An equivalence check for SIGNEXTEND opcode

Checking the implementation of SIGNEXTEND using Z3's native SignExt and Extract

test/formal/signextend_equivalence.py

@@ -0,0 +1,24 @@
+from rule import Rule
+from opcodes import *
+
+"""
+Checking the implementation of SIGNEXTEND using Z3's native SignExt and Extract
+"""
+
+rule = Rule()
+n_bits = 256
+
+x = BitVec('X', n_bits)
+
+def SIGNEXTEND_native(i, x):
+    return SignExt(256 - 8 * i - 8, Extract(8 * i + 7, 0, x))
+
+for i in range(0, 32):
+    rule.check(
+        SIGNEXTEND(BitVecVal(i, n_bits), x),
+        SIGNEXTEND_native(i, x)
+    )
+
+i = BitVec('I', n_bits)
+rule.require(UGT(i, BitVecVal(31, n_bits)))
+rule.check(SIGNEXTEND(i, x), x)