diff --git a/docs/metaref.rst b/docs/metaref.rst
index 4ac81096cc..8df75d80a2 100644
--- a/docs/metaref.rst
+++ b/docs/metaref.rst
@@ -25,6 +25,45 @@ The main driver for this source code generation process is the
 :file:`meta/build.py` script which is invoked as part of the build process if
 anything in the :file:`meta` directory has changed since the last build.
 
+
+Settings
+========
+
+Settings are used by the environment embedding Cretonne to control the details
+of code generation. Each setting is defined in the meta language so a compact
+and consistent Rust representation can be generated. Shared settings are defined
+in the :mod:`cretonne.settings` module. Some settings are specific to a target
+ISA, and defined in a `settings` module under the appropriate :file:`meta/isa/*`
+directory.
+
+Settings can take boolean on/off values, small numbers, or explicitly enumerated
+symbolic values. Each type is represented by a sub-class of :class:`Setting`:
+
+.. inheritance-diagram:: Setting BoolSetting NumSetting EnumSetting
+    :parts: 1
+
+.. autoclass:: Setting
+.. autoclass:: BoolSetting
+.. autoclass:: NumSetting
+.. autoclass:: EnumSetting
+
+All settings must belong to a *group*, represented by a :class:`SettingGroup`
+object.
+
+.. autoclass:: SettingGroup
+
+Normally, a setting group corresponds to all settings defined in a module. Such
+a module looks like this::
+
+    group = SettingGroup('example')
+
+    foo = BoolSetting('use the foo')
+    bar = BoolSetting('enable bars', True)
+    opt = EnumSetting('optimization level', 'Debug', 'Release')
+
+    group.close(globals())
+
+
 Instruction descriptions
 ========================
 
diff --git a/meta/build.py b/meta/build.py
index 5dadc112e2..6ca4425823 100644
--- a/meta/build.py
+++ b/meta/build.py
@@ -5,6 +5,7 @@
 import argparse
 import isa
 import gen_instr
+import gen_settings
 import gen_build_deps
 
 parser = argparse.ArgumentParser(description='Generate sources for Cretonne.')
@@ -16,4 +17,5 @@ out_dir = args.out_dir
 isas = isa.all_isas()
 
 gen_instr.generate(isas, out_dir)
+gen_settings.generate(isas, out_dir)
 gen_build_deps.generate()
diff --git a/meta/cretonne/__init__.py b/meta/cretonne/__init__.py
index ee523cfec5..54f20cc678 100644
--- a/meta/cretonne/__init__.py
+++ b/meta/cretonne/__init__.py
@@ -18,6 +18,146 @@ def camel_case(s):
     return camel_re.sub(lambda m: m.group(2).upper(), s)
 
