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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 divide functions from TS 18661-1 to
glibc's libm: fdiv, fdivl, ddivl, f32divf64, f32divf32x, f32xdivf64
for all configurations; f32divf64x, f32divf128, f64divf64x,
f64divf128, f32xdivf64x, f32xdivf128, f64xdivf128 for configurations
with _Float64x and _Float128; __nldbl_ddivl for ldbl-opt.
The changes are mostly essentially the same as for the other narrowing
functions, so the description of those generally applies to this patch
as well.
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 div.
(libm-test-funcs-narrow): Likewise.
* math/Versions (GLIBC_2.28): Add narrowing divide functions.
* math/bits/mathcalls-narrow.h (div): Use __MATHCALL_NARROW.
* math/gen-auto-libm-tests.c (test_functions): Add div.
* math/math-narrow.h (CHECK_NARROW_DIV): New macro.
(NARROW_DIV_ROUND_TO_ODD): Likewise.
(NARROW_DIV_TRIVIAL): Likewise.
* sysdeps/ieee754/float128/float128_private.h (__fdivl): New
macro.
(__ddivl): Likewise.
* sysdeps/ieee754/ldbl-opt/Makefile (libnldbl-calls): Add fdiv and
ddiv.
(CFLAGS-nldbl-ddiv.c): New variable.
(CFLAGS-nldbl-fdiv.c): Likewise.
* sysdeps/ieee754/ldbl-opt/Versions (GLIBC_2.28): Add
__nldbl_ddivl.
* sysdeps/ieee754/ldbl-opt/nldbl-compat.h (__nldbl_ddivl): New
prototype.
* manual/arith.texi (Misc FP Arithmetic): Document fdiv, fdivl,
ddivl, fMdivfN, fMdivfNx, fMxdivfN and fMxdivfNx.
* math/auto-libm-test-in: Add tests of div.
* math/auto-libm-test-out-narrow-div: New generated file.
* math/libm-test-narrow-div.inc: New file.
* sysdeps/i386/fpu/s_f32xdivf64.c: Likewise.
* sysdeps/ieee754/dbl-64/s_f32xdivf64.c: Likewise.
* sysdeps/ieee754/dbl-64/s_fdiv.c: Likewise.
* sysdeps/ieee754/float128/s_f32divf128.c: Likewise.
* sysdeps/ieee754/float128/s_f64divf128.c: Likewise.
* sysdeps/ieee754/float128/s_f64xdivf128.c: Likewise.
* sysdeps/ieee754/ldbl-128/s_ddivl.c: Likewise.
* sysdeps/ieee754/ldbl-128/s_f64xdivf128.c: Likewise.
* sysdeps/ieee754/ldbl-128/s_fdivl.c: Likewise.
* sysdeps/ieee754/ldbl-128ibm/s_ddivl.c: Likewise.
* sysdeps/ieee754/ldbl-128ibm/s_fdivl.c: Likewise.
* sysdeps/ieee754/ldbl-96/s_ddivl.c: Likewise.
* sysdeps/ieee754/ldbl-96/s_fdivl.c: Likewise.
* sysdeps/ieee754/ldbl-opt/nldbl-ddiv.c: Likewise.
* sysdeps/ieee754/ldbl-opt/nldbl-fdiv.c: Likewise.
* sysdeps/ieee754/soft-fp/s_ddivl.c: Likewise.
* sysdeps/ieee754/soft-fp/s_fdiv.c: Likewise.
* sysdeps/ieee754/soft-fp/s_fdivl.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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