of bits extracted
from the instruction.

The new method referenced_fields() on all the Field classes returns a
list of fields that this field references.  This just passes through,
except for the new NamedField class.

We can then use referenced_fields() to:
 * construct a list of 'dangling references' for a format or
   pattern, which is the fields that the format/pattern uses but
   doesn't define itself
 * do a topological sort, so that we output "field = value"
   assignments in an order that means that we assign a field before
   we reference it in a subsequent assignment
 * check when we output the code for a pattern whether we need to
   fill in the format fields before or after the pattern fields, and
   do other error checking

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230523120447.728365-6-peter.maydell@linaro.org>

scripts/decodetree.py

@@ -290,6 +290,9 @@ class Field:
         s = 's' if self.sign else ''
         return f'{s}extract{bitop_width}(insn, {self.pos}, {self.len})'
 
+    def referenced_fields(self):
+        return []
+
     def __eq__(self, other):
         return self.sign == other.sign and self.mask == other.mask
 
@@ -321,6 +324,12 @@ class MultiField:
             pos += f.len
         return ret
 
+    def referenced_fields(self):
+        l = []
+        for f in self.subs:
+            l.extend(f.referenced_fields())
+        return l
+
     def __ne__(self, other):
         if len(self.subs) != len(other.subs):
             return True
@@ -347,6 +356,9 @@ class ConstField:
     def str_extract(self, lvalue_formatter):
         return str(self.value)
 
+    def referenced_fields(self):
+        return []
+
     def __cmp__(self, other):
         return self.value - other.value
 # end ConstField
@@ -367,6 +379,9 @@ class FunctionField:
         return (self.func + '(ctx, '
                 + self.base.str_extract(lvalue_formatter) + ')')
 
+    def referenced_fields(self):
+        return self.base.referenced_fields()
+
     def __eq__(self, other):
         return self.func == other.func and self.base == other.base
 
@@ -388,6 +403,9 @@ class ParameterField:
     def str_extract(self, lvalue_formatter):
         return self.func + '(ctx)'
 
+    def referenced_fields(self):
+        return []
+
     def __eq__(self, other):
         return self.func == other.func
 
@@ -395,6 +413,32 @@ class ParameterField:
         return not self.__eq__(other)
 # end ParameterField
 
+class NamedField:
+    """Class representing a field already named in the pattern"""
+    def __init__(self, name, sign, len):
+        self.mask = 0
+        self.sign = sign
+        self.len = len
+        self.name = name
+
+    def __str__(self):
+        return self.name
+
+    def str_extract(self, lvalue_formatter):
+        global bitop_width
+        s = 's' if self.sign else ''
+        lvalue = lvalue_formatter(self.name)
+        return f'{s}extract{bitop_width}({lvalue}, 0, {self.len})'
+
+    def referenced_fields(self):
+        return [self.name]
+
+    def __eq__(self, other):
+        return self.name == other.name
+
+    def __ne__(self, other):
+        return not self.__eq__(other)
+# end NamedField
 
 class Arguments:
     """Class representing the extracted fields of a format"""
@@ -418,7 +462,6 @@ class Arguments:
             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, w):
@@ -432,6 +475,7 @@ class General:
         self.fieldmask = fldm
         self.fields = flds
         self.width = w
+        self.dangling = None
 
     def __str__(self):
         return self.name + ' ' + str_match_bits(self.fixedbits, self.fixedmask)
@@ -439,10 +483,51 @@ class General:
     def str1(self, i):
         return str_indent(i) + self.__str__()
 
