[PATCH 2/6] x86-64: Don't generate cmov in vread_tsc

[Andy Lutomirski]

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@xxxxxxx>
---
 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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