[RFT/PATCH v2 3/6] x86-64: Don't generate cmov in vread_tsc

Andy Lutomirski
2011-04-07T02:07:03Z

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 | 19 +++++++++++++++----
1 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 858c084..69ff619 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -794,14 +794,25 @@ static cycle_t __vsyscall_fn vread_tsc(void)
*/

/*
- * 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 *)&VVAR(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. I don't barrier() because
+ * we don't actually need a barrier, and if this function
+ * ever gets inlined it will generate worse code.
+ */
+ asm volatile ("");
+ return last;
}
#endif

--
1.7.4