+    def dangling_references(self):
+        # Return a list of all named references which aren't satisfied
+        # directly by this format/pattern. This will be either:
+        #  * a format referring to a field which is specified by the
+        #    pattern(s) using it
+        #  * a pattern referring to a field which is specified by the
+        #    format it uses
+        #  * a user error (referring to a field that doesn't exist at all)
+        if self.dangling is None:
+            # Compute this once and cache the answer
+            dangling = []
+            for n, f in self.fields.items():
+                for r in f.referenced_fields():
+                    if r not in self.fields:
+                        dangling.append(r)
+            self.dangling = dangling
+        return self.dangling
+
     def output_fields(self, indent, lvalue_formatter):
+        # We use a topological sort to ensure that any use of NamedField
+        # comes after the initialization of the field it is referencing.
+        graph = {}
         for n, f in self.fields.items():
-            output(indent, lvalue_formatter(n), ' = ',
-                   f.str_extract(lvalue_formatter), ';\n')
+            refs = f.referenced_fields()
+            graph[n] = refs
+
+        try:
+            ts = TopologicalSorter(graph)
+            for n in ts.static_order():
+                # We only want to emit assignments for the keys
+                # in our fields list, not for anything that ends up
+                # in the tsort graph only because it was referenced as
+                # a NamedField.
+                try:
+                    f = self.fields[n]
+                    output(indent, lvalue_formatter(n), ' = ',
+                           f.str_extract(lvalue_formatter), ';\n')
+                except KeyError:
+                    pass
+        except CycleError as e:
+            # The second element of args is a list of nodes which form
+            # a cycle (there might be others too, but only one is reported).
+            # Pretty-print it to tell the user.
+            cycle = ' => '.join(e.args[1])
+            error(self.lineno, 'field definitions form a cycle: ' + cycle)
 # end General
 
 
@@ -477,10 +562,36 @@ class Pattern(General):
         ind = str_indent(i)
         arg = self.base.base.name
         output(ind, '/* ', self.file, ':', str(self.lineno), ' */\n')
+        # We might have named references in the format that refer to fields
+        # in the pattern, or named references in the pattern that refer
+        # to fields in the format. This affects whether we extract the fields
+        # for the format before or after the ones for the pattern.
+        # For simplicity we don't allow cross references in both directions.
+        # This is also where we catch the syntax error of referring to
+        # a nonexistent field.
+        fmt_refs = self.base.dangling_references()
+        for r in fmt_refs:
+            if r not in self.fields:
+                error(self.lineno, f'format refers to undefined field {r}')
+        pat_refs = self.dangling_references()
+        for r in pat_refs:
+            if r not in self.base.fields:
+                error(self.lineno, f'pattern refers to undefined field {r}')
+        if pat_refs and fmt_refs:
+            error(self.lineno, ('pattern that uses fields defined in format '
+                                'cannot use format that uses fields defined '
+                                'in pattern'))
+        if fmt_refs:
+            # pattern fields first
+            self.output_fields(ind, lambda n: 'u.f_' + arg + '.' + n)
+            assert not extracted, "dangling fmt refs but it was already extracted"
         if not extracted:
             output(ind, self.base.extract_name(),
                    '(ctx, &u.f_', arg, ', insn);\n')
-        self.output_fields(ind, lambda n: 'u.f_' + arg + '.' + n)
+        if not fmt_refs:
+            # pattern fields last
+            self.output_fields(ind, lambda n: 'u.f_' + arg + '.' + n)
+
         output(ind, 'if (', translate_prefix, '_', self.name,
                '(ctx, &u.f_', arg, ')) return true;\n')
 
@@ -626,8 +737,10 @@ class Tree:
         ind = str_indent(i)
 
         # If we identified all nodes below have the same format,
-        # extract the fields now.
-        if not extracted and self.base:
+        # extract the fields now. But don't do it if the format relies
+        # on named fields from the insn pattern, as those won't have
+        # been initialised at this point.
+        if not extracted and self.base and not self.base.dangling_references():
             output(ind, self.base.extract_name(),
                    '(ctx, &u.f_', self.base.base.name, ', insn);\n')
             extracted = True
@@ -749,6 +862,7 @@ def parse_field(lineno, name, toks):
     """Parse one instruction field from TOKS at LINENO"""
     global fields
     global insnwidth
+    global re_C_ident
 
     # A "simple" field will have only one entry;
     # a "multifield" will have several.
@@ -763,6 +877,25 @@ def parse_field(lineno, name, toks):
             func = func[1]
             continue
 
+        if re.fullmatch(re_C_ident + ':s[0-9]+', t):
+            # Signed named field
+            subtoks = t.split(':')
+            n = subtoks[0]
+            le = int(subtoks[1])
+            f = NamedField(n, True, le)
+            subs.append(f)
+            width += le
+            continue
+        if re.fullmatch(re_C_ident + ':[0-9]+', t):
+            # Unsigned named field
+            subtoks = t.split(':')
+            n = subtoks[0]
+            le = int(subtoks[1])
+            f = NamedField(n, False, le)
+            subs.append(f)
+            width += le
+            continue
+
         if re.fullmatch('[0-9]+:s[0-9]+', t):
             # Signed field extract
             subtoks = t.split(':s')