Add more expansion patterns.

RISC-V does not have a flags register, and thus no add-with-carry
instructions. Neither does MIPS.

Add expansions of these instructions in terms of iadd and icmp.

lib/cretonne/meta/cretonne/legalize.py

@@ -7,19 +7,43 @@ patterns that describe how base instructions can be transformed to other base
 instructions that are legal.
 """
 from __future__ import absolute_import
-from .base import iadd, iadd_cout, iadd_cin, isplit_lohi, iconcat_lohi
+from .base import iadd, iadd_cout, iadd_cin, iadd_carry
-from .base import isub, isub_bin, isub_bout
+from .base import isub, isub_bin, isub_bout, isub_borrow
+from .base import bor, isplit_lohi, iconcat_lohi
+from .base import icmp
 from .ast import Var
 from .xform import Rtl, XFormGroup
 
 
-narrow = XFormGroup()
+narrow = XFormGroup("""
+        Legalize instructions by narrowing.
+
+        The transformations in the 'narrow' group work by expressing
+        instructions in terms of smaller types. Operations on vector types are
+        expressed in terms of vector types with fewer lanes, and integer
+        operations are expressed in terms of smaller integer types.
+        """)
+
+expand = XFormGroup("""
+        Legalize instructions by expansion.
+
+        Rewrite instructions in terms of other instructions, generally
+        operating on the same types as the original instructions.
+        """)
 
 x = Var('x')
 y = Var('y')
 a = Var('a')
+a1 = Var('a1')
+a2 = Var('a2')
 b = Var('b')
+b1 = Var('b1')
+b2 = Var('b2')
+b_in = Var('b_in')
 c = Var('c')
+c1 = Var('c1')
+c2 = Var('c2')
+c_in = Var('c_in')
 xl = Var('xl')
 xh = Var('xh')
 yl = Var('yl')
@@ -46,3 +70,49 @@ narrow.legalize(
             ah << isub_bin(xh, yh, b),
             a << iconcat_lohi(al, ah)
         ))
+
+# Expand integer operations with carry for RISC architectures that don't have
+# the flags.
+expand.legalize(
+        (a, c) << iadd_cout(x, y),
+        Rtl(
+            a << iadd(x, y),
+            c << icmp('ult', a, x)
+        ))
+
+expand.legalize(
+        (a, b) << isub_bout(x, y),
+        Rtl(
+            a << isub(x, y),
+            b << icmp('ugt', a, x)
+        ))
+
+expand.legalize(
+        a << iadd_cin(x, y, c),
+        Rtl(
+            a1 << iadd(x, y),
+            a << iadd(a1, c)
+        ))
+
+expand.legalize(
+        a << isub_bin(x, y, b),
+        Rtl(
+            a1 << isub(x, y),
+            a << isub(a1, b)
+        ))
+
+expand.legalize(
+        (a, c) << iadd_carry(x, y, c_in),
+        Rtl(
+            (a1, c1) << iadd_cout(x, y),
+            (a, c2) << iadd_cout(a1, c_in),
+            c << bor(c1, c2)
+        ))
+
+expand.legalize(
+        (a, b) << isub_borrow(x, y, b_in),
+        Rtl(
+            (a1, b1) << isub_bout(x, y),
+            (a, b2) << isub_bout(a1, b_in),
+            c << bor(c1, c2)
+        ))

lib/cretonne/meta/cretonne/xform.py

@@ -169,8 +169,9 @@ class XFormGroup(object):
     A group of related transformations.
     """
 
-    def __init__(self):
+    def __init__(self, doc):
         self.xforms = list()
+        self.__doc__ = doc
 
     def legalize(self, src, dst):
         """