Date: Fri, 17 Aug 2007 16:49:04 +1000 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Simun Mikecin <numisemis@yahoo.com> Cc: freebsd-gnats-submit@freebsd.org, freebsd-amd64@freebsd.org Subject: Re: amd64/115581: [fix] -mfancy-math-387 has no effect Message-ID: <20070817150656.V27406@delplex.bde.org> In-Reply-To: <200708161642.l7GGg0Vu056249@www.freebsd.org> References: <200708161642.l7GGg0Vu056249@www.freebsd.org>
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On Thu, 16 Aug 2007, Simun Mikecin wrote: >> Description: > 32-bit compatibility libraries on FreeBSD/amd64 are compiled using -mfancy-math-387 gcc option. As stated in gcc(1): > > -mno-fancy-math-387 > Some 387 emulators do not support the "sin", "cos" and "sqrt" > instructions for the 387. Specify this option to avoid generating > those instructions. This option is the default on FreeBSD, OpenBSD > and NetBSD. This option is overridden when -march indicates that > the target cpu will always have an FPU and so the instruction will > not need emulation. As of revision 2.6.1, these instructions are > not generated unless you also use the -funsafe-math-optimizations > switch. > > So, using just -mfancy-math-387 has no effect. It should be used in combination with -funsafe-math-optimizations or it should not be used. It should not be used, especially on amd64 systems since basic FP instructions are relatively fast compared with the fancy instructions (except for sqrt). The 64-bit amd64 libm intentionally never uses the fancy instructions (except for sqrt), partly because they are not much faster and partly because they are much less accurate. The fancy instructions are not used for float precision (unless you pessimize things using -mfancy-math-387) since they are about 3 times slower than the library versions on small args. Is -mno-fancy-math-387 still actually the default on FreeBSD (with FreeBSD's config/i386/freebsd.h which is quite different (mostly gratuitously different) from the distribution one)? FreeBSD hasn't supported the math emulator for about 10 years. > --- Makefile.inc1.orig Tue Jul 10 18:39:36 2007 > +++ Makefile.inc1 Thu Aug 16 18:30:44 2007 > @@ -238,7 +238,7 @@ > .else > LIB32CPUTYPE= ${TARGET_CPUTYPE} > .endif > -LIB32FLAGS= -m32 -march=${LIB32CPUTYPE} -mfancy-math-387 -DCOMPAT_32BIT \ > +LIB32FLAGS= -m32 -march=${LIB32CPUTYPE} -mfancy-math-387 -funsafe-math-optimizations -DCOMPAT_32BIT \ > -iprefix ${LIB32TMP}/usr/ \ > -L${LIB32TMP}/usr/lib32 \ > -B${LIB32TMP}/usr/lib32 -unsafe-math-optimizations should be named -broken-floating-point-optimizations and should almost never be used. It should never be used for compiling FreeBSD's math library, since the library depends on floating point not being very broken. gcc-4.2 still says the above, but doesn't actually do the above for sqrt. It inlines sqrt (but not cos or sin) without -funsafe-math-optimizations. I think the difference is just due to inlining sqrt not actually being unsafe and the documentation of -ffancy-math-387 being many years out of date (this difference is not new). Inlining cos and sin would be safe if the inline code were large enough to detect the unsafe cases, but the inline code only checks for the result being a NaN (?), which is more than enough for sqrt but not enough for cos or sin. gcc (at least in 4.2) has a -fno-math-errno option which defaults to the wrong thing for FreeBSD (-fmath-errno) but the correct think on Darwin. -funsafe-math-optimizations apparently has the apparently-undocumented effect of turning off on -fno-math-errno. So -march=pentium4 gives the following inlining of the fancy functions: Default: sqrt: inlined, bogus errno handing cos, sin: not inlined with -fno-math-errno: sqrt: inlined, optimal cos, sin: not inlined with -funsafe-math-optimizations: sqrt: inlined, optimal cos, sin: inlined, broken (additional breakage only for large args) cosf: inlined, optimal cosf, sinf: inlined, broken (for large args, and small args near a multiple of pi/2), pessimized (only for small args) Bruce
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