The default sysdeps/ieee754 fma implementations rely on exceptions and
rounding modes to achieve correct results through internal use of
round-to-odd. Thus, glibc configurations without support for
exceptions and rounding modes instead need to use implementations of
fma based on soft-fp.
At present, this is achieved via having implementation files in
soft-fp/ that are #included by sysdeps files for each glibc
configuration that needs them. In general this means such a
configuration has its own s_fma.c and s_fmaf.c.
TS 18661-1 adds functions that do an operation (+ - * / sqrt fma) on
arguments wider than the return type, with a single rounding of the
infinite-precision result to that return type. These are also
naturally implemented using round-to-odd on platforms with hardware
support for rounding modes and exceptions but lacking hardware support
for these narrowing operations themselves. (Platforms that have
direct hardware support for such narrowing operations include at least
ia64, and Power ISA 2.07 or later, which I think means POWER8 or
later.)
So adding the remaining TS 18661-1 functions would mean at least six
narrowing function implementations (fadd fsub fmul fdiv ffma fsqrt),
with aliases for other types and further implementations in some
configurations, that need to be overridden for configurations lacking
hardware exceptions and rounding modes. Requiring all such
configurations (currently seven of them) to have their own source
files for all those functions seems undesirable.
Thus, this patch adds a directory sysdeps/ieee754/soft-fp to contain
libm function implementations based on soft-fp. This directory is
then used via Implies from all the configurations that need it, so no
more files need adding to every such configuration when adding more
functions with soft-fp implementations. A configuration can still
selectively #include a particular file from this directory if desired;
thus, the MIPS #include of the fmal implementation is retained, since
that's appropriate even for hard float (because long double is always
implementated in software for MIPS64, so the soft-fp implementation of
fmal is better than the ldbl-128 one).
This also provides additional motivation for my recent patch removing
--with-fp / --without-fp: previously there was no need for correct use
of --without-fp for no-FPU ARM or SH3, and now we have autodetection
nofpu/ sysdeps directories can be used by this patch for those
configurations without imposing any new requirements on how glibc is
configured.
(The mips64/*/fpu/s_fma.c files added by this patch are needed to keep
the dbl-64 version of fma for double, rather than the ldbl-128 one,
used in that case.)
Tested with build-many-glibcs.py that installed stripped shared
libraries are unchanged by this patch.
projects / thirdparty / glibc.git / commit
