Presumably due to -fno-strict-aliasing, gcc doesn't know that the stores through vdata do not alter the value of tk->tkr_mono.shift. So e.g. in the loop normalizing vdata->monotonic_time_snsec, gcc reloads tk->tkr_mono.shift and recomputes the ((u64)NSEC_PER_SEC) << tk->tkr_mono.shift constant. While that loop is usually never entered, so the constant is only used once, we might as well make the generated code a little smaller and microscopically faster when we do enter the loop. So make a local copy of shift and use that.
Do not do the assignment in the declaration, because there's another shift operation (1 << vclock_mode) before we get to use the shift variable, and there's no point in having gcc load it to some non-%ecx register, or, on 32 bit, immediately spill it. As a bonus, two awkward line breaks vanish. bloat-o-meter: Function old new delta update_vsyscall 480 398 -82 (i386) update_vsyscall 304 281 -23 (x86_64) Signed-off-by: Rasmus Villemoes <[email protected]> --- Other tricks are possible (reuse the initial monotonic_time computation for the _coarse version, maybe alias vdata->wall_time{,_coarse}_sec via a union (and rearrange to make sure both are in the same cache line as their nsec buddy)), but I'm not sure even this one is worth it. arch/x86/entry/vsyscall/vsyscall_gtod.c | 13 ++++++------- 1 file changed, 6 insertions(+), 7 deletions(-) diff --git a/arch/x86/entry/vsyscall/vsyscall_gtod.c b/arch/x86/entry/vsyscall/vsyscall_gtod.c index e1216dd95c04..550447a43a50 100644 --- a/arch/x86/entry/vsyscall/vsyscall_gtod.c +++ b/arch/x86/entry/vsyscall/vsyscall_gtod.c @@ -31,6 +31,7 @@ void update_vsyscall(struct timekeeper *tk) { int vclock_mode = tk->tkr_mono.clock->archdata.vclock_mode; struct vsyscall_gtod_data *vdata = &vsyscall_gtod_data; + int shift; /* Mark the new vclock used. */ BUILD_BUG_ON(VCLOCK_MAX >= 32); @@ -43,7 +44,7 @@ void update_vsyscall(struct timekeeper *tk) vdata->cycle_last = tk->tkr_mono.cycle_last; vdata->mask = tk->tkr_mono.mask; vdata->mult = tk->tkr_mono.mult; - vdata->shift = tk->tkr_mono.shift; + vdata->shift = shift = tk->tkr_mono.shift; vdata->wall_time_sec = tk->xtime_sec; vdata->wall_time_snsec = tk->tkr_mono.xtime_nsec; @@ -52,17 +53,15 @@ void update_vsyscall(struct timekeeper *tk) + tk->wall_to_monotonic.tv_sec; vdata->monotonic_time_snsec = tk->tkr_mono.xtime_nsec + ((u64)tk->wall_to_monotonic.tv_nsec - << tk->tkr_mono.shift); - while (vdata->monotonic_time_snsec >= - (((u64)NSEC_PER_SEC) << tk->tkr_mono.shift)) { - vdata->monotonic_time_snsec -= - ((u64)NSEC_PER_SEC) << tk->tkr_mono.shift; + << shift); + while (vdata->monotonic_time_snsec >= (((u64)NSEC_PER_SEC) << shift)) { + vdata->monotonic_time_snsec -= ((u64)NSEC_PER_SEC) << shift; vdata->monotonic_time_sec++; } vdata->wall_time_coarse_sec = tk->xtime_sec; vdata->wall_time_coarse_nsec = (long)(tk->tkr_mono.xtime_nsec >> - tk->tkr_mono.shift); + shift); vdata->monotonic_time_coarse_sec = vdata->wall_time_coarse_sec + tk->wall_to_monotonic.tv_sec; -- 2.15.1
