[RISCV] Incorrect disassembly of c.lui when imm=0 #133446
Description
context:
During disassembly, a binary can contain an invalid instruction and objdump tells that this instruction is unknown.
For example, Bytecode 0x6101 represents instruction c.addi16sp x2, 0 (where c.addi16sp shares the opcode with c.lui, but has a destination field of x2) and c.addi16sp is only valid when the immediate is not equal to zero.
If I try to disassemble a binary with Bytecode 0x6101, I get:
$ cat disasm.s
.insn 0x6101 # c.addi16sp x2, 0
$ clang -c --target=riscv32 -march=rv32imafdc disasm.s -o disasm.o
$ llvm-objdump -M no-aliases -d disasm.o
disasm.o: file format elf32-littleriscv
Disassembly of section .text:
00000000 <.text>:
0: 6101 <unknown>
description of the problem:
According to RISC-V Instruction Set Manual, instruction c.lui rd, imm is only valid when rd != {x0, x2}, and when the immediate is not equal to zero.
As a consequence, the following instruction is invalid:
$ cat compile.s
c.lui x1, 0
llvm verifies this before compilation:
$ clang -c --target=riscv32 -march=rv32imafdc compile.s -o compile.o
compile.s:1:11: error: immediate must be in [0xfffe0, 0xfffff] or [1, 31]
c.lui x1, 0
^
But, if a binary contains such invalid instruction, objdump says that this is valid:
$ cat problem.s
.insn 0x6081 # c.lui x1, 0
$ clang -c --target=riscv32 -march=rv32imafdc problem.s -o problem.o
$ llvm-objdump -M no-aliases -d problem.o
problem.o: file format elf32-littleriscv
Disassembly of section .text:
00000000 <.text>:
0: 6081 c.lui ra, 0x0
The problem occurs with the 31 following Bytecodes:
- 0x6001 is disassembled to
c.lui zero, 0x0 - 0x6081 is disassembled to
c.lui ra, 0x0 - 0x6181 is disassembled to
c.lui gp, 0x0 - 0x6201 is disassembled to
c.lui tp, 0x0 - 0x6281 is disassembled to
c.lui t0, 0x0 - 0x6301 is disassembled to
c.lui t1, 0x0 - 0x6381 is disassembled to
c.lui t2, 0x0 - 0x6401 is disassembled to
c.lui s0, 0x0 - 0x6481 is disassembled to
c.lui s1, 0x0 - 0x6501 is disassembled to
c.lui a0, 0x0 - 0x6581 is disassembled to
c.lui a1, 0x0 - 0x6601 is disassembled to
c.lui a2, 0x0 - 0x6681 is disassembled to
c.lui a3, 0x0 - 0x6701 is disassembled to
c.lui a4, 0x0 - 0x6781 is disassembled to
c.lui a5, 0x0 - 0x6801 is disassembled to
c.lui a6, 0x0 - 0x6881 is disassembled to
c.lui a7, 0x0 - 0x6901 is disassembled to
c.lui s2, 0x0 - 0x6981 is disassembled to
c.lui s3, 0x0 - 0x6a01 is disassembled to
c.lui s4, 0x0 - 0x6a81 is disassembled to
c.lui s5, 0x0 - 0x6b01 is disassembled to
c.lui s6, 0x0 - 0x6b81 is disassembled to
c.lui s7, 0x0 - 0x6c01 is disassembled to
c.lui s8, 0x0 - 0x6c81 is disassembled to
c.lui s9, 0x0 - 0x6d01 is disassembled to
c.lui s10, 0x0 - 0x6d81 is disassembled to
c.lui s11, 0x0 - 0x6e01 is disassembled to
c.lui t3, 0x0 - 0x6e81 is disassembled to
c.lui t4, 0x0 - 0x6f01 is disassembled to
c.lui t5, 0x0 - 0x6f81 is disassembled to
c.lui t6, 0x0
expected result:
Since these 31 Bytecodes represent invalid instructions, objdump should say that the resulting instructions are unknown.