Fix f64xdivf128, f64xmulf128 spurious underflows (bug 28358)

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.

1 files changed, 28 insertions, 17 deletions

diff --git a/math/math-narrow.h b/math/math-narrow.h
index 93d1b4c52a..0194f8c97e 100644
--- a/math/math-narrow.h
+++ b/math/math-narrow.h
@@ -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;							\