[Bug 1820686] Re: risc-v: 'c.unimp' instruction decoded as 'c.addi4spn fp, 0'

[Peter Maydell]

Since this bug isn't present in the decodetree version of the riscv
decoder, we might as well just close this as fix-released; we won't be
doing more point-releases of QEMU versions as old as 3.1.

** Changed in: qemu
       Status: New => Fix Released

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https://bugs.launchpad.net/bugs/1820686

Title:
  risc-v: 'c.unimp' instruction decoded as 'c.addi4spn fp, 0'

Status in QEMU:
  Fix Released

Bug description:
  QEMU 3.1 incorrectly decodes the "c.unimp" instruction (opcode 0x0000)
  as an "addi4spn fp, 0" when either of the two following bytes are non-
  zero. This is because the ctx->opcode value used when decoding the
  instruction is actually filled with a 32-bit load (to handle normal
  uncompressed instructions) but when a compressed instruction is found
  only the low 16 bits are valid. Other reserved/illegal bit patterns
  with the addi4spn opcode are also incorrectly decoded.

  I believe that the switch to decodetree on master happened to fix this
  issue, but hopefully it is helpful to have this recorded somewhere.
  I've included a simple one line patch if anyone wants to backport
  this.

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