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

[Paul Clarke]

Public bug reported:

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.

** Affects: qemu
     Importance: Undecided
         Status: New

** Attachment added: "testcase reporting exceptions set by simple floating 
point multiply-add"
   https://bugs.launchpad.net/bugs/1841491/+attachment/5284808/+files/test-fma.c

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

Title:
  floating point emulation can fail to set FE_UNDERFLOW

Status in QEMU:
  New

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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