Hi Paul, > Here's a proposed patch to 'signbit' to have it use 'copysign' if > available.
Globally, I think this is backwards: 'copysign' is a more general function than 'signbit', since it takes 2 arguments. And the usual way to return a boolean result is in an integer register of the CPU, not in a floating- point register. Therefore I think 'signbit' is the primitive function, and I'm going to create modules for 'copysign' that rely on 'signbit'. The complexity translates into code size: gnulib's signbit macro, defined in the <math.h> replacement, expands into a bit test that will always be faster than a call to a function that calls a function that manipulates 2 floating-point numbers. > Normally the 'signbit' implementation relies on undefined behavior, as > it accesses the "wrong" member of a union This is the point of a 'union'. The code is not casting pointers; it is using 'union's for the purpose they were defined for. And the bit positions have been validated in the autoconf test. So where do you see undefined behaviour? > This is safer for traditional platforms like Solaris 9 that > have copysign but not signbit. What do you mean by "safer"? Solaris runs on machines with IEEE floats; on these the signbit.m4 autoconf test will determine the right bit positions. > 2007-04-06 Paul Eggert <[EMAIL PROTECTED]> > > * lib/signbitd.c (gl_signbitd): Use copysign if available. > This avoids relying on undefined behavior. > * lib/signbitf.c (gl_signbitf): Use copysignf if available. > * lib/signbitl.c (gl_signbitl): Use copysignl if available. > * m4/signbit.m4 (gl_SIGNBIT): Test for copysignf, copysign, > copysignl. This patch has two problems: 1) It relies on functions only available in libm. The 'signbit' module should not rely on libm: its module description has no "Link:" section, and it relies on isnan*-nolibm to avoid needing to link with extra libraries. $ ./show-portability copysignf libc macosx-10.3 libcygwin cygwin libm cygwin libm freebsd-5.2.1 libm freebsd-6.0 libm hpux-11.00 libm hpux-11.11 libm netbsd-3.0 libm openbsd-3.8 libm osf1-4.0d libm osf1-5.1a libm solaris-2.10 libroot beos MISSING in aix-4.3.2 aix-5.1.0 irix-6.5 mingw nsk-G06 solaris-2.4 solaris-2.5.1 solaris-2.6 solaris-2.7 solaris-2.8 solaris-2.9 $ ./show-portability copysign libc macosx-10.3 libc nsk-G06 libcygwin cygwin libm aix-4.3.2 libm aix-5.1.0 libm aix-5.1.0 libm cygwin libm freebsd-5.2.1 libm freebsd-6.0 libm hpux-11.00 libm hpux-11.11 libm irix-6.5 libm netbsd-3.0 libm openbsd-3.8 libm osf1-4.0d libm osf1-5.1a libm solaris-2.10 libm solaris-2.4 libm solaris-2.5.1 libm solaris-2.6 libm solaris-2.7 libm solaris-2.8 libm solaris-2.9 libroot beos MISSING in mingw $ ./show-portability copysignl libc macosx-10.3 libm freebsd-6.0 libm osf1-4.0d libm osf1-5.1a libm solaris-2.10 libroot beos MISSING in aix-4.3.2 aix-5.1.0 cygwin freebsd-5.2.1 hpux-11.00 hpux-11.11 irix-6.5 mingw netbsd-3.0 nsk-G06 openbsd-3.8 solaris-2.4 solaris-2.5.1 solaris-2.6 solaris-2.7 solaris-2.8 solaris-2.9 So, instead of checking whether the functions are declared, the autoconf test should use an AC_TRY_LINK check that does not use extra libraries. The C macro name should reflect this: HAVE_COPYSIGN_IN_LIBC, to be consistent with isnan.m4 and printf-frexp.m4. 2) The copysign-based implementation should be used as second alternative: - not as first alternative, for performance reasons, - but before the memcmp-based alternative, because the latter does not do the right thing for NaNs. Bruno
