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18a218b74c
This patch refines the math.texi documentation of the goals for when libm function raise the inexact and underflow exceptions. The previous text was problematic in some cases around the underflow threshold. * Strictly, it would have meant that if the mathematical result of pow was very slightly below DBL_MIN, for example, it was required to raise the underflow exception; although normally a few ulps error would be OK, if that error meant the computed value was slightly above DBL_MIN it would fail the previously described underflow exception goal. * Similarly, strict IEEE semantics would imply that sin (DBL_MIN), in round-to-nearest mode, underflows on before-rounding but not after-rounding architectures, while returning DBL_MIN; the previous wording would have required an underflow exception, so preventing checks for a result with absolute value below DBL_MIN from being sufficient checks to determine whether the exception is required. (Under the previous wording, checks for a result with absolute value <= DBL_MIN wouldn't have been sufficient either, because in FE_TOWARDZERO mode a result of DBL_MIN definitely does not result from an underflowing infinite-precision result.) * The previous wording about rounding infinite-precision values could be taken to mean all exceptions including "inexact" must be consistent with some such value. That would mean that a result of DBL_MIN in FE_UPWARD mode with "inexact" raised must also have "underflow" raised on before-rounding architectures. Again, that would cause problems for computing a result (possibly with spurious "inexact" exceptions) and then using a rounding-mode-independent test for results with absolute value below DBL_MIN to determine whether an underflow exception must be forced in case the underflows from intermediate computations happened to be exact. By refining the documentation, this patch avoids stating goals for accuracy close to the underflow threshold that were stricter than applied anywhere else, and allows the implementation strategy of: compute a result within a few ulps, taking care to avoid underflows in intermediate computations, then force an underflow exception if that result was subnormal. Only fully-defined functions such as fma need to take greater care about the exact underflow threshold (including its dependence on whether the architecture is before-rounding or after-rounding, and on the rounding mode on after-rounding architectures). (If the rounding mode is changed as part of the computation, it's still necessary to ensure that not just intermediate computations, but the final computation of the result to be returned, do not raise underflow if that result is the least normal value and underflow would be inconsistent with the original rounding mode. Since such code can readily discard exceptions as part of saving and restoring the rounding mode - SET_RESTORE_ROUND_NOEX etc. - I don't think that should be a problem in practice.) * manual/math.texi (Errors in Math Functions): Clarify goals regarding inexact and underflow exceptions. |
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This directory contains the sources of the GNU C Library. See the file "version.h" for what release version you have. The GNU C Library is the standard system C library for all GNU systems, and is an important part of what makes up a GNU system. It provides the system API for all programs written in C and C-compatible languages such as C++ and Objective C; the runtime facilities of other programming languages use the C library to access the underlying operating system. In GNU/Linux systems, the C library works with the Linux kernel to implement the operating system behavior seen by user applications. In GNU/Hurd systems, it works with a microkernel and Hurd servers. The GNU C Library implements much of the POSIX.1 functionality in the GNU/Hurd system, using configurations i[4567]86-*-gnu. The current GNU/Hurd support requires out-of-tree patches that will eventually be incorporated into an official GNU C Library release. When working with Linux kernels, this version of the GNU C Library requires Linux kernel version 2.6.32 or later. Also note that the shared version of the libgcc_s library must be installed for the pthread library to work correctly. The GNU C Library supports these configurations for using Linux kernels: aarch64*-*-linux-gnu alpha*-*-linux-gnu arm-*-linux-gnueabi hppa-*-linux-gnu Not currently functional without patches. i[4567]86-*-linux-gnu x86_64-*-linux-gnu Can build either x86_64 or x32 ia64-*-linux-gnu m68k-*-linux-gnu microblaze*-*-linux-gnu mips-*-linux-gnu mips64-*-linux-gnu powerpc-*-linux-gnu Hardware or software floating point, BE only. powerpc64*-*-linux-gnu Big-endian and little-endian. s390-*-linux-gnu s390x-*-linux-gnu sh[34]-*-linux-gnu sparc*-*-linux-gnu sparc64*-*-linux-gnu tilegx-*-linux-gnu tilepro-*-linux-gnu If you are interested in doing a port, please contact the glibc maintainers; see http://www.gnu.org/software/libc/ for more information. See the file INSTALL to find out how to configure, build, and install the GNU C Library. You might also consider reading the WWW pages for the C library at http://www.gnu.org/software/libc/. The GNU C Library is (almost) completely documented by the Texinfo manual found in the `manual/' subdirectory. The manual is still being updated and contains some known errors and omissions; we regret that we do not have the resources to work on the manual as much as we would like. For corrections to the manual, please file a bug in the `manual' component, following the bug-reporting instructions below. Please be sure to check the manual in the current development sources to see if your problem has already been corrected. Please see http://www.gnu.org/software/libc/bugs.html for bug reporting information. We are now using the Bugzilla system to track all bug reports. This web page gives detailed information on how to report bugs properly. The GNU C Library is free software. See the file COPYING.LIB for copying conditions, and LICENSES for notices about a few contributions that require these additional notices to be distributed. License copyright years may be listed using range notation, e.g., 1996-2015, indicating that every year in the range, inclusive, is a copyrightable year that would otherwise be listed individually.