On Fri, Nov 27 2015, Arnd Bergmann <arnd@xxxxxxxx> wrote: > On Friday 27 November 2015 09:53:50 Rasmus Villemoes wrote: >> >> It seems that gcc happily compiles >> >> for (i = 0; i < 1000000000; ++i) ; >> >> into simply >> >> i = 1000000000; >> >> (which is then usually eliminated as a dead store). At least at -O2, and >> when i is not declared volatile. So it would seem that the loops at >> >> arch/mips/pci/pci-rt2880.c:235 >> arch/mips/pmcs-msp71xx/msp_setup.c:80 >> arch/mips/sni/reset.c:35 >> >> actually don't do anything. (In the middle one, i is 'register', but >> that doesn't change anything.) Is mips compiled with some special flags >> that would make gcc actually emit code for the above? >> > > I remember that gcc used to not optimize code that looked like a > delay loop such as the above, and my tests show that this was still > the case in gcc-4.0.3, but starting with gcc-4.1 it opimtized away > that loop. OK, thanks. That's a very very long time ago. FWIW, the remaining instances that my trivial coccinelle script found are ./arch/alpha/boot/main.c:187:1-4: no-op delay loop ./arch/m68k/68000/m68VZ328.c:86:10-13: no-op delay loop ./arch/m68k/bvme6000/config.c:338:2-5: no-op delay loop ./arch/m68k/coldfire/m53xx.c:533:1-4: no-op delay loop ./drivers/cpufreq/cris-artpec3-cpufreq.c:85:3-6: no-op delay loop ./drivers/cpufreq/cris-etraxfs-cpufreq.c:85:3-6: no-op delay loop ./drivers/tty/hvc/hvc_opal.c:313:3-6: no-op delay loop ./drivers/tty/hvc/hvc_vio.c:289:3-6: no-op delay loop (cc += a few people). The tty ones use volatile, so they probably work, though one might still want to use the *delay API. Rasmus
