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Avoid costly 64-bit division in grub_get_time_ms on most platforms.

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This commit is contained in:
Vladimir 'phcoder' Serbinenko 2013-03-10 17:45:38 +01:00
parent fc3ff2a2ad
commit 5341c0fbfc
10 changed files with 70 additions and 93 deletions
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28
grub-core/loader/i386/xnu.c
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@ -125,9 +125,6 @@ guessfsb (void)
{
const grub_uint64_t sane_value = 100000000;
grub_uint32_t manufacturer[3], max_cpuid, capabilities, msrlow;
grub_uint64_t start_tsc;
grub_uint64_t end_tsc;
grub_uint64_t tsc_ticks_per_ms;
if (! grub_cpu_is_cpuid_supported ())
return sane_value;
@ -192,14 +189,6 @@ guessfsb (void)
if (! (capabilities & (1 << 7)))
return sane_value;
/* Calibrate the TSC rate. */
start_tsc = grub_get_tsc ();
grub_pit_wait (0xffff);
end_tsc = grub_get_tsc ();
tsc_ticks_per_ms = grub_divmod64 (end_tsc - start_tsc, 55, 0);
/* Read the multiplier. */
asm volatile ("movl $0x198, %%ecx\n"
"rdmsr"
@ -207,8 +196,21 @@ guessfsb (void)
:
: "%ecx", "%eax");
return grub_divmod64 (2000 * tsc_ticks_per_ms,
((msrlow >> 7) & 0x3e) + ((msrlow >> 14) & 1), 0);
grub_uint64_t v;
grub_uint32_t r;
/* (2000ULL << 32) / grub_tsc_rate */
/* Assumption: TSC frequency is over 2 MHz. */
v = 0xffffffff / grub_tsc_rate;
v *= 2000;
/* v is at most 2000 off from (2000ULL << 32) / grub_tsc_rate.
Since grub_tsc_rate < 2^32/2^11=2^21, so no overflow.
*/
r = (2000ULL << 32) - v * grub_tsc_rate;
v += r / grub_tsc_rate;
return grub_divmod64 (v, ((msrlow >> 7) & 0x3e) | ((msrlow >> 14) & 1),
0);
}
struct property_descriptor