Support for pseudo-instructions.

[Pawan Kumar Sanjaya]

Problem

All coverpoints are currently defined over the opcode field in the cgf. Since isac deals with the mnemonics of the instruction rather than the actual encoding, dealing with pseudo-instructions needs additional thought. The same instruction instance of the base op should satisfy/trigger the coverpoints of both the base op and the pseudo instruction for accurate coverage reporting.

Potential Solution

To support this, the nodes in the cgf will have to undergo a overhaul. The opcode field can be renamed to mnemonics and 2 new fields base_op and p_op_cond are added to the covergroups. The base_op and p_op_cond are optional fields which specify the base operation and the condition over the different fields of the instruction, which when satisfied results in the instruction being recognised as the pseudo op. For example, zext.h is a pseudo op of pack in RV64 and packw in RV32 with the rs2 field hardwired to x0. The nodes for zext.h will be expressed as follows:

zext.h_32:
    config: 
      - check ISA:=regex(.*RV32.*B.*)
      - check ISA:=regex(.*RV32.*Zbb.*)
    mnemonics: 
      zext.h: 0
    base_op: packw
    p_op_cond: rs2 == x0
    ...

zext.h_64:
    config: 
      - check ISA:=regex(.*RV64.*B.*)
      - check ISA:=regex(.*RV64.*Zbb.*)
    mnemonics: 
      zext.h: 0
    base_op: pack
    p_op_cond: rs2 == x0
    ...

Schema

• The p_op_cond node is relevant only if the base_op node has been defined. Otherwise both the nodes remain empty.
• The mnemonics node is allowed to have multiple entries only if the base_op node is empty. This introduces a restriction that a pseudo-op cannot be combined with other instructions into a single covergroup.

Changes required

• This change will need an overhaul of all the cgfs defined so far( a simple replace of opcode with mnemonics).
• Changes in ISAC and CTG to support this.

Caveats

• Pseudo-ops cannot be combined with other instructions in a covergroup
• Decoder always detects an encoding as the relevant base-op(i.e pack even if the encoding satisfies conditions for zext.h).
• Coverpoints are defined over the fields of the base_op always.

[Pawan Kumar Sanjaya]

In the interest of backward compatibility, the opcode node should also be supported in the cgfs(with a deprecation warning). Possibly as a part of the normalise function, so that the older cgfs can still be used with isac.