 
+class Setting(object):
+    """
+    A named setting variable that can be configured externally to Cretonne.
+
+    Settings are normally not named when they are created. They get their name
+    from the `extract_names` method.
+    """
+
+    def __init__(self, doc):
+        self.name = None  # Assigned later by `extract_names()`.
+        self.__doc__ = doc
+        # Offset of byte in settings vector containing this setting.
+        self.byte_offset = None
+        SettingGroup.append(self)
+
+    @staticmethod
+    def extract_names(globs):
+        """
+        Given a dict mapping name -> object as returned by `globals()`, find
+        all the Setting objects and set their name from the dict key. This is
+        used to name a bunch of global variables in a module.
+        """
+        for name, obj in globs.iteritems():
+            if isinstance(obj, Setting):
+                assert obj.name is None
+                obj.name = name
+
+
+class BoolSetting(Setting):
+    """
+    A named setting with a boolean on/off value.
+
+    :param doc: Documentation string.
+    :param default: The default value of this setting.
+    """
+
+    def __init__(self, doc, default=False):
+        super(BoolSetting, self).__init__(doc)
+        self.default = default
+
+    def default_byte(self):
+        """
+        Get the default value of this setting, as a byte that can be bitwise
+        or'ed with the other booleans sharing the same byte.
+        """
+        if self.default:
+            return 1 << self.bit_offset
+        else:
+            return 0
+
+
+class NumSetting(Setting):
+    """
+    A named setting with an integral value in the range 0--255.
+
+    :param doc: Documentation string.
+    :param default: The default value of this setting.
+    """
+
+    def __init__(self, doc, default=0):
+        super(NumSetting, self).__init__(doc)
+        assert default == int(default)
+        assert default >= 0 and default <= 255
+        self.default = default
+
+    def default_byte(self):
+        return self.default
+
+
+class EnumSetting(Setting):
+    """
+    A named setting with an enumerated set of possible values.
+
+    The default value is always the first enumerator.
+
+    :param doc: Documentation string.
+    :param args: Tuple of unique strings representing the possible values.
+    """
+
+    def __init__(self, doc, *args):
+        super(EnumSetting, self).__init__(doc)
+        assert len(args) > 0, "EnumSetting must have at least one value"
+        self.values = tuple(str(x) for x in args)
+        self.default = self.values[0]
+
+    def default_byte(self):
+        return 0
+
+
+class SettingGroup(object):
+    """
+    A group of settings.
+
+    Whenever a :class:`Setting` object is created, it is added to the currently
+    open group. A setting group must be closed explicitly before another can be
+    opened.
+
+    :param name: Short mnemonic name for setting group.
+    """
+
+    # The currently open setting group.
+    _current = None
+
+    def __init__(self, name):
+        self.name = name
+        self.settings = []
+        self.open()
+
+    def open(self):
+        """
+        Open this setting group such that future new settings are added to this
+        group.
+        """
+        assert SettingGroup._current is None, (
+                "Can't open {} since {} is already open"
+                .format(self, SettingGroup._current))
+        SettingGroup._current = self
+
+    def close(self, globs=None):
+        """
+        Close this setting group. This function must be called before opening
+        another setting group.
+
+        :param globs: Pass in `globals()` to run `extract_names` on all
+            settings defined in the module.
+        """
+        assert SettingGroup._current is self, (
+                "Can't close {}, the open setting group is {}"
+                .format(self, SettingGroup._current))
+        SettingGroup._current = None
+        if globs:
+            Setting.extract_names(globs)
+
+    @staticmethod
+    def append(setting):
+        assert SettingGroup._current, \
+                "Open a setting group before defining settings."
+        SettingGroup._current.settings.append(setting)
+
+
 # Kinds of operands.
 #
 # Each instruction has an opcode and a number of operands. The opcode
@@ -689,7 +829,7 @@ class BoundInstruction(object):
         assert len(typevars) <= 1 + len(inst.other_typevars)
 
     def __str__(self):
-        return '.'.join([self.inst.name,] + map(str, self.typevars))
+        return '.'.join([self.inst.name, ] + map(str, self.typevars))
 
