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

[Richard Henderson]

> Interesting. Did you run qemu-aarch64 on aarch64? If so, it may have been
> using "hardfloat". I ran "qemu-system-ppc64" on x86_64 and "qemu-x86_64"
> on ppc64le to ensure I was using "softfloat".

That's not how that works.  Indeed, the point of the hardfloat path is to
accelerate fpu emulation for a non-native guest.

That said, qemu-system-ppc64 will *never* use hardfloat, because ppc always
need the current and correct result of inexact for emulation of the FI bit,
which requires that we use the softfloat path.

Also, use of the hardfloat path requires normal inputs.  For this test case
we're passing the minimum denormal input: 0x0000_0000_0000_0001.  So we will
always use the softfloat path for this.


** Changed in: qemu
       Status: New => Confirmed

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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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