http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55760
--- Comment #4 from Dominique d'Humieres <dominiq at lps dot ens.fr> 2012-12-20 16:07:11 UTC --- > is there any reason why rsqrtss and rcpss are not used for scalar code while > rsqrtps and rcpps are used for loops? Yep! I don't have the patience to dig the bugzilla archive right now, but the main reason is related to a loss of accuracy (especially 1/2.0 != 0.5) leading to problems in some codes (see gas_dyn.f90 in the polyhedron tests). You can pass options to force the use of rsqrtss and rcpss for scalars: -mrecip This option enables use of RCPSS and RSQRTSS instructions (and their vectorized variants RCPPS and RSQRTPS) with an additional Newton-Raphson step to increase precision instead of DIVSS and SQRTSS (and their vectorized variants) for single-precision floating-point arguments. These instructions are generated only when -funsafe-math-optimizations is enabled together with -finite-math-only and -fno-trapping-math. Note that while the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994). Note that GCC implements 1.0f/sqrtf(x) in terms of RSQRTSS (or RSQRTPS) already with -ffast-math (or the above option combination), and doesn't need -mrecip. Also note that GCC emits the above sequence with additional Newton-Raphson step for vectorized single-float division and vectorized sqrtf(x) already with -ffast-math (or the above option combination), and doesn't need -mrecip. -mrecip=opt This option controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, which may be preceded by a `!' to invert the option: `all' Enable all estimate instructions. `default' Enable the default instructions, equivalent to -mrecip. `none' Disable all estimate instructions, equivalent to -mno-recip. `div' Enable the approximation for scalar division. `vec-div' Enable the approximation for vectorized division. `sqrt' Enable the approximation for scalar square root. `vec-sqrt' Enable the approximation for vectorized square root. So, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations, except for square root.
