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Commit 99c19e6a authored by Andy Lutomirski's avatar Andy Lutomirski Committed by Thomas Gleixner
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x86/vdso: Rearrange do_hres() to improve code generation



vgetcyc() is full of barriers, so fetching values out of the vvar
page before vgetcyc() for use after vgetcyc() results in poor code
generation.  Put vgetcyc() first to avoid this problem.

Also, pull the tv_sec division into the loop and put all the ts
writes together.  The old code wrote ts->tv_sec on each iteration
before the syscall fallback check and then added in the offset
afterwards, which forced the compiler to pointlessly copy base->sec
to ts->tv_sec on each iteration.  The new version seems to generate
sensible code.

Saves several cycles.  With this patch applied, the result is faster
than before the clock_gettime() rewrite.

Signed-off-by: default avatarAndy Lutomirski <luto@kernel.org>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/3c05644d010b72216aa286a6d20b5078d5fae5cd.1538762487.git.luto@kernel.org
parent bcc4a62a
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