On Sun, Jun 06, 2021 at 05:42:30PM +1200, Michael Schmitz wrote: > >Would (port & ~1) be faster than (port - 1) here? > > Perhaps it would - I'd hope the compiler will pick the most efficient > solution here. I'm pretty sure the compiler couldn't do that optimization. Without more context, those two statements are not equivalent. I don't see how the compiler could know that. Looking at the 68040 manual, the instruction timing of ANDI and SUBQ is mostly the same (except for immediate addressing, where ANDI is slower). The ANDI is also going to take up extra bytes in the code to put the bitmask in an extension word. BCLR can be faster than ANDI or SUBQ in some cases, but it's slower in others. It also needs an extension word when using an immediate value for the bit number. If you presume port is in a data register, then ANDI and SUBQ have the same timing and BCLR is slower. At that point, the difference between ANDI and SUBQ is the size of the instruction which would favor using SUBQ. I haven't checked if other m68k chips have similar timings. Unless I missed something, I don't see a better way to do the equivalent of the and with a constant bitmask. Brad Boyer flar@xxxxxxxxxxxxx
