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[Bug 1912934] [NEW] QEMU emulation of fmadds instruction on powerpc64le


From: Bruno Haible
Subject: [Bug 1912934] [NEW] QEMU emulation of fmadds instruction on powerpc64le is buggy
Date: Sun, 24 Jan 2021 08:40:41 -0000

Public bug reported:

The attached program test-fmadds.c tests the fmadds instruction on
powerpc64le.

Result on real hardware (POWER8E processor):
$ ./a.out ; echo $?
0

Result in Alpine Linux 3.13/powerpcle, emulated by QEMU 5.0.0 on Ubuntu 16.04:
$ ./a.out ; echo $?
32

Result in Debian 8.6.0/ppc64el, emulated by QEMU 2.9.0 on Ubuntu 16.04:
$ ./a.out ; echo $?
32

Through 'nm --dynamic qemu-system-ppc64 | grep fma' I can see that QEMU
is NOT using the fmaf() or fma() function from the host system's libc;
this function is working fine in glibc of the host system (see
https://www.gnu.org/software/gnulib/manual/html_node/fmaf.html ).

** Affects: qemu
     Importance: Undecided
         Status: New

** Attachment added: "Test case"
   
https://bugs.launchpad.net/bugs/1912934/+attachment/5456225/+files/test-fmadds.c

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https://bugs.launchpad.net/bugs/1912934

Title:
  QEMU emulation of fmadds instruction on powerpc64le is buggy

Status in QEMU:
  New

Bug description:
  The attached program test-fmadds.c tests the fmadds instruction on
  powerpc64le.

  Result on real hardware (POWER8E processor):
  $ ./a.out ; echo $?
  0

  Result in Alpine Linux 3.13/powerpcle, emulated by QEMU 5.0.0 on Ubuntu 16.04:
  $ ./a.out ; echo $?
  32

  Result in Debian 8.6.0/ppc64el, emulated by QEMU 2.9.0 on Ubuntu 16.04:
  $ ./a.out ; echo $?
  32

  Through 'nm --dynamic qemu-system-ppc64 | grep fma' I can see that
  QEMU is NOT using the fmaf() or fma() function from the host system's
  libc; this function is working fine in glibc of the host system (see
  https://www.gnu.org/software/gnulib/manual/html_node/fmaf.html ).

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