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
Reserved bits in the Floating-Point Control and Status Register (FCSR)
should not be implicitly cleared by fedisableexcept or feenableexcept,
there is no reason to. Among these are the 8 condition codes and one of
the two bits reserved for architecture implementers (bits #22 & #21).
As to the latter, there is no reason to treat any of them as reserved
either, they should be user controllable and settable via __fpu_control
override as the user sees fit. For example in processors implemented by
MIPS Technologies, such as the 5Kf or the 24Kf, these bits are used to
change the treatment of denormalised operands and tiny results: bit #22
is Flush Override (FO) and bit #21 is Flush to Nearest (FN). They cause
non-IEEE-compliant behaviour, but some programs may have a use for such
modes of operation; the library should not obstruct such use just as it
does not for the architectural Flush to Zero (FS) bit (bit #24).
Therefore the change adjusts the reserved mask accordingly and also
documents the distinction between bits 22:21 and 20:18.
Resolves: #15465
The program name may be unavailable if the user application tampers
with argc and argv[]. Some parts of the dynamic linker caters for
this while others don't, so this patch consolidates the check and
fallback into a single macro and updates all users.
[BZ #15442] This adds support for the inverse interpretation of the
quiet bit of IEEE 754 floating-point NaN data that some processors
use. This includes in particular MIPS architecture processors; the
payload used for the canonical qNaN encoding is updated accordingly
so as not to interfere with the quiet bit.
Steve Ellcey
Maciej W. Rozycki
Ondřej Bílka
Joseph Myers
Ryan S. Arnold
Siddhesh Poyarekar
Thomas Schwinge
Maxim Kuvyrkov