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floating point emulation can fail to set FE_UNDERFLOW
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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