[Bug 1841990] Re: instruction 'denbcdq' misbehaving

[Mark Cave-Ayland]

Great! It looks as if I can't add Stefan to the bug report in launchpad
since he isn't registered there, so I'll send a quick email to qemu-
devel and add him as CC.

In the meantime whilst your test setup is working and everything is
fresh, I'll have a quick go at switching the BCD_DIG_BYTE bits over to
use the VsrX() macros to abstract out more host endian behaviour...

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https://bugs.launchpad.net/bugs/1841990

Title:
  instruction 'denbcdq' misbehaving

Status in QEMU:
  New

Bug description:
  Instruction 'denbcdq' appears to have no effect.  Test case attached.

  On ppc64le native:
  --
  gcc -g -O -mcpu=power9 bcdcfsq.c test-denbcdq.c -o test-denbcdq
  $ ./test-denbcdq
  0x00000000000000000000000000000000
  0x0000000000000000000000000000000c
  0x22080000000000000000000000000000
  $ ./test-denbcdq 1
  0x00000000000000000000000000000001
  0x0000000000000000000000000000001c
  0x22080000000000000000000000000001
  $ ./test-denbcdq $(seq 0 99)
  0x00000000000000000000000000000064
  0x0000000000000000000000000000100c
  0x22080000000000000000000000000080
  --

  With "qemu-ppc64le -cpu power9"
  --
  $ qemu-ppc64le -cpu power9 -L [...] ./test-denbcdq
  0x00000000000000000000000000000000
  0x0000000000000000000000000000000c
  0x0000000000000000000000000000000c
  $ qemu-ppc64le -cpu power9 -L [...] ./test-denbcdq 1
  0x00000000000000000000000000000001
  0x0000000000000000000000000000001c
  0x0000000000000000000000000000001c
  $ qemu-ppc64le -cpu power9 -L [...] ./test-denbcdq $(seq 100)
  0x00000000000000000000000000000064
  0x0000000000000000000000000000100c
  0x0000000000000000000000000000100c
  --

  I started looking at the code, but I got confused rather quickly.
  Could be related to endianness? I think denbcdq arrived on the scene
  before little-endian was a big deal.  Maybe something to do with
  utilizing implicit floating-point register pairs...  I don't think the
  right data is getting to helper_denbcdq, which would point back to the
  gen_fprp_ptr uses in dfp-impl.inc.c (GEN_DFP_T_FPR_I32_Rc).  (Maybe?)

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