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Fix f64xdivf128, f64xmulf128 spurious underflows (bug 28358)

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As described in bug 28358, the round-to-odd computations used in the
libm functions that round their results to a narrower format can yield
spurious underflow exceptions in the following circumstances: the
narrowing only narrows the precision of the type and not the exponent
range (i.e., it's narrowing _Float128 to _Float64x on x86_64, x86 or
ia64), the architecture does after-rounding tininess detection (which
applies to all those architectures), the result is inexact, tiny
before rounding but not tiny after rounding (with the chosen rounding
mode) for _Float64x (which is possible for narrowing mul, div and fma,
not for narrowing add, sub or sqrt), so the underflow exception
resulting from the toward-zero computation in _Float128 is spurious
for _Float64x.

Fixed by making ROUND_TO_ODD call feclearexcept (FE_UNDERFLOW) in the
problem cases (as indicated by an extra argument to the macro); there
is never any need to preserve underflow exceptions from this part of
the computation, because the conversion of the round-to-odd value to
the narrower type will underflow in exactly the cases in which the
function should raise that exception, but it may be more efficient to
avoid the extra manipulation of the floating-point environment when
not needed.

Tested for x86_64 and x86, and with build-many-glibcs.py.
This commit is contained in:
Joseph Myers 2021-09-21 21:54:37 +00:00
parent 0b5ca7c3e5
commit 1356f38df5
18 changed files with 9604 additions and 31 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

30
math/auto-libm-test-in
View File

@ -4870,6 +4870,21 @@ div 0x1.0000000000008001000000000001p0 0x1.0000000000008p0
# Similar, for double rounding to 64-bit of a division of 53-bit values.
div 0x1ffe1p0 0xfffp0
# Cases where there is underflow before rounding (for some format) but
# might not be after rounding, depending on the rounding mode.
div 0x1p-126 0x1.0000001p0
div 0x1p-126 -0x1.0000001p0
div -0x1p-126 0x1.0000001p0
div -0x1p-126 -0x1.0000001p0
div 0x1p-1022 0x1.00000000000001p0
div 0x1p-1022 -0x1.00000000000001p0
div -0x1p-1022 0x1.00000000000001p0
div -0x1p-1022 -0x1.00000000000001p0
div 0x1p-16382 0x1.00000000000000001p0
div 0x1p-16382 -0x1.00000000000000001p0
div -0x1p-16382 0x1.00000000000000001p0
div -0x1p-16382 -0x1.00000000000000001p0
erf 0
erf -0
erf 0.125
@ -6645,6 +6660,21 @@ mul 0x50000000000000005p-64 0xcccccccccccccccccccccccccccdp-114
# This product equals 2^64 + 2^11 + 1.
mul 97689974585 188829449
# Cases where there is underflow before rounding (for some format) but
# might not be after rounding, depending on the rounding mode.
mul 0x0.ffffff8p-126 0x1.0000001p0
mul 0x0.ffffff8p-126 -0x1.0000001p0
mul -0x0.ffffff8p-126 0x1.0000001p0
mul -0x0.ffffff8p-126 -0x1.0000001p0
mul 0x0.fffffffffffffcp-1022 0x1.00000000000001p0
mul 0x0.fffffffffffffcp-1022 -0x1.00000000000001p0
mul -0x0.fffffffffffffcp-1022 0x1.00000000000001p0
mul -0x0.fffffffffffffcp-1022 -0x1.00000000000001p0
mul 0x0.ffffffffffffffff8p-16382 0x1.00000000000000001p0
mul 0x0.ffffffffffffffff8p-16382 -0x1.00000000000000001p0
mul -0x0.ffffffffffffffff8p-16382 0x1.00000000000000001p0
mul -0x0.ffffffffffffffff8p-16382 -0x1.00000000000000001p0
pow 0 0
pow 0 -0
pow -0 0

