Fix: c.addi4spn with imm=0 and rd'!=0 should be reserved

[ngc7331]

Related issue: not applicable

Type of change: bug report

Impact: API modification

Development Phase: implementation

Release Notes

According to unprivileged specification section 26.5.2:

C.ADDI4SPN is a CIW-format instruction that adds a zero-extended non-zero immediate, scaled by
4, to the stack pointer, x2, and writes the result to rd′. This instruction is used to generate pointers to
stack-allocated variables, and expands to addi rd′, x2, nzuimm[9:2]. C.ADDI4SPN is only valid
when nzuimm≠0; the code points with nzuimm=0 are reserved.

C.ADDI4SPN with imm=0 (i.e. x(12,5) === 0.U) should be reserved (illegal for now), whether or not rd'=0 (i.e. x(4,2) === 0.U)

[linux-foundation-easycla]

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