fix SIGNEXTEND opcode

nimbus/vm/interpreter/opcodes_impl.nim

@@ -121,10 +121,11 @@ op signExtend, inline = false, bits, value:
   var res: UInt256
   if bits <= 31.u256:
     let
+      one = 1.u256
       testBit = bits.toInt * 8 + 7
-      bitPos = (1 shl testBit)
-      mask = u256(bitPos - 1)
-    if not isZero(value and bitPos.u256):
+      bitPos = one shl testBit
+      mask = bitPos - one
+    if not isZero(value and bitPos):
       res = value or (not mask)
     else:
       res = value and mask

tests/test_op_arith.nim

@@ -341,3 +341,12 @@ suite "Arithmetic Opcodes":
       SMOD
     success: false
     stack: "0x000000000000000000000000000000000000000000000000000000000000001E"
+
+  # real case, EVM bug, integer over flow
+  assembler: # SIGNEXTEND OP
+    title: "SIGNEXTEND_1"
+    code:
+      PUSH32 "0x000000000000000000000000000000003f9b347132d29b62d161117bca8c7307"
+      PUSH1 "0x0F"
+      SIGNEXTEND
+    stack: "0x000000000000000000000000000000003f9b347132d29b62d161117bca8c7307"

tests/test_op_env.nim

@@ -280,8 +280,6 @@ suite "Environmental Information Opcodes":
       "0x5E"
       "0x07"
 
-  # macro_assembler automatically insert STOP instruction at the end,
-  # so codesize become 63, not 62 like ethereumj
   assembler: # CODESIZE OP
     title: "CODESIZE_1"
     code:
@@ -289,7 +287,7 @@ suite "Environmental Information Opcodes":
       "0x602001600a5254516040016014525451606001601e5254516080016028525460"
       "0xa052546016604860003960166000f26000603f556103e75660005460005360200235"
     stack:
-      "0x0000000000000000000000000000000000000000000000000000000000000063"
+      "0x0000000000000000000000000000000000000000000000000000000000000062"
     success: false
 
   # 0x94 == 148 bytes