Date: Wed, 26 Sep 2012 06:46:54 +1000 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Steve Kargl <sgk@troutmask.apl.washington.edu> Cc: numerics@freebsd.org Subject: Re: LDBL_MAX broken on i386 Message-ID: <20120926063517.N3207@besplex.bde.org> In-Reply-To: <20120924231208.GA22960@troutmask.apl.washington.edu> References: <20120925035402.C1433@besplex.bde.org> <20120924195121.GA22138@troutmask.apl.washington.edu> <20120925073508.M2077@besplex.bde.org> <20120924231208.GA22960@troutmask.apl.washington.edu>
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On Mon, 24 Sep 2012, Steve Kargl wrote: > On Tue, Sep 25, 2012 at 08:28:34AM +1000, Bruce Evans wrote: > ... >> LDBL_MAX stopped working with this change to gcc. From gcc/ChangeLog- >> 2003: >> >> % 2003-07-01 Richard Henderson <rth@redhat.com> >> % (blame to: Loren James Rittle <ljrittle@acm.org>) >> % >> % * real.h (ieee_extended_intel_96_round_53_format): New. >> % * real.c (ieee_extended_intel_96_round_53_format): New. >> % * config/i386/freebsd.h (SUBTARGET_OVERRIDE_OPTIONS): Use it >> % for XFmode and TFmode. >> >> This was well enough discussed in FreeBSD lists before it was committed >> (or even developed?), and I agreed with it at the time, but didn't >> understand its full awfulness. gcc-4.2.1 still hard-configures the >> flag that enables this: from gcc/config/freebsd.h: >> >> % /* FreeBSD sets the rounding precision of the FPU to 53 bits. Let the >> % compiler get the contents of <float.h> and std::numeric_limits correct. */ >> % #undef TARGET_96_ROUND_53_LONG_DOUBLE >> % #define TARGET_96_ROUND_53_LONG_DOUBLE (!TARGET_64BIT) > > I just checked the head of gcc, the above is still present. > This suggests to me that i386 FreeBSD will never be free of > the npx feature of setting the FPU to 53 bits. We can always change it in our version. >> so i386 always gets it and amd64 never does. There seems to be no way to >> get 53-bit rounding for expressions without getting it for literals. > > I think you're correct about literals. gcc, since about version > 4.5.x, uses MPFR to do constant-folding and it does this in the > precision of literal constant as determined by gcc. On the bright > side, MPFR claims to correctly round the folding. I thought it already used something sophisticated with correct rounding to do constant folding. > ... >> So gcc's values are perfectly consistent, but are correct if no one >> ever creates values with the lower 11 bits set. > > So, if I understand the above, should we try to correct float.h > to have 21 (36) digits on ld80 (ld128)? Doing we limit LDBL_MAX > on i386 to 2**(emax - 11)? 17 digits are enough on i386. Extras are harmless if correctly rounded. ld128 has 1 extra but I think with correct rounding. The most critical change is the last one. Except it is wrong for clang. The predefines can be used to agreee with the compiler, but I prefer to see the actual values in float.h. That means idefs are required for clang. Several other ifdefs are required for clang. Brucehome | help
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