[Bug 1888165] [NEW] loopz/loopnz clearing previous instruction's modified flags on cx -> 0

[Jeffrey]

Public bug reported:

If you run QBasic in qemu, printing a double-type single-digit number
will print an extra decimal point (e.g. PRINT CDBL(3) prints "3.") that
does not appear when running on a real CPU (or on qemu with -enable-
kvm). I tracked this down to the state of the status flags after a
loopnz instruction.

After executing a sequence like this in qemu:

        mov bx,1
        mov cx,1
        dec bx    ; sets Z bit in flags
A:      loopnz A  ; should not modify flags

Z is incorrectly clear afterwards. loopz does the same thing (but not
plain loop). Interestingly, inserting pushf+popf after dec results in Z
set, so loopnz/loopz does not always clear Z itself but is rather
interfering with the previous instruction's flag setting.

Version 5.1.0-rc0, x86-64 host.

** Affects: qemu
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1888165

Title:
  loopz/loopnz clearing previous instruction's modified flags on cx -> 0

Status in QEMU:
  New

Bug description:
  If you run QBasic in qemu, printing a double-type single-digit number
  will print an extra decimal point (e.g. PRINT CDBL(3) prints "3.")
  that does not appear when running on a real CPU (or on qemu with
  -enable-kvm). I tracked this down to the state of the status flags
  after a loopnz instruction.

  After executing a sequence like this in qemu:

        mov bx,1
        mov cx,1
        dec bx    ; sets Z bit in flags
  A:    loopnz A  ; should not modify flags

  Z is incorrectly clear afterwards. loopz does the same thing (but not
  plain loop). Interestingly, inserting pushf+popf after dec results in
  Z set, so loopnz/loopz does not always clear Z itself but is rather
  interfering with the previous instruction's flag setting.

  Version 5.1.0-rc0, x86-64 host.

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