[BUG] : Decoder Bitmanip instructions

Created by: AyoubJalali

Is there an existing CVA6 bug for this?

• I have searched the existing bug issues

Bug Description

Hello, this is a RTL bug in the cva6 Decoder, the problem appears in this code

there's 2 problems here :

1. there's no condition for [24:23] bits, according to the spec these should be always 0.
2. there's no else for illegal instruction if we didn't enter any if.

so if we try using a directed test to execute an ctz instruction with [24-23] != 0 , the RTL will execute it, that's the wrong behavior !!