Date: Thu, 14 Mar 2019 12:59:14 -0700 From: John Baldwin <jhb@FreeBSD.org> To: Konstantin Belousov <kostikbel@gmail.com>, Peter Jeremy <peter@rulingia.com> Cc: Steve Kargl <sgk@troutmask.apl.washington.edu>, freebsd-toolchain@freebsd.org, freebsd-current@freebsd.org Subject: Re: Optimization bug with floating-point? Message-ID: <99ad567f-a1de-b3be-af4b-456df116bee7@FreeBSD.org> In-Reply-To: <20190314192000.GI2492@kib.kiev.ua> 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> <20190314192000.GI2492@kib.kiev.ua>
next in thread | previous in thread | raw e-mail | index | archive | help
On 3/14/19 12:20 PM, Konstantin Belousov wrote: > 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. So what I don't understand then is what is gcc doing different than clang in this case. I assume neither GCC _nor_ clang are adjusting the FPU in compiler-generated code, and in fact as Steve's earlier tests shows, the precision is set to PD by default when a clang-built binary is run. -- John Baldwin
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?99ad567f-a1de-b3be-af4b-456df116bee7>