[Qemu-devel] [Bug 1841442] Re: floating point emulation can fail to set FE_INEXACT

[Richard Henderson]

Patch for ppc:
https://lists.gnu.org/archive/html/qemu-devel/2019-08/msg05532.html

Fixing x86_64 is significantly harder, as support for fp exceptions
is completely lacking in the code currently.

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https://bugs.launchpad.net/bugs/1841442

Title:
  floating point emulation can fail to set FE_INEXACT

Status in QEMU:
  Confirmed

Bug description:
  Floating point emulation can fail to set FE_INEXACT in some
  circumstances. This shows up quite often in glibc's "math" tests.  A
  similar test is attached.

  On ppc64le native:
  --
  $ gcc nextafter.c -o nextafter -lm
  $ ./nextafter $(./nextafter)
  0x0000000000000001 0.000000
  0x0

  0xa000000
  FE_INEXACT FE_UNDERFLOW
  0x0000000000000000 0.000000
  --

  On x86_64:
  --
  $ gcc nextafter.c -o nextafter -lm
  $ ./nextafter $(./nextafter)
  0x0000000000000001 0.000000
  0x0

  0x30
  FE_INEXACT FE_UNDERFLOW 
  0x0000000000000000 0.000000
  --

  Using qemu-system-ppc64
  --
  $ ./nextafter $(./nextafter)
  0x0000000000000001 0.000000
  0x0

  0x8000000
  FE_UNDERFLOW 
  0x0000000000000000 0.000000
  --

  Using qemu-x86_64:
  --
  $ ./nextafter $(./nextafter)
  0x0000000000000001 0.000000
  0x0

  0x0

  0x0000000000000000 0.000000
  --

  QEMU versions vary, but not too much, and are pretty close to git HEAD:
  - 586f3dced9 (HEAD -> master, origin/master, origin/HEAD) Merge 
remote-tracking branch 'remotes/cohuck/tags/s390x-20190822' into staging
  - 864ab31 Update version for v4.1.0-rc4 release

  Since the issue happens nearly identically on different targets, I
  suspect the issue lies somewhere in fpu/softfloat.c.

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