Date: Fri, 05 Jun 2015 09:09:26 +0200 From: Hans Petter Selasky <hps@selasky.org> To: FreeBSD Current <freebsd-current@freebsd.org> Subject: [CFR] Replacing while loops with proper division and multiplication Message-ID: <55714B26.6060802@selasky.org>
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Hi,
I was going through some timer code and found some unnecessary while
loops in kern/kern_clocksource.c .
I added some prints and found that during boot, "runs" can exceed 2000,
while during regular usage runs is typically 1. Do you think it is worth
to convert these loops into division and multiplications? It might make
the CPU pipeline a tiny bit faster, having to skip some conditionals?
And also possibly improve readability?
What do you think?
--HPS
> Index: kern/kern_clocksource.c
> ===================================================================
> --- kern/kern_clocksource.c (revision 283606)
> +++ kern/kern_clocksource.c (working copy)
> @@ -155,10 +155,11 @@
> handleevents(sbintime_t now, int fake)
> {
> sbintime_t t, *hct;
> + sbintime_t runs;
> struct trapframe *frame;
> struct pcpu_state *state;
> int usermode;
> - int done, runs;
> + int done;
>
> CTR3(KTR_SPARE2, "handle at %d: now %d.%08x",
> curcpu, (int)(now >> 32), (u_int)(now & 0xffffffff));
> @@ -173,12 +174,10 @@
>
> state = DPCPU_PTR(timerstate);
>
> - runs = 0;
> - while (now >= state->nexthard) {
> - state->nexthard += tick_sbt;
> - runs++;
> - }
> - if (runs) {
> + runs = (now - state->nexthard) / tick_sbt;
> + if (runs > 0) {
> + printf("R%d ", (int)runs);
> + state->nexthard += tick_sbt * runs;
> hct = DPCPU_PTR(hardclocktime);
> *hct = state->nexthard - tick_sbt;
> if (fake < 2) {
> @@ -186,25 +185,25 @@
> done = 1;
> }
> }
> - runs = 0;
> - while (now >= state->nextstat) {
> - state->nextstat += statperiod;
> - runs++;
> + runs = (now - state->nextstat) / statperiod;
> + if (runs > 0) {
> + printf("S%d ", (int)runs);
> + state->nextstat += statperiod * runs;
> + if (fake < 2) {
> + statclock_cnt(runs, usermode);
> + done = 1;
> + }
> }
> - if (runs && fake < 2) {
> - statclock_cnt(runs, usermode);
> - done = 1;
> - }
> if (profiling) {
> - runs = 0;
> - while (now >= state->nextprof) {
> - state->nextprof += profperiod;
> - runs++;
> + runs = (now - state->nextprof) / profperiod;
> + if (runs > 0) {
> + printf("T%d ", (int)runs);
> + state->nextprof += profperiod * runs;
> + if (!fake) {
> + profclock_cnt(runs, usermode, TRAPF_PC(frame));
> + done = 1;
> + }
> }
> - if (runs && !fake) {
> - profclock_cnt(runs, usermode, TRAPF_PC(frame));
> - done = 1;
> - }
> } else
> state->nextprof = state->nextstat;
> if (now >= state->nextcallopt) {
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