From: lkcl <lkcl@web>
Date: Sun, 6 Mar 2022 07:52:27 +0000 (+0000)
Subject: (no commit message)
X-Git-Tag: opf_rfc_ls005_v1~3149
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=bbbd1296e042ffbfd4cb94e4749f106d25d2ac4a;p=libreriscv.git

---

diff --git a/openpower/sv/bitmanip.mdwn b/openpower/sv/bitmanip.mdwn
index 80bfeeccb..b7a54c7ff 100644
--- a/openpower/sv/bitmanip.mdwn
+++ b/openpower/sv/bitmanip.mdwn
@@ -562,14 +562,24 @@ where the SimpleV variant may override RS-as-src differently from RS-as-dest
 ```
 from functools import reduce
 
+def gf_degree(a) :
+  res = 0
+  a >>= 1
+  while (a != 0) :
+    a >>= 1;
+    res += 1;
+  return res
+
 # constants used in the multGF2 function
 mask1 = mask2 = polyred = None
 
-def setGF2(degree, irPoly):
+def setGF2(irPoly):
     """Define parameters of binary finite field GF(2^m)/g(x)
-       - degree: extension degree of binary field
        - irPoly: coefficients of irreducible polynomial g(x)
     """
+    # degree: extension degree of binary field
+    degree = gf_degree(irPoly)
+
     def i2P(sInt):
         """Convert an integer into a polynomial"""
         return [(sInt >> i) & 1
@@ -595,14 +605,14 @@ def multGF2(p1, p2):
 if __name__ == "__main__":
   
     # Define binary field GF(2^3)/x^3 + x + 1
-    setGF2(3, 0b1011)
+    setGF2(0b1011) # degree 3
 
     # Evaluate the product (x^2 + x + 1)(x^2 + 1)
     print("{:02x}".format(multGF2(0b111, 0b101)))
     
     # Define binary field GF(2^8)/x^8 + x^4 + x^3 + x + 1
     # (used in the Advanced Encryption Standard-AES)
-    setGF2(8, 0b100011011)
+    setGF2(0b100011011) # degree 8
     
     # Evaluate the product (x^7)(x^7 + x + 1)
     print("{:02x}".format(multGF2(0b10000000, 0b10000011)))