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Author SHA1 Message Date
Joseph Myers
54142c44e9 Use math_narrow_eval more consistently. 
Where glibc code needs to avoid excess range and precision in
floating-point arithmetic, code variously uses either asms or volatile
to force the results of that arithmetic to memory; mostly this is
conditional on FLT_EVAL_METHOD, but in the case of lrint / llrint
functions some use of volatile is unconditional (and is present
unnecessarily in versions for long double).  This patch make such code
use the recently-added math_narrow_eval macro consistently, removing
the unnecessary uses of volatile in long double lrint / llrint
implementations completely.

Tested for x86_64, x86, mips64 and powerpc.

	* math/s_nexttowardf.c (__nexttowardf): Use math_narrow_eval.
	* stdlib/strtod_l.c: Include <math_private.h>.
	(overflow_value): Use math_narrow_eval.
	(underflow_value): Likewise.
	* sysdeps/i386/fpu/s_nexttoward.c (__nexttoward): Likewise.
	* sysdeps/i386/fpu/s_nexttowardf.c (__nexttowardf): Likewise.
	* sysdeps/ieee754/dbl-64/e_gamma_r.c (gamma_positive): Likewise.
	(__ieee754_gamma_r): Likewise.
	* sysdeps/ieee754/dbl-64/gamma_productf.c (__gamma_productf):
	Likewise.
	* sysdeps/ieee754/dbl-64/k_rem_pio2.c (__kernel_rem_pio2):
	Likewise.
	* sysdeps/ieee754/dbl-64/lgamma_neg.c (__lgamma_neg): Likewise.
	* sysdeps/ieee754/dbl-64/s_erf.c (__erfc): Likewise.
	* sysdeps/ieee754/dbl-64/s_llrint.c (__llrint): Likewise.
	* sysdeps/ieee754/dbl-64/s_lrint.c (__lrint): Likewise.
	* sysdeps/ieee754/flt-32/e_gammaf_r.c (gammaf_positive): Likewise.
	(__ieee754_gammaf_r): Likewise.
	* sysdeps/ieee754/flt-32/k_rem_pio2f.c (__kernel_rem_pio2f):
	Likewise.
	* sysdeps/ieee754/flt-32/lgamma_negf.c (__lgamma_negf): Likewise.
	* sysdeps/ieee754/flt-32/s_erff.c (__erfcf): Likewise.
	* sysdeps/ieee754/flt-32/s_llrintf.c (__llrintf): Likewise.
	* sysdeps/ieee754/flt-32/s_lrintf.c (__lrintf): Likewise.
	* sysdeps/ieee754/ldbl-128/s_llrintl.c (__llrintl): Do not use
	volatile.
	* sysdeps/ieee754/ldbl-128/s_lrintl.c (__lrintl): Likewise.
	* sysdeps/ieee754/ldbl-128/s_nexttoward.c (__nexttoward): Use
	math_narrow_eval.
	* sysdeps/ieee754/ldbl-128ibm/s_nexttoward.c (__nexttoward):
	Likewise.
	* sysdeps/ieee754/ldbl-128ibm/s_nexttowardf.c (__nexttowardf):
	Likewise.
	* sysdeps/ieee754/ldbl-96/gamma_product.c (__gamma_product):
	Likewise.
	* sysdeps/ieee754/ldbl-96/s_llrintl.c (__llrintl): Do not use
	volatile.
	* sysdeps/ieee754/ldbl-96/s_lrintl.c (__lrintl): Likewise.
	* sysdeps/ieee754/ldbl-96/s_nexttoward.c (__nexttoward): Use
	math_narrow_eval.
	* sysdeps/ieee754/ldbl-96/s_nexttowardf.c (__nexttowardf):
	Likewise.
	* sysdeps/ieee754/ldbl-opt/s_nexttowardfd.c (__nldbl_nexttowardf):
	Likewise.
 2015-09-23 18:14:57 +00:00
Joseph Myers
050f29c188 Fix lgamma (negative) inaccuracy (bug 2542, bug 2543, bug 2558). 
The existing implementations of lgamma functions (except for the ia64
versions) use the reflection formula for negative arguments.  This
suffers large inaccuracy from cancellation near zeros of lgamma (near
where the gamma function is +/- 1).

