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Teach the compiler how to shift TSC value efficiently. As noted in r220631,

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some times compiler inserts redundant instructions to preserve unused upper
32 bits even when it is casted to a 32-bit value.  Unfortunately, it seems
the problem becomes more serious when it is shifted, especially on amd64.
This commit is contained in:
jkim 2011-06-17 21:41:06 +00:00
parent 93d2d762ed
commit 8a9fdbb838
1 changed files with 5 additions and 2 deletions
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7
sys/x86/x86/tsc.c
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@ -461,7 +461,7 @@ init_TSC_tc(void)
tsc_timecounter.tc_quality = 1000;
init:
for (shift = 0; shift < 32 && (tsc_freq >> shift) > max_freq; shift++)
for (shift = 0; shift < 31 && (tsc_freq >> shift) > max_freq; shift++)
;
if (shift > 0) {
tsc_timecounter.tc_get_timecount = tsc_get_timecount_low;
@ -579,6 +579,9 @@ tsc_get_timecount(struct timecounter *tc __unused)
static u_int
tsc_get_timecount_low(struct timecounter *tc)
{
uint32_t rv;
return (rdtsc() >> (int)(intptr_t)tc->tc_priv);
__asm __volatile("rdtsc; shrd %%cl, %%edx, %0"
: "=a" (rv) : "c" ((int)(intptr_t)tc->tc_priv) : "edx");
return (rv);
}