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* [Bug target/59666] New: IBM long double arithmetic results invalid in non-default rounding modes
@ 2014-01-03 16:19 jsm28 at gcc dot gnu.org
2023-05-01 18:06 ` [Bug target/59666] " vital.had at gmail dot com
2023-05-31 6:05 ` egallager at gcc dot gnu.org
0 siblings, 2 replies; 3+ messages in thread
From: jsm28 at gcc dot gnu.org @ 2014-01-03 16:19 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59666
Bug ID: 59666
Summary: IBM long double arithmetic results invalid in
non-default rounding modes
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: jsm28 at gcc dot gnu.org
CC: dje at gcc dot gnu.org
Target: powerpc*-*-linux*
The IBM long double functions in ibm-ldouble.c, when called in rounding modes
other than round-to-nearest, can produce results that are invalid (do not
satisfy the requirements in ibm-ldouble-format on what pairs of doubles make a
valid long double value, in particular as regards the high part being equal to
the sum of the two parts rounded to nearest). For example:
#include <fenv.h>
#include <float.h>
#include <stdio.h>
volatile long double a = LDBL_MAX;
int
main (void)
{
fesetround (FE_TOWARDZERO);
union u { long double ld; double d[2]; } x;
volatile long double r = a * a;
x.ld = a;
printf ("LDBL_MAX: %a %a\n", x.d[0], x.d[1]);
x.ld = r;
printf ("LDBL_MAX * LDBL_MAX: %a %a\n", x.d[0], x.d[1]);
return 0;
}
prints
LDBL_MAX: 0x1.fffffffffffffp+1023 0x1.ffffffffffffep+969
LDBL_MAX * LDBL_MAX: 0x1.fffffffffffffp+1023 0x1.fffffffffffffp+1023
where the value for LDBL_MAX * LDBL_MAX is not a valid long double at all.
(This isn't limited to overflow cases, although they may produce the greatest
errors; e.g. 0x1.fffffffffffffp-1L * 0x1.fffffffffffffp-1L in FE_UPWARD mode
produces (0x1.fffffffffffffp-1, -0x1.fffffffffffffp-54), where the high part is
not the sum of the two parts rounded to nearest.)
ISO C does not allow for arithmetic operations simply not working - producing
invalid results - for some types and rounding modes, although for non-IEEE
types they need not be correctly rounding.
I think the right approach for a fix will probably involve setting
round-to-nearest temporarily within the functions, then adjusting overflowing
and underflowing results based on the original rounding mode. I don't know
what performance impact that might have, and whether it might be best to avoid
that performance impact in the common case by having separate __gcc_*_round
functions that deal with saving / restoring the rounding mode and are only used
if -frounding-math, with the existing functions (not handling rounding modes)
being used otherwise.
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug target/59666] IBM long double arithmetic results invalid in non-default rounding modes
2014-01-03 16:19 [Bug target/59666] New: IBM long double arithmetic results invalid in non-default rounding modes jsm28 at gcc dot gnu.org
@ 2023-05-01 18:06 ` vital.had at gmail dot com
2023-05-31 6:05 ` egallager at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: vital.had at gmail dot com @ 2023-05-01 18:06 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59666
Sergey Fedorov <vital.had at gmail dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |vital.had at gmail dot com
--- Comment #8 from Sergey Fedorov <vital.had at gmail dot com> ---
(In reply to Vincent Lefèvre from comment #1)
> (In reply to Joseph S. Myers from comment #0)
> It seems to be like that "by design" (though this is not satisfactory) and
> part of the ppc64 ABI for instance:
>
> http://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi.html#PREC
>
> "The software support is restricted to round-to-nearest mode. Programs that
> use extended precision must ensure that this rounding mode is in effect when
> extended-precision calculations are performed."
Also true for AIX:
https://www.ibm.com/docs/sr/xcafbg/9.0.0?topic=SS3KZ4_9.0.0/com.ibm.xlf111.bg.doc/xlfopg/fp-overview.html
Does anyone know whether this is also true for Darwin on PowerPC though?
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug target/59666] IBM long double arithmetic results invalid in non-default rounding modes
2014-01-03 16:19 [Bug target/59666] New: IBM long double arithmetic results invalid in non-default rounding modes jsm28 at gcc dot gnu.org
2023-05-01 18:06 ` [Bug target/59666] " vital.had at gmail dot com
@ 2023-05-31 6:05 ` egallager at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: egallager at gcc dot gnu.org @ 2023-05-31 6:05 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59666
Eric Gallager <egallager at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |egallager at gcc dot gnu.org,
| |iains at gcc dot gnu.org
--- Comment #9 from Eric Gallager <egallager at gcc dot gnu.org> ---
(In reply to Sergey Fedorov from comment #8)
> (In reply to Vincent Lefèvre from comment #1)
> > (In reply to Joseph S. Myers from comment #0)
> > It seems to be like that "by design" (though this is not satisfactory) and
> > part of the ppc64 ABI for instance:
> >
> > http://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi.html#PREC
> >
> > "The software support is restricted to round-to-nearest mode. Programs that
> > use extended precision must ensure that this rounding mode is in effect when
> > extended-precision calculations are performed."
>
> Also true for AIX:
> https://www.ibm.com/docs/sr/xcafbg/9.0.0?topic=SS3KZ4_9.0.0/com.ibm.xlf111.
> bg.doc/xlfopg/fp-overview.html
>
> Does anyone know whether this is also true for Darwin on PowerPC though?
I don't, but Iain might...
^ permalink raw reply [flat|nested] 3+ messages in thread
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2014-01-03 16:19 [Bug target/59666] New: IBM long double arithmetic results invalid in non-default rounding modes jsm28 at gcc dot gnu.org
2023-05-01 18:06 ` [Bug target/59666] " vital.had at gmail dot com
2023-05-31 6:05 ` egallager at gcc dot gnu.org
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