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 >> <s...@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 _______________________________________________ freebsd-current@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/freebsd-current To unsubscribe, send any mail to "freebsd-current-unsubscr...@freebsd.org"
