Implement C23 pown

C23 adds various <math.h> function families originally defined in TS
18661-4.  Add the pown functions, which are like pow but with an
integer exponent.  That exponent has type long long int in C23; it was
intmax_t in TS 18661-4, and as with other interfaces changed after
their initial appearance in the TS, I don't think we need to support
the original version of the interface.  The test inputs are based on
the subset of test inputs for pow that use integer exponents that fit
in long long.

As the first such template implementation that saves and restores the
rounding mode internally (to avoid possible issues with directed
rounding and intermediate overflows or underflows in the wrong
rounding mode), support also needed to be added for using
SET_RESTORE_ROUND* in such template function implementations.  This
required math-type-macros-float128.h to include <fenv_private.h>, so
it can tell whether SET_RESTORE_ROUNDF128 is defined.  In turn, the
include order with <fenv_private.h> included before <math_private.h>
broke loongarch builds, showing up that
sysdeps/loongarch/math_private.h is really a fenv_private.h file
(maybe implemented internally before the consistent split of those
headers in 2018?) and needed to be renamed to fenv_private.h to avoid
errors with duplicate macro definitions if <math_private.h> is
included after <fenv_private.h>.

The underlying implementation uses __ieee754_pow functions (called
more than once in some cases, where the exponent does not fit in the
floating type).  I expect a custom implementation for a given format,
that only handles integer exponents but handles larger exponents
directly, could be faster and more accurate in some cases.

I encourage searching for worst cases for ulps error for these
implementations (necessarily non-exhaustively, given the size of the
input space).

Tested for x86_64 and x86, and with build-many-glibcs.py.

5 files changed, 16 insertions, 0 deletions

diff --git a/sysdeps/generic/math-type-macros-double.h b/sysdeps/generic/math-type-macros-double.h
index 1e84d3f..f03aea1 100644
--- a/sysdeps/generic/math-type-macros-double.h
+++ b/sysdeps/generic/math-type-macros-double.h
@@ -28,6 +28,8 @@
 #define M_STRTO_NAN __strtod_nan
 #define M_USE_BUILTIN(c) USE_ ##c ##_BUILTIN
 
+#define M_SET_RESTORE_ROUND(RM) SET_RESTORE_ROUND (RM)
+
 #include <libm-alias-double.h>
 #include <math-nan-payload-double.h>
 
diff --git a/sysdeps/generic/math-type-macros-float.h b/sysdeps/generic/math-type-macros-float.h
index 4aac524..445535a 100644
--- a/sysdeps/generic/math-type-macros-float.h
+++ b/sysdeps/generic/math-type-macros-float.h
@@ -30,6 +30,8 @@
 /* GNU extension float constant macros.  */
 #define M_MLIT(c) c ## f
 
+#define M_SET_RESTORE_ROUND(RM) SET_RESTORE_ROUNDF (RM)
+
 #include <libm-alias-float.h>
 #include <math-nan-payload-float.h>
 
diff --git a/sysdeps/generic/math-type-macros-float128.h b/sysdeps/generic/math-type-macros-float128.h
index ad2310b..f64c1d3 100644
--- a/sysdeps/generic/math-type-macros-float128.h
+++ b/sysdeps/generic/math-type-macros-float128.h
@@ -19,6 +19,8 @@
 #ifndef _MATH_TYPE_MACROS_FLOAT128
 #define _MATH_TYPE_MACROS_FLOAT128
 
+#include <fenv_private.h>
+
 #define M_LIT(c) __f128 (c)
 #define M_PFX FLT128
 #define M_SUF(c) c ## f128
@@ -30,6 +32,13 @@
 
 #define M_MLIT(c) c ## f128
 
+/* fenv_private.h may not define SET_RESTORE_ROUNDF128.  */
+#ifdef SET_RESTORE_ROUNDF128
+# define M_SET_RESTORE_ROUND(RM) SET_RESTORE_ROUNDF128 (RM)
+#else
+# define M_SET_RESTORE_ROUND(RM) SET_RESTORE_ROUNDL (RM)
+#endif
+
 #include <libm-alias-float128.h>
 #include <math-nan-payload-float128.h>
 
diff --git a/sysdeps/generic/math-type-macros-ldouble.h b/sysdeps/generic/math-type-macros-ldouble.h
index 931d5ec..00309cb 100644
--- a/sysdeps/generic/math-type-macros-ldouble.h
+++ b/sysdeps/generic/math-type-macros-ldouble.h
@@ -28,6 +28,8 @@
 #define M_STRTO_NAN __strtold_nan
 #define M_USE_BUILTIN(c) USE_ ##c ##L_BUILTIN
 
+#define M_SET_RESTORE_ROUND(RM) SET_RESTORE_ROUNDL (RM)
+
 #include <libm-alias-ldouble.h>
 #include <math-nan-payload-ldouble.h>
 
diff --git a/sysdeps/generic/math-type-macros.h b/sysdeps/generic/math-type-macros.h
index e3224a0..21c5fee 100644
--- a/sysdeps/generic/math-type-macros.h
+++ b/sysdeps/generic/math-type-macros.h
@@ -33,6 +33,7 @@
    M_STRTO_NAN - Resolves to the internal libc function which
 		converts a string into the appropriate FLOAT nan
 		value.
+   M_SET_RESTORE_ROUND - Resolves to a SET_RESTORE_ROUND call for M_TYPE.
 
   declare_mgen_alias(from,to)
       This exposes the appropriate symbol(s) for a