From: Arnd Bergmann <arnd@xxxxxxxx> Date: Wed, 25 May 2016 23:01:06 +0200 > On Wednesday, May 25, 2016 1:50:39 PM CEST David Miller wrote: >> From: Arnd Bergmann <arnd@xxxxxxxx> >> Date: Wed, 25 May 2016 22:47:33 +0200 >> >> > If we use the normal calling conventions, we could remove these overrides >> > along with the respective special-case handling in glibc. None of them >> > look particularly performance-sensitive, but I could be wrong there. >> >> You could set the lowest bit in the system call entry pointer to indicate >> the upper-half clears should be elided. > > Right, but that would introduce an extra conditional branch in the syscall > hotpath, and likely eliminate the gains from passing the loff_t arguments > in a single register instead of a pair. Ok, then, how much are you really gaining from avoiding a 'shift' and an 'or' to build the full 64-bit value? 3 cycles? Maybe 4? And the executing the wrappers, those have a non-trivial cost too. Cost wise, this seems like it all cancels out in the end, but what do I know? -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