3372
math/auto-libm-test-out-narrow-div
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File diff suppressed because it is too large Load Diff

6156
math/auto-libm-test-out-narrow-mul
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File diff suppressed because it is too large Load Diff

45
math/math-narrow.h
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@ -28,6 +28,7 @@
#include <math_private.h>
#include <fenv_private.h>
#include <math-narrow-alias.h>
#include <stdbool.h>
/* Carry out a computation using round-to-odd. The computation is
EXPR; the union type in which to store the result is UNION and the
@ -37,11 +38,15 @@
function rather than a C operator is used when argument and result
types are the same) and the libc_fe* macros to ensure that the
correct rounding mode is used, for platforms with multiple rounding
modes where those macros set only the relevant mode. This macro
does not work correctly if the sign of an exact zero result depends
on the rounding mode, so that case must be checked for
separately. */
#define ROUND_TO_ODD(EXPR, UNION, SUFFIX, MANTISSA) \
modes where those macros set only the relevant mode.
CLEAR_UNDERFLOW indicates whether underflow exceptions must be
cleared (in the case where a round-toward-zero underflow might not
indicate an underflow after narrowing, when that narrowing only
reduces precision not exponent range and the architecture uses
before-rounding tininess detection). This macro does not work
correctly if the sign of an exact zero result depends on the
rounding mode, so that case must be checked for separately. */
#define ROUND_TO_ODD(EXPR, UNION, SUFFIX, MANTISSA, CLEAR_UNDERFLOW) \
({ \
fenv_t env; \
UNION u; \
@ -49,6 +54,8 @@
libc_feholdexcept_setround ## SUFFIX (&env, FE_TOWARDZERO); \
u.d = (EXPR); \
math_force_eval (u.d); \
if (CLEAR_UNDERFLOW) \
feclearexcept (FE_UNDERFLOW); \
u.ieee.MANTISSA \
|= libc_feupdateenv_test ## SUFFIX (&env, FE_INEXACT) != 0; \
\
@ -91,7 +98,7 @@
ret = (TYPE) ((X) + (Y)); \
else \
ret = (TYPE) ROUND_TO_ODD (math_opt_barrier (X) + (Y), \
UNION, SUFFIX, MANTISSA); \
UNION, SUFFIX, MANTISSA, false); \
\
CHECK_NARROW_ADD (ret, (X), (Y)); \
return ret; \
@ -149,7 +156,7 @@
ret = (TYPE) ((X) - (Y)); \
else \
ret = (TYPE) ROUND_TO_ODD (math_opt_barrier (X) - (Y), \
UNION, SUFFIX, MANTISSA); \
UNION, SUFFIX, MANTISSA, false); \
\
CHECK_NARROW_SUB (ret, (X), (Y)); \
return ret; \
@ -194,15 +201,17 @@
while (0)
/* Implement narrowing multiply using round-to-odd. The arguments are
X and Y, the return type is TYPE and UNION, MANTISSA and SUFFIX are
as for ROUND_TO_ODD. */
#define NARROW_MUL_ROUND_TO_ODD(X, Y, TYPE, UNION, SUFFIX, MANTISSA) \
X and Y, the return type is TYPE and UNION, MANTISSA, SUFFIX and
CLEAR_UNDERFLOW are as for ROUND_TO_ODD. */
#define NARROW_MUL_ROUND_TO_ODD(X, Y, TYPE, UNION, SUFFIX, MANTISSA, \
CLEAR_UNDERFLOW) \
do \
{ \
TYPE ret; \
\
ret = (TYPE) ROUND_TO_ODD (math_opt_barrier (X) * (Y), \
UNION, SUFFIX, MANTISSA); \
UNION, SUFFIX, MANTISSA, \
CLEAR_UNDERFLOW); \
\
CHECK_NARROW_MUL (ret, (X), (Y)); \
return ret; \
@ -246,16 +255,18 @@
} \
while (0)
/* Implement narrowing divide using round-to-odd. The arguments are
X and Y, the return type is TYPE and UNION, MANTISSA and SUFFIX are
as for ROUND_TO_ODD. */
#define NARROW_DIV_ROUND_TO_ODD(X, Y, TYPE, UNION, SUFFIX, MANTISSA) \
/* Implement narrowing divide using round-to-odd. The arguments are X
and Y, the return type is TYPE and UNION, MANTISSA, SUFFIX and
CLEAR_UNDERFLOW are as for ROUND_TO_ODD. */
#define NARROW_DIV_ROUND_TO_ODD(X, Y, TYPE, UNION, SUFFIX, MANTISSA, \
CLEAR_UNDERFLOW) \
do \
{ \
TYPE ret; \
\
ret = (TYPE) ROUND_TO_ODD (math_opt_barrier (X) / (Y), \
UNION, SUFFIX, MANTISSA); \
UNION, SUFFIX, MANTISSA, \
CLEAR_UNDERFLOW); \
\
CHECK_NARROW_DIV (ret, (X), (Y)); \
return ret; \
@ -308,7 +319,7 @@
TYPE ret; \
\
ret = (TYPE) ROUND_TO_ODD (sqrt ## SUFFIX (math_opt_barrier (X)), \
UNION, SUFFIX, MANTISSA); \
UNION, SUFFIX, MANTISSA, false); \
\
CHECK_NARROW_SQRT (ret, (X)); \
return ret; \

