[Qemu-devel] [Bug 1841491] Re: floating point emulation can fail to set FE_UNDERFLOW

[Richard Henderson]

The float test failure is part of a larger problem for target/powerpc in
which all float routines are implemented incorrectly.  They are all
implemented as double operations with rounding to float as a second
step.  Which not only produces incorrect exceptions, as in this case,
but incorrect numerical results from the double rounding.

This should probably be split to a separate bug...

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

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https://bugs.launchpad.net/bugs/1841491

Title:
  floating point emulation can fail to set FE_UNDERFLOW

Status in QEMU:
  Confirmed

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

  This is similar to bug #1841442, but not the same problem, and I don't
  think the fix will be in the same code.

  On ppc64le native:
  --
  $ gcc -c -O2 fma.c
  $ gcc -O2 test-fma.c fma.o -lm -o test-fma
  $ ./test-fma $(./test-fma)
  fma(0x1.ffffffffffffcp-1022, 0x1.0000000000001p-1, 0x0.0000000000001p-1022)
  0x0

  0xa000000
  FE_INEXACT FE_UNDERFLOW 
  0x1p-1022
  --

  On qemu-system-ppc64:
  --
  $ ./test-fma $(./test-fma)
  fma(0x1.ffffffffffffcp-1022, 0x1.0000000000001p-1, 0x0.0000000000001p-1022)
  0x0

  0x2000000
  FE_INEXACT 
  0x1p-1022
  --

  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

  There are worse symptoms on qemu-x86_64, but this is apparently not
  surprising per
  https://bugs.launchpad.net/qemu/+bug/1841442/comments/6.

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