Date: Thu, 14 Mar 2019 21:20:00 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Peter Jeremy <peter@rulingia.com> Cc: Steve Kargl <sgk@troutmask.apl.washington.edu>, John Baldwin <jhb@freebsd.org>, freebsd-toolchain@freebsd.org, freebsd-current@freebsd.org Subject: Re: Optimization bug with floating-point? Message-ID: <20190314192000.GI2492@kib.kiev.ua> In-Reply-To: <20190314185037.GI87064@server.rulingia.com> References: <20190313024506.GA31746@troutmask.apl.washington.edu> <20190313151635.GA34757@troutmask.apl.washington.edu> <e290b68f-7a1d-2456-4a0c-9f7dfd303f55@FreeBSD.org> <20190313164039.GA35340@troutmask.apl.washington.edu> <e638e0bf-b8d0-9bd1-d034-8acc6eef872c@FreeBSD.org> <20190313212455.GA37717@troutmask.apl.washington.edu> <20190314063007.GA41995@troutmask.apl.washington.edu> <20190314185037.GI87064@server.rulingia.com>
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On Fri, Mar 15, 2019 at 05:50:37AM +1100, Peter Jeremy wrote: > On 2019-Mar-13 23:30:07 -0700, Steve Kargl <sgk@troutmask.apl.washington.edu> wrote: > >AFAICT, all libm float routines need to be modified to conditional > >include ieeefp.h and call fpsetprec(FP_PD). This will work around > >issues is FP and libm. FreeBSD needs to issue an erratum about > >the numerical issues with clang. > > I vaguely recall looking into the x87 initialisation a long time ago > and STR that the startup code (either crtX or in the kernel) does > a fninit() to set the precision. I don't recall exactly where. At boot, a clean initial FPU state is stored in fpu_initialstate. Then on first FPU access from userspace (first for the given process context), this saved state is copied into hardware registers. The quirk is that for i386 binaries on amd64, we adjust fpu control word to what is expected by i386 binaries. > > IMO, calling fpsetprec() in every libm float function is overkill. It > should be enough to fpsetprec() before main() and add a note in the > man pages that libm is built to use the default FPU configuration and > changing the configuration (precision or rounding) may result in larger > errors. Changing default precision in crt1 would break the ABI.
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