On Wed, 12 Aug 2020 17:35:17 +0300 Konstantin Belousov <kostik...@gmail.com> wrote: > On Wed, Aug 12, 2020 at 01:41:58PM +0200, Tijl Coosemans wrote: >> StgWord64 is uint64_t which is unsigned long long which is 4 byte >> aligned on i386. Clang wants 8 byte alignment to use the fildll >> instruction. > This all is very strange. > > How could code use fildll to load 8 bytes as bit-value ? FILDLL converts > single and double precision fp into long-double fp, so it would change > the bit-value.
FILDLL loads long long and converts it to long double which has 64 bit mantissa so the value is unchanged. > Also, both ISA and x86 psABI only require 4-byte alignment for the > double precision fp variables. > > If the variable memory spans over two cache lines, then SDM states that > the access can be not atomic, but I believe it cannot happen for any > existing CPU. It might be slow. For some future CPUs, Intel provides > control which would cause such accesses to trap. Well, it seems clang follows the SDM and assumes 8 byte alignment is needed because then the variable never crosses a cache line boundary. _______________________________________________ freebsd-toolchain@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/freebsd-toolchain To unsubscribe, send any mail to "freebsd-toolchain-unsubscr...@freebsd.org"