     def bind(self, *args):
         """
@@ -711,7 +851,9 @@ class BoundInstruction(object):
         `(inst, typevars)` pair.
         """
         if len(self.typevars) < 1 + len(self.inst.other_typevars):
-            unb = ', '.join(str(tv) for tv in self.inst.other_typevars[len(self.typevars) - 1:])
+            unb = ', '.join(
+                    str(tv) for tv in
+                    self.inst.other_typevars[len(self.typevars) - 1:])
             raise AssertionError("Unbound typevar {} in {}".format(unb, self))
         assert len(self.typevars) == 1 + len(self.inst.other_typevars)
         return (self.inst, self.typevars)
diff --git a/meta/cretonne/settings.py b/meta/cretonne/settings.py
new file mode 100644
index 0000000000..f06c69bfef
--- /dev/null
+++ b/meta/cretonne/settings.py
@@ -0,0 +1,13 @@
+"""
+Cretonne shared settings.
+
+This module defines settings are are relevant for all code generators.
+"""
+
+from . import SettingGroup, BoolSetting
+
+group = SettingGroup('shared')
+
+enable_simd = BoolSetting("Enable the use of SIMD instructions", default=True)
+
+group.close(globals())
diff --git a/meta/gen_settings.py b/meta/gen_settings.py
new file mode 100644
index 0000000000..accd739cf0
--- /dev/null
+++ b/meta/gen_settings.py
@@ -0,0 +1,104 @@
+"""
+Generate sources with settings.
+"""
+
+import srcgen
+from cretonne import BoolSetting, NumSetting, settings
+
+
+def layout_group(sgrp):
+    """
+    Layout the settings in sgrp, assigning byte and bit offsets.
+
+    Return the next unused byte offset.
+    """
+    # Byte offset where booleans are allocated.
+    bool_byte = -1
+    # Next available bit number in bool_byte.
+    bool_bit = 10
+    # Next available whole byte.
+    next_byte = 0
+
+    for setting in sgrp.settings:
+        if isinstance(setting, BoolSetting):
+            # Allocate a bit from bool_byte.
+            if bool_bit > 7:
+                bool_byte = next_byte
+                next_byte += 1
+                bool_bit = 0
+            setting.byte_offset = bool_byte
+            setting.bit_offset = bool_bit
+            bool_bit += 1
+        else:
+            # This is a numerical or enumerated setting. Allocate a single
+            # byte.
+            setting.byte_offset = next_byte
+            next_byte += 1
+
+    return next_byte
+
+
+def gen_getter(setting, fmt):
+    """
+    Emit a getter function for `setting`.
+    """
+    fmt.doc_comment(setting.__doc__ + '.')
+
+    if isinstance(setting, BoolSetting):
+        proto = 'pub fn {}(&self) -> bool'.format(setting.name)
+        with fmt.indented(proto + ' {', '}'):
+            fmt.line('(self.bytes[{}] & (1 << {})) != 0'.format(
+                setting.byte_offset,
+                setting.bit_offset))
+    elif isinstance(setting, NumSetting):
+        proto = 'pub fn {}(&self) -> u8'.format(setting.name)
+        with fmt.indented(proto + ' {', '}'):
+            fmt.line('self.bytes[{}]'.format(setting.byte_offset))
+    else:
+        raise AssertionError("Unknown setting kind")
+
+
+def gen_getters(sgrp, fmt):
+    """
+    Emit getter functions for all the settings in fmt.
+    """
+    fmt.doc_comment("User-defined settings.")
+    with fmt.indented('impl Settings {', '}'):
+        for setting in sgrp.settings:
+            gen_getter(setting, fmt)
+
+
+def gen_default(sgrp, byte_size, fmt):
+    """
+    Emit a Default impl for Settings.
+    """
+    v = [0] * byte_size
+    for setting in sgrp.settings:
+        v[setting.byte_offset] |= setting.default_byte()
+
+    with fmt.indented('impl Default for Settings {', '}'):
+        fmt.doc_comment('Return a `Settings` object with default values.')
+        with fmt.indented('fn default() -> Settings {', '}'):
+            with fmt.indented('Settings {', '}'):
+                vs = ', '.join('{:#04x}'.format(x) for x in v)
+                fmt.line('bytes: [ {} ],'.format(vs))
+
+
+def gen_group(sgrp, fmt):
+    """
+    Generate a Settings struct representing `sgrp`.
+    """
+    byte_size = layout_group(sgrp)
+
+    fmt.doc_comment('Settings group `{}`.'.format(sgrp.name))
+    with fmt.indented('pub struct Settings {', '}'):
+        fmt.line('bytes: [u8; {}],'.format(byte_size))
+
+    gen_getters(sgrp, fmt)
+    gen_default(sgrp, byte_size, fmt)
+
+
+def generate(isas, out_dir):
+    fmt = srcgen.Formatter()
+    gen_group(settings.group, fmt)
+    fmt.update_file('settings.rs', out_dir)
diff --git a/src/libcretonne/lib.rs b/src/libcretonne/lib.rs
index 9d696fab53..766c33abcf 100644
--- a/src/libcretonne/lib.rs
+++ b/src/libcretonne/lib.rs
@@ -13,5 +13,6 @@ pub mod write;
 pub mod cfg;
 pub mod dominator_tree;
 pub mod entity_map;
+pub mod settings;
 
 #[cfg(test)]pub mod test_utils;
diff --git a/src/libcretonne/settings.rs b/src/libcretonne/settings.rs
new file mode 100644
index 0000000000..0b21bc2eb0
--- /dev/null
+++ b/src/libcretonne/settings.rs
@@ -0,0 +1,7 @@
+//! Shared settings module.
+//!
+//! This module defines data structures to access the settings defined in the meta language.
+
+// Include code generated by `meta/gen_settings.py`. This file contains a public `Settings` struct
+// with an impl for all of the settings defined in `meta/cretonne/settings.py`.
+include!(concat!(env!("OUT_DIR"), "/settings.rs"));