This patch fixes this inaccuracy.  For arguments above -2, there are
no zeros and no large cancellation, while for sufficiently large
negative arguments the zeros are so close to integers that even for
integers +/- 1ulp the log(gamma(1-x)) term dominates and cancellation
is not significant.  Thus, it is only necessary to take special care
about cancellation for arguments around a limited number of zeros.

Accordingly, this patch uses precomputed tables of relevant zeros,
expressed as the sum of two floating-point values.  The log of the
ratio of two sines can be computed accurately using log1p in cases
where log would lose accuracy.  The log of the ratio of two gamma(1-x)
values can be computed using Stirling's approximation (the difference
between two values of that approximation to lgamma being computable
without computing the two values and then subtracting), with
appropriate adjustments (which don't reduce accuracy too much) in
cases where 1-x is too small to use Stirling's approximation directly.

In the interval from -3 to -2, using the ratios of sines and of
gamma(1-x) can still produce too much cancellation between those two
parts of the computation (and that interval is also the worst interval
for computing the ratio between gamma(1-x) values, which computation
becomes more accurate, while being less critical for the final result,
for larger 1-x).  Because this can result in errors slightly above
those accepted in glibc, this interval is instead dealt with by
polynomial approximations.  Separate polynomial approximations to
(|gamma(x)|-1)(x-n)/(x-x0) are used for each interval of length 1/8
from -3 to -2, where n (-3 or -2) is the nearest integer to the
1/8-interval and x0 is the zero of lgamma in the relevant half-integer
interval (-3 to -2.5 or -2.5 to -2).

Together, the two approaches are intended to give sufficient accuracy
for all negative arguments in the problem range.  Outside that range,
the previous implementation continues to be used.

Tested for x86_64, x86, mips64 and powerpc.  The mips64 and powerpc
testing shows up pre-existing problems for ldbl-128 and ldbl-128ibm
with large negative arguments giving spurious "invalid" exceptions
(exposed by newly added tests for cases this patch doesn't affect the
logic for); I'll address those problems separately.

	[BZ #2542]
	[BZ #2543]
	[BZ #2558]
	* sysdeps/ieee754/dbl-64/e_lgamma_r.c (__ieee754_lgamma_r): Call
	__lgamma_neg for arguments from -28.0 to -2.0.
	* sysdeps/ieee754/flt-32/e_lgammaf_r.c (__ieee754_lgammaf_r): Call
	__lgamma_negf for arguments from -15.0 to -2.0.
	* sysdeps/ieee754/ldbl-128/e_lgammal_r.c (__ieee754_lgammal_r):
	Call __lgamma_negl for arguments from -48.0 or -50.0 to -2.0.
	* sysdeps/ieee754/ldbl-96/e_lgammal_r.c (__ieee754_lgammal_r):
	Call __lgamma_negl for arguments from -33.0 to -2.0.
	* sysdeps/ieee754/dbl-64/lgamma_neg.c: New file.
	* sysdeps/ieee754/dbl-64/lgamma_product.c: Likewise.
	* sysdeps/ieee754/flt-32/lgamma_negf.c: Likewise.
	* sysdeps/ieee754/flt-32/lgamma_productf.c: Likewise.
	* sysdeps/ieee754/ldbl-128/lgamma_negl.c: Likewise.
	* sysdeps/ieee754/ldbl-128/lgamma_productl.c: Likewise.
	* sysdeps/ieee754/ldbl-128ibm/lgamma_negl.c: Likewise.
	* sysdeps/ieee754/ldbl-128ibm/lgamma_productl.c: Likewise.
	* sysdeps/ieee754/ldbl-96/lgamma_negl.c: Likewise.
	* sysdeps/ieee754/ldbl-96/lgamma_product.c: Likewise.
	* sysdeps/ieee754/ldbl-96/lgamma_productl.c: Likewise.
	* sysdeps/generic/math_private.h (__lgamma_negf): New prototype.
	(__lgamma_neg): Likewise.
	(__lgamma_negl): Likewise.
	(__lgamma_product): Likewise.
	(__lgamma_productl): Likewise.
	* math/Makefile (libm-calls): Add lgamma_neg and lgamma_product.
	* math/auto-libm-test-in: Add more tests of lgamma.
	* math/auto-libm-test-out: Regenerated.
	* sysdeps/i386/fpu/libm-test-ulps: Update.
	* sysdeps/x86_64/fpu/libm-test-ulps: Likewise.
 2015-09-10 22:27:58 +00:00