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

[Andy Lutomirski <luto@MIT.EDU>]

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