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From: "ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/98384] [11 Regression] new test case
 20_util/to_chars/long_double.cc in r11-6249 fails on powerpc64 BE
Date: Tue, 12 Jan 2021 16:35:22 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98384

--- Comment #8 from Patrick Palka <ppalka at gcc dot gnu.org> ---
(In reply to ro@CeBiTec.Uni-Bielefeld.DE from comment #7)
> > --- Comment #6 from Patrick Palka <ppalka at gcc dot gnu.org> ---
> [...]
> > Thanks for testing!  Hmm, that execute failure is surprising.  I wonder=
 just
> > how much we're diverging from the output of printf here.  If possible, =
could
> > you let me know the value of the locals 'to_chars_buffer', 'printf_buff=
er',
> > 'precision' and 'testcase' (ideally in hex form) at the point of the as=
sertion
> > failure?
>=20
> I'm seeing exactly the same failure mode on Solaris, both sparc and x86:
> 32-bit compilation PASSes, execution XFAILed, 64-bit compilation PASSes,
> too, but execution FAILs with the same assertion failure.
>=20
> Here's what gdb prints for those variables (with the test compiled with
> -g3 -O0 where the assertion still fails):
>=20
> (gdb) p to_chars_buffer
> $1 =3D "f.", 'f' <repeats 14 times>, "ep+5380", '\000' <repeats 1000 time=
s>
> (gdb) p printf_buffer
> $2 =3D "0x1.", 'f' <repeats 15 times>, "cp+5383", '\000' <repeats 997 tim=
es>
> (gdb) p precision
> No symbol "precision" in current context.
> (gdb) p testcase
> $3 =3D 5.56540347525605847154e+1620
> (gdb) p/x testcase
> $5 =3D 0x7fffffffffffffff

Thanks for this helpful info.=20

>>From what I can tell, the difference in output here is basically harmless. =
 The
two hexadecimal forms (in to_chars_buffer and printf_buffer) are equivalent,
though ours is one digit shorter.  Both hexadecimal forms are also valid
results of printf's %La specifier for this value.

Since the standard underspecifies the result of printf's %a specifier, and
since there's apparent implementation divergence, we shouldn't be verifying=
 the
hex output of std::to_chars by comparing it with that of printf.  Instead we
should compare with the known correct value, or perhaps verify that applying
std::from_chars on the output yields the original value.=