[Qemu-devel] [Bug 1841990] Re: instruction 'denbcdq' misbehaving

[Philippe Mathieu-Daudé]

I tried to compile your test program with 2 different GCC versions but
it keeps failing, do you need a special/recent version? Meanwhile can
you attach a statically linked binary?

$ gcc -v
gcc version 6.3.0 20170516 (Debian 6.3.0-18) 

$ gcc -g -O -mcpu=power9 test-denbcdq.c -o test-denbcdq
test-denbcdq.c: In function 'bcdcfsq':
test-denbcdq.c:7:2: error: impossible register constraint in 'asm'
  asm volatile ( "bcdcfsq. %0,%1,0" : "=v" (r) : "v" (i128) );
  ^~~

--

$ gcc version 8.1.1 20180626 (Red Hat Cross 8.1.1-3) (GCC)

$ gcc -g -O -mcpu=power9 test-denbcdq.c -o test-denbcdq
test-denbcdq.c: In function ‘main’:
test-denbcdq.c:15:3: error: decimal floating point not supported for this target
   _Decimal128 d128;
   ^~~~~~~~~~~

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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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