Hello. As mentioned in [1], we does not fully cover all i386 target options in [2]. Apart from that, flag explanation is more brief than we've got in [3].
One example: [3]: -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 -ffinite-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. [2]: ‘recip’ ‘no-recip’ Enable/disable the generation of RCPSS, RCPPS, RSQRTSS and RSQRTPS instructions followed an additional Newton-Raphson step instead of doing a floating-point division. I'm thinking whether it would be possible to tell that 'target (options)' accepts all options as mentioned in [3], expect ... If it's acceptable opinion, I can prepare a patch. Thanks, Martin [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71652#c4 [2] https://gcc.gnu.org/onlinedocs/gcc/x86-Function-Attributes.html#x86-Function-Attributes [3] https://gcc.gnu.org/onlinedocs/gcc-6.1.0/gcc/x86-Options.html#x86-Options
