The c.lui rd=x0 HINT code point is rejected as a reserved encoding

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
88/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
rust
Domain
compilers

Research direction

Start at the decode_q1 entry point and inspect the existing compressed-instruction tests, including the JIT boundary test. Verify that c.lui with rd=x0 and nzimm!=0 executes as a no-op, that a nonzero rd remains valid, and that nzimm==0 still traps as reserved.

Written by the indexing model from the issue text.

Description

Summary

The compressed c.lui code point with rd=x0 and nzimm!=0 is a HINT per the Zca specification (insns-c_lui: "The code points with rd=x0 and imm!=0 are HINTs"); HINTs execute as no-ops and must never trap. RAX's decode_q1 rejects the encoding as reserved (if v == 0 || rd == 0), so the legal HINT c.lui x0, 1 (0x6005) raises an illegal-instruction exception. Only the nzimm==0 code point is reserved.

Steps to reproduce

let mut c = cpu64();
let r = exec_rvc(&mut c, 0x6005, Xlen::Rv64, &Isa::rv64gc()); // c.lui x0, 1
assert!(r.is_err()); // defect: legal HINT rejected as reserved

Expected result

c.lui x0, imm (imm!=0) decodes as lui x0, imm, a no-op that executes normally; c.lui x0, 0 remains reserved.

Observed result

The HINT code point traps as an illegal instruction.

Environment

RAX master 7f162b19aeaa47825c82fb1c77ca042f535124e3 (checked 2026-08-23).

Suggested fix

Keep only the nzimm==0 reserved check in decode_q1 and let rd=x0 with nzimm!=0 decode as a normal (no-op) lui. A regression test covers the HINT decoding, a rd!=0 control, and the nzimm==0 reserved case; the JIT boundary test drops the now-legal C.LUI rd=x0 case.

I searched the issue tracker and found no existing report for c.lui rd=x0 HINT being rejected.

Dominant language
Rust
Stars
218
Forks
10
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