2
sysdeps/i386/fpu/s_f32xdivf64.c
View File

@ -24,6 +24,6 @@ __f32xdivf64 (_Float64 x, _Float64 y)
{
/* To avoid double rounding, use round-to-odd on long double. */
NARROW_DIV_ROUND_TO_ODD ((long double) x, (long double) y, double,
union ieee854_long_double, l, mantissa1);
union ieee854_long_double, l, mantissa1, false);
}
libm_alias_float32x_float64 (div)

2
sysdeps/i386/fpu/s_f32xmulf64.c
View File

@ -24,6 +24,6 @@ __f32xmulf64 (_Float64 x, _Float64 y)
{
/* To avoid double rounding, use round-to-odd on long double. */
NARROW_MUL_ROUND_TO_ODD ((long double) x, (long double) y, double,
union ieee854_long_double, l, mantissa1);
union ieee854_long_double, l, mantissa1, false);
}
libm_alias_float32x_float64 (mul)

3
sysdeps/ieee754/dbl-64/s_fdiv.c
View File

@ -29,6 +29,7 @@
float
__fdiv (double x, double y)
{
NARROW_DIV_ROUND_TO_ODD (x, y, float, union ieee754_double, , mantissa1);
NARROW_DIV_ROUND_TO_ODD (x, y, float, union ieee754_double, , mantissa1,
false);
}
libm_alias_float_double (div)

3
sysdeps/ieee754/dbl-64/s_fmul.c
View File

@ -29,6 +29,7 @@
float
__fmul (double x, double y)
{
NARROW_MUL_ROUND_TO_ODD (x, y, float, union ieee754_double, , mantissa1);
NARROW_MUL_ROUND_TO_ODD (x, y, float, union ieee754_double, , mantissa1,
false);
}
libm_alias_float_double (mul)

2
sysdeps/ieee754/ldbl-128/s_ddivl.c
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@ -32,6 +32,6 @@ double
__ddivl (_Float128 x, _Float128 y)
{
NARROW_DIV_ROUND_TO_ODD (x, y, double, union ieee854_long_double, l,
mantissa3);
mantissa3, false);
}
libm_alias_double_ldouble (div)

2
sysdeps/ieee754/ldbl-128/s_dmull.c
View File

@ -32,6 +32,6 @@ double
__dmull (_Float128 x, _Float128 y)
{
NARROW_MUL_ROUND_TO_ODD (x, y, double, union ieee854_long_double, l,
mantissa3);
mantissa3, false);
}
libm_alias_double_ldouble (mul)

