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As described in bug 28358, the round-to-odd computations used in the
libm functions that round their results to a narrower format can yield
spurious underflow exceptions in the following circumstances: the
narrowing only narrows the precision of the type and not the exponent
range (i.e., it's narrowing _Float128 to _Float64x on x86_64, x86 or
ia64), the architecture does after-rounding tininess detection (which
applies to all those architectures), the result is inexact, tiny
before rounding but not tiny after rounding (with the chosen rounding
mode) for _Float64x (which is possible for narrowing mul, div and fma,
not for narrowing add, sub or sqrt), so the underflow exception
resulting from the toward-zero computation in _Float128 is spurious
for _Float64x.
Fixed by making ROUND_TO_ODD call feclearexcept (FE_UNDERFLOW) in the
problem cases (as indicated by an extra argument to the macro); there
is never any need to preserve underflow exceptions from this part of
the computation, because the conversion of the round-to-odd value to
the narrower type will underflow in exactly the cases in which the
function should raise that exception, but it may be more efficient to
avoid the extra manipulation of the floating-point environment when
not needed.
Tested for x86_64 and x86, and with build-many-glibcs.py.
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This patch adds the narrowing multiply functions from TS 18661-1 to
glibc's libm: fmul, fmull, dmull, f32mulf64, f32mulf32x, f32xmulf64
for all configurations; f32mulf64x, f32mulf128, f64mulf64x,
f64mulf128, f32xmulf64x, f32xmulf128, f64xmulf128 for configurations
with _Float64x and _Float128; __nldbl_dmull for ldbl-opt.
The changes are mostly essentially the same as for the narrowing add
functions, so the description of those generally applies to this patch
as well. f32xmulf64 for i386 cannot use precision control as used for
add and subtract, because that would result in double rounding for
subnormal results, so that uses round-to-odd with long double
intermediate result instead. The soft-fp support involves adding a
new FP_TRUNC_COOKED since soft-fp multiplication uses cooked inputs
and outputs.
Tested for x86_64, x86, mips64 (all three ABIs, both hard and soft
float) and powerpc, and with build-many-glibcs.py.
* math/Makefile (libm-narrow-fns): Add mul.
(libm-test-funcs-narrow): Likewise.
* math/Versions (GLIBC_2.28): Add narrowing multiply functions.
* math/bits/mathcalls-narrow.h (mul): Use __MATHCALL_NARROW.
* math/gen-auto-libm-tests.c (test_functions): Add mul.
* math/math-narrow.h (CHECK_NARROW_MUL): New macro.
(NARROW_MUL_ROUND_TO_ODD): Likewise.
(NARROW_MUL_TRIVIAL): Likewise.
* soft-fp/op-common.h (FP_TRUNC_COOKED): Likewise.
* sysdeps/ieee754/float128/float128_private.h (__fmull): New
macro.
(__dmull): Likewise.
* sysdeps/ieee754/ldbl-opt/Makefile (libnldbl-calls): Add fmul and
dmul.
(CFLAGS-nldbl-dmul.c): New variable.
(CFLAGS-nldbl-fmul.c): Likewise.
* sysdeps/ieee754/ldbl-opt/Versions (GLIBC_2.28): Add
__nldbl_dmull.
* sysdeps/ieee754/ldbl-opt/nldbl-compat.h (__nldbl_dmull): New
prototype.
* manual/arith.texi (Misc FP Arithmetic): Document fmul, fmull,
dmull, fMmulfN, fMmulfNx, fMxmulfN and fMxmulfNx.
* math/auto-libm-test-in: Add tests of mul.
* math/auto-libm-test-out-narrow-mul: New generated file.
* math/libm-test-narrow-mul.inc: New file.
* sysdeps/i386/fpu/s_f32xmulf64.c: Likewise.
* sysdeps/ieee754/dbl-64/s_f32xmulf64.c: Likewise.
* sysdeps/ieee754/dbl-64/s_fmul.c: Likewise.
* sysdeps/ieee754/float128/s_f32mulf128.c: Likewise.
* sysdeps/ieee754/float128/s_f64mulf128.c: Likewise.
* sysdeps/ieee754/float128/s_f64xmulf128.c: Likewise.
* sysdeps/ieee754/ldbl-128/s_dmull.c: Likewise.
* sysdeps/ieee754/ldbl-128/s_f64xmulf128.c: Likewise.
* sysdeps/ieee754/ldbl-128/s_fmull.c: Likewise.
* sysdeps/ieee754/ldbl-128ibm/s_dmull.c: Likewise.
* sysdeps/ieee754/ldbl-128ibm/s_fmull.c: Likewise.
* sysdeps/ieee754/ldbl-96/s_dmull.c: Likewise.
* sysdeps/ieee754/ldbl-96/s_fmull.c: Likewise.
* sysdeps/ieee754/ldbl-opt/nldbl-dmul.c: Likewise.
* sysdeps/ieee754/ldbl-opt/nldbl-fmul.c: Likewise.
* sysdeps/ieee754/soft-fp/s_dmull.c: Likewise.
* sysdeps/ieee754/soft-fp/s_fmul.c: Likewise.
* sysdeps/ieee754/soft-fp/s_fmull.c: Likewise.
* sysdeps/powerpc/fpu/libm-test-ulps: Update.
* sysdeps/mach/hurd/i386/libm.abilist: Likewise.
* sysdeps/unix/sysv/linux/aarch64/libm.abilist: Likewise.
* sysdeps/unix/sysv/linux/alpha/libm.abilist: Likewise.
* sysdeps/unix/sysv/linux/arm/libm.abilist: Likewise.
* sysdeps/unix/sysv/linux/hppa/libm.abilist: Likewise.
* sysdeps/unix/sysv/linux/i386/libm.abilist: Likewise.
* sysdeps/unix/sysv/linux/ia64/libm.abilist: Likewise.
* sysdeps/unix/sysv/linux/m68k/coldfire/libm.abilist: Likewise.
* sysdeps/unix/sysv/linux/m68k/m680x0/libm.abilist: Likewise.
* sysdeps/unix/sysv/linux/microblaze/libm.abilist: Likewise.
* sysdeps/unix/sysv/linux/mips/mips32/libm.abilist: Likewise.
* sysdeps/unix/sysv/linux/mips/mips64/libm.abilist: Likewise.
* sysdeps/unix/sysv/linux/nios2/libm.abilist: Likewise.
* sysdeps/unix/sysv/linux/powerpc/powerpc32/fpu/libm.abilist: Likewise.
* sysdeps/unix/sysv/linux/powerpc/powerpc32/nofpu/libm.abilist: Likewise.
* sysdeps/unix/sysv/linux/powerpc/powerpc64/libm-le.abilist: Likewise.
* sysdeps/unix/sysv/linux/powerpc/powerpc64/libm.abilist: Likewise.
* sysdeps/unix/sysv/linux/riscv/rv64/libm.abilist: Likewise.
* sysdeps/unix/sysv/linux/s390/s390-32/libm.abilist: Likewise.
* sysdeps/unix/sysv/linux/s390/s390-64/libm.abilist: Likewise.
* sysdeps/unix/sysv/linux/sh/libm.abilist: Likewise.
* sysdeps/unix/sysv/linux/sparc/sparc32/libm.abilist: Likewise.
* sysdeps/unix/sysv/linux/sparc/sparc64/libm.abilist: Likewise.
* sysdeps/unix/sysv/linux/x86_64/64/libm.abilist: Likewise.
* sysdeps/unix/sysv/linux/x86_64/x32/libm.abilist: Likewise.
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