Reduces LUT needed by mul extension

bench/tests/op/mul.s

@@ -1,22 +1,54 @@
-# expected: 0000000000000000, 00000000000dead0, c1b080e900, -dead0, 54a029
     .text
 _start:
-    li x2, 0xdead0
-    li x3, 0x0
-    li x4, 0x1
-    li x5, (-1)
-    li x6, 0xdead0
-    li x7, (-2355)
+    li x2,  0xdead0        # +912080
+    li x3,  0              # zero
+    li x4,  1              # one
+    li x5, -1              # -1
+    li x6,  0xdead0        # duplicate of x2
+    li x7, -2355           # negative
+    li x8,  123456         # positive
+    li x9, -765432         # negative
+    li x10, 0x7fffffffffffffff  # max positive 64b
+    li x11, 0x8000000000000000  # min negative 64b
 
-    # Mul 0
+    # --------------------------------------------------------------
+    # 1. MUL : a * 0
     mul x31, x2, x3
+    # expected: 0000000000000000
 
-    # Basic multiplications
+    # 2. MUL : a * 1
     mul x31, x2, x4
+    # expected: 00000000000dead0
+
+    # 3. MUL : a * a
     mul x31, x2, x6
+    # expected: 0000000c1b080e900
 
-    # Overflow
+    # 4. MUL : -1 * a
     mul x31, x5, x2
+    # expected: -00000000000dead0
 
-    # Signed * signed
+    # 5. MUL : negative * negative
     mul x31, x7, x7
+    # expected: 000000000054a029
+
+    # 6. MUL : negative * positive
+    mul x31, x7, x8
+    # expected: -00000000115452c0
+
+    # 7. MUL : positive * negative
+    mul x31, x8, x9
+    # expected: -0000001600786e00
+
+    # 8. MUL : overflow example (max * 2)
+    li x12, 2
+    mul x31, x10, x12
+    # expected: fffffffffffffffe
+
+    # 9. MUL : max * max
+    mul x31, x10, x10
+    # expected: 0000000000000001
+
+    # 10. MUL : min * min
+    mul x31, x11, x11
+    # expected: 0000000000000000