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Subject[PATCH 2/6] x86-64: Don't generate cmov in vread_tsc
Date
vread_tsc checks whether rdtsc returns something less than
cycle_last, which is an extremely predictable branch. GCC likes
to generate a cmov anyway, which is several cycles slower than
a predicted branch. This saves a couple of nanoseconds.

Signed-off-by: Andy Lutomirski <luto@mit.edu>
---
arch/x86/kernel/tsc.c | 15 +++++++++++----
1 files changed, 11 insertions(+), 4 deletions(-)
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 80e6017..a159fba 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -791,14 +791,21 @@ static cycle_t __vsyscall_fn vread_tsc(void)
* ensures that rdtsc is ordered wrt all later loads.
*/

- /* This doesn't multiply 'zero' by anything, which *should*
- * generate nicer code, except that gcc cleverly embeds the
- * dereference into the cmp and the cmovae. Oh, well.
+ /* This doesn't multiply 'zero' by anything, which generates
+ * very slightly nicer code than multiplying it by 8.
*/
last = *( (cycle_t *)
((char *)&__vsyscall_gtod_data.clock.cycle_last + zero) );

- return ret >= last ? ret : last;
+ if (likely(ret >= last))
+ return ret;
+
+ /* GCC likes to generate cmov here, but this branch is extremely
+ predictable (it's just a funciton of time and the likely is
+ very likely) and there's a data dependence, so force GCC
+ to generate a branch instead. */
+ asm volatile ("");
+ return last;
}
#endif

--
1.7.4


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