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It has been a long practice for software using IEEE 754 floating-point arithmetic run on MIPS processors to use an encoding of Not-a-Number (NaN) data different to one used by software run on other processors. And as of IEEE 754-2008 revision [1] this encoding does not follow one recommended in the standard, as specified in section 6.2.1, where it is stated that quiet NaNs should have the first bit (d1) of their significand set to 1 while signalling NaNs should have that bit set to 0, but MIPS software interprets the two bits in the opposite manner. As from revision 3.50 [2][3] the MIPS Architecture provides for processors that support the IEEE 754-2008 preferred NaN encoding format. As the two formats (further referred to as "legacy NaN" and "2008 NaN") are incompatible to each other, tools have to provide support for the two formats to help people avoid using incompatible binary modules. The change is comprised of two functional groups of features, both of which are required for correct support. 1. Dynamic linker support. To enforce the NaN encoding requirement in dynamic linking a new ELF file header flag has been defined. This flag is set for 2008-NaN shared modules and executables and clear for legacy-NaN ones. The dynamic linker silently ignores any incompatible modules it encounters in dependency processing. To avoid unnecessary processing of incompatible modules in the presence of a shared module cache, a set of new cache flags has been defined to mark 2008-NaN modules for the three ABIs supported. Changes to sysdeps/unix/sysv/linux/mips/readelflib.c have been made following an earlier code quality suggestion made here: http://sourceware.org/ml/libc-ports/2009-03/msg00036.html and are therefore a little bit more extensive than the minimum required. Finally a new name has been defined for the dynamic linker so that 2008-NaN and legacy-NaN binaries can coexist on a single system that supports dual-mode operation and that a legacy dynamic linker that does not support verifying the 2008-NaN ELF file header flag is not chosen to interpret a 2008-NaN binary by accident. 2. Floating environment support. IEEE 754-2008 features are controlled in the Floating-Point Control and Status (FCSR) register and updates are needed to floating environment support so that the 2008-NaN flag is set correctly and the kernel default, inferred from the 2008-NaN ELF file header flag at the time an executable is loaded, respected. As the NaN encoding format is a property of GCC code generation that is both a user-selected GCC configuration default and can be overridden with GCC options, code that needs to know what NaN encoding standard it has been configured for checks for the __mips_nan2008 macro that is defined internally by GCC whenever the 2008-NaN mode has been selected. This mode is determined at the glibc configuration time and therefore a few consistency checks have been added to catch cases where compilation flags have been overridden by the user. The 2008 NaN set of features relies on kernel support as the in-kernel floating-point emulator needs to be aware of the NaN encoding used even on hard-float processors and configure the FPU context according to the value of the 2008 NaN ELF file header flag of the executable being started. As at this time work on kernel support is still in progress and the relevant changes have not made their way yet to linux.org master repository. Therefore the minimum version supported has been artificially set to 10.0.0 so that 2008-NaN code is not accidentally run on a Linux kernel that does not suppport it. It is anticipated that the version is adjusted later on to the actual initial linux.org kernel version to support this feature. Legacy NaN encoding support is unaffected, older kernel versions remain supported. [1] "IEEE Standard for Floating-Point Arithmetic", IEEE Computer Society, IEEE Std 754-2008, 29 August 2008 [2] "MIPS Architecture For Programmers, Volume I-A: Introduction to the MIPS32 Architecture", MIPS Technologies, Inc., Document Number: MD00082, Revision 3.50, September 20, 2012 [3] "MIPS Architecture For Programmers, Volume I-A: Introduction to the MIPS64 Architecture", MIPS Technologies, Inc., Document Number: MD00083, Revision 3.50, September 20, 2012 |
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