Quadruple-precision floating-point format: Difference between revisions

Revision as of 07:56, 3 July 2025

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For the [[C (programming language)|C programming language]], ISO/IEC TS 18661-3 (floating-point extensions for C, interchange and extended types) specifies <code>_Float128</code> as the type implementing the IEEE 754 quadruple-precision format (binary128).<ref>{{cite web|title=ISO/IEC TS 18661-3|url=https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1945.pdf|date=2015-06-10|access-date=2019-09-22}}</ref> Alternatively, in [[C (programming language)|C]]/[[C++]] with a few systems and compilers, quadruple precision may be specified by the [[long double]] type, but this is not required by the language (which only requires <code>long double</code> to be at least as precise as <code>double</code>), nor is it common.

As of [[C++23]], the C++ language defines a <code><stdfloat></code> header that contains fixed width floating point types. Implementations of these are optional, but if an implementation supports it, <code>std::float128_t</code>. corresponds to Quadruple-precision.

On x86 and x86-64, the most common C/C++ compilers implement <code>long double</code> as either 80-bit [[extended precision]] (e.g. the [[GNU C Compiler]] gcc<ref>[https://web.archive.org/web/20080713131713/https://gcc.gnu.org/onlinedocs/gcc/i386-and-x86_002d64-Options.html i386 and x86-64 Options (archived copy on web.archive.org)], ''Using the GNU Compiler Collection''.</ref> and the [[Intel C++ Compiler]] with a <code>/Qlong&#8209;double</code> switch<ref>[http://software.intel.com/en-us/articles/size-of-long-integer-type-on-different-architecture-and-os/ Intel Developer Site].</ref>) or simply as being synonymous with double precision (e.g. [[Microsoft Visual C++]]<ref>[http://msdn.microsoft.com/en-us/library/9cx8xs15.aspx MSDN homepage, about Visual C++ compiler].</ref>), rather than as quadruple precision. The procedure call standard for the [[ARM architecture#AArch64|ARM 64-bit architecture]] (AArch64) specifies that <code>long double</code> corresponds to the IEEE 754 quadruple-precision format.<ref>{{cite web|title=Procedure Call Standard for the ARM 64-bit Architecture (AArch64)|url=http://infocenter.arm.com/help/topic/com.arm.doc.ihi0055b/IHI0055B_aapcs64.pdf|date=2013-05-22|access-date=2019-09-22|archive-url=https://web.archive.org/web/20191016000704/http://infocenter.arm.com/help/topic/com.arm.doc.ihi0055b/IHI0055B_aapcs64.pdf|archive-date=2019-10-16|url-status=dead}}</ref> On a few other architectures, some C/C++ compilers implement <code>long double</code> as quadruple precision, e.g. gcc on [[PowerPC]] (as double-double<ref>[https://gcc.gnu.org/onlinedocs/gcc/RS_002f6000-and-PowerPC-Options.html RS/6000 and PowerPC Options], ''Using the GNU Compiler Collection''.</ref><ref>[https://developer.apple.com/legacy/mac/library/documentation/Performance/Conceptual/Mac_OSX_Numerics/Mac_OSX_Numerics.pdf Inside Macintosh – PowerPC Numerics]. {{webarchive|url=https://web.archive.org/web/20121009191824/http://developer.apple.com/legacy/mac/library/documentation/Performance/Conceptual/Mac_OSX_Numerics/Mac_OSX_Numerics.pdf|date=October 9, 2012}}.</ref><ref>[https://opensource.apple.com/source/gcc/gcc-5646/gcc/config/rs6000/darwin-ldouble.c 128-bit long double support routines for Darwin].</ref>) and [[SPARC]],<ref>[https://gcc.gnu.org/onlinedocs/gcc/SPARC-Options.html SPARC Options], ''Using the GNU Compiler Collection''.</ref> or the [[Sun Studio (software)|Sun Studio compilers]] on SPARC.<ref>[http://docs.oracle.com/cd/E19422-01/819-3693/ncg_lib.html The Math Libraries], Sun Studio 11 ''Numerical Computation Guide'' (2005).</ref> Even if <code>long double</code> is not quadruple precision, however, some C/C++ compilers provide a nonstandard quadruple-precision type as an extension. For example, gcc provides a quadruple-precision type called <code>__float128</code> for x86, x86-64 and [[Itanium]] CPUs,<ref>[https://gcc.gnu.org/onlinedocs/gcc/Floating-Types.html Additional Floating Types], ''Using the GNU Compiler Collection''</ref> and on [[PowerPC]] as IEEE 128-bit floating-point using the -mfloat128-hardware or -mfloat128 options;<ref name=gcc6changes>{{cite web|title=GCC 6 Release Series - Changes, New Features, and Fixes|url=https://gcc.gnu.org/gcc-6/changes.html|access-date=2016-09-13}}</ref> and some versions of Intel's C/C++ compiler for x86 and x86-64 supply a nonstandard quadruple-precision type called <code>_Quad</code>.<ref>[http://software.intel.com/en-us/forums/showthread.php?t=56359 Intel C++ Forums] (2007).</ref>