3
sysdeps/ieee754/ldbl-128/s_f64xdivf128.c
View File

@ -18,6 +18,7 @@
#include <math.h>
#include <math-narrow.h>
#include <tininess.h>
/* math_ldbl.h defines _Float128 to long double for this directory,
but when they are different, this function must be defined with
@ -30,7 +31,7 @@ __f64xdivf128 (_Float128 x, _Float128 y)
{
#if __HAVE_FLOAT64X_LONG_DOUBLE && __HAVE_DISTINCT_FLOAT128
NARROW_DIV_ROUND_TO_ODD (x, y, _Float64x, union ieee854_long_double, l,
mantissa3);
mantissa3, TININESS_AFTER_ROUNDING);
#else
NARROW_DIV_TRIVIAL (x, y, _Float64x);
#endif

3
sysdeps/ieee754/ldbl-128/s_f64xmulf128.c
View File

@ -18,6 +18,7 @@
#include <math.h>
#include <math-narrow.h>
#include <tininess.h>
/* math_ldbl.h defines _Float128 to long double for this directory,
but when they are different, this function must be defined with
@ -30,7 +31,7 @@ __f64xmulf128 (_Float128 x, _Float128 y)
{
#if __HAVE_FLOAT64X_LONG_DOUBLE && __HAVE_DISTINCT_FLOAT128
NARROW_MUL_ROUND_TO_ODD (x, y, _Float64x, union ieee854_long_double, l,
mantissa3);
mantissa3, TININESS_AFTER_ROUNDING);
#else
NARROW_MUL_TRIVIAL (x, y, _Float64x);
#endif

2
sysdeps/ieee754/ldbl-128/s_fdivl.c
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@ -28,6 +28,6 @@ float
__fdivl (_Float128 x, _Float128 y)
{
NARROW_DIV_ROUND_TO_ODD (x, y, float, union ieee854_long_double, l,
mantissa3);
mantissa3, false);
}
libm_alias_float_ldouble (div)

2
sysdeps/ieee754/ldbl-128/s_fmull.c
View File

@ -28,6 +28,6 @@ float
__fmull (_Float128 x, _Float128 y)
{
NARROW_MUL_ROUND_TO_ODD (x, y, float, union ieee854_long_double, l,
mantissa3);
mantissa3, false);
}
libm_alias_float_ldouble (mul)

2
sysdeps/ieee754/ldbl-96/s_ddivl.c
View File

@ -28,6 +28,6 @@ double
__ddivl (long double x, long double y)
{
NARROW_DIV_ROUND_TO_ODD (x, y, double, union ieee854_long_double, l,
mantissa1);
mantissa1, false);
}
libm_alias_double_ldouble (div)

2
sysdeps/ieee754/ldbl-96/s_dmull.c
View File

@ -28,6 +28,6 @@ double
__dmull (long double x, long double y)
{
NARROW_MUL_ROUND_TO_ODD (x, y, double, union ieee854_long_double, l,
mantissa1);
mantissa1, false);
}
libm_alias_double_ldouble (mul)

2
sysdeps/ieee754/ldbl-96/s_fdivl.c
View File

@ -26,6 +26,6 @@ float
__fdivl (long double x, long double y)
{
NARROW_DIV_ROUND_TO_ODD (x, y, float, union ieee854_long_double, l,
mantissa1);
mantissa1, false);
}
libm_alias_float_ldouble (div)

2
sysdeps/ieee754/ldbl-96/s_fmull.c
View File

@ -26,6 +26,6 @@ float
__fmull (long double x, long double y)
{
NARROW_MUL_ROUND_TO_ODD (x, y, float, union ieee854_long_double, l,
mantissa1);
mantissa1, false);
}
libm_alias_float_ldouble (mul)