MIPS: IEEE 754-2008 NaN encoding support

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

1 files changed, 21 insertions, 18 deletions

diff --git a/sysdeps/generic/ldconfig.h b/sysdeps/generic/ldconfig.h
index d96089d492..037f959954 100644
--- a/sysdeps/generic/ldconfig.h
+++ b/sysdeps/generic/ldconfig.h
@@ -21,24 +21,27 @@
 
 #include <stdint.h>
 
-#define FLAG_ANY		-1
-#define FLAG_TYPE_MASK		0x00ff
-#define FLAG_LIBC4		0x0000
-#define FLAG_ELF		0x0001
-#define FLAG_ELF_LIBC5		0x0002
-#define FLAG_ELF_LIBC6		0x0003
-#define FLAG_REQUIRED_MASK	0xff00
-#define FLAG_SPARC_LIB64	0x0100
-#define FLAG_IA64_LIB64		0x0200
-#define FLAG_X8664_LIB64	0x0300
-#define FLAG_S390_LIB64		0x0400
-#define FLAG_POWERPC_LIB64	0x0500
-#define FLAG_MIPS64_LIBN32	0x0600
-#define FLAG_MIPS64_LIBN64	0x0700
-#define FLAG_X8664_LIBX32	0x0800
-#define FLAG_ARM_LIBHF		0x0900
-#define FLAG_AARCH64_LIB64	0x0a00
-#define FLAG_ARM_LIBSF		0x0b00
+#define FLAG_ANY			-1
+#define FLAG_TYPE_MASK			0x00ff
+#define FLAG_LIBC4			0x0000
+#define FLAG_ELF			0x0001
+#define FLAG_ELF_LIBC5			0x0002
+#define FLAG_ELF_LIBC6			0x0003
+#define FLAG_REQUIRED_MASK		0xff00
+#define FLAG_SPARC_LIB64		0x0100
+#define FLAG_IA64_LIB64			0x0200
+#define FLAG_X8664_LIB64		0x0300
+#define FLAG_S390_LIB64			0x0400
+#define FLAG_POWERPC_LIB64		0x0500
+#define FLAG_MIPS64_LIBN32		0x0600
+#define FLAG_MIPS64_LIBN64		0x0700
+#define FLAG_X8664_LIBX32		0x0800
+#define FLAG_ARM_LIBHF			0x0900
+#define FLAG_AARCH64_LIB64		0x0a00
+#define FLAG_ARM_LIBSF			0x0b00
+#define FLAG_MIPS_LIB32_NAN2008		0x0c00
+#define FLAG_MIPS64_LIBN32_NAN2008	0x0d00
+#define FLAG_MIPS64_LIBN64_NAN2008	0x0e00
 
 /* Name of auxiliary cache.  */
 #define _PATH_LDCONFIG_AUX_CACHE "/var/cache/ldconfig/aux-cache"