3 ms·

Thanks for the info. Realizing that the two sides of the "if" statement are effectively stored as a 1-bit of information, and then using instructions to store

Thanks for the info. Realizing that the two sides of the "if" statement are effectively stored as a 1-bit of information, and then using instructions to store that 1-bit into the carry flag, and then using cmovcc / sbbl instructions to manipulate that 1-bit of information is pretty advanced. I could imagine a compiler making use of all that information, but only maybe with some programmer assistance (IE: code in the right format for easier optimizing). Or you know... an assembly programmer who just outright explicitly writes that kind of assembly language.

brigade 5y ago
Yeah, this sort of conversion is pretty common for the core of arithmetic/range decoders, and I definitely don't expect a compiler to be capable of the sort of transformations needed to convert the branchy form to branchless. Though I'd expect compilers to be at least 90% as good with the ternary equivalent to the asm, e.g.: tmp = code < bound ? ~0 : 0; range = code < bound ? bound : range; compiles tmp to xor -> setl -> neg (clang) or setge -> movzx -> sub (gcc) instead of sbb, but on arm64 they use just one csetm (clang) or csinv (gcc) which is basically optimal, and in all cases the comparison is reused for the cmov. https://godbolt.org/z/jPbo3qc1Y https://godbolt.org/z/jPbo3qc1Y