x86-64: Don't generate cmov in vread_tsc

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>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Borislav Petkov <bp@amd64.org>
Link: http://lkml.kernel.org/r/%3C561280649519de41352fcb620684dfb22bad6bac.1306156808.git.luto%40mit.edu%3E
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 1e6244202612d1588fcf3deba28f13e935c28b58..24249a5360b6a2eb7f5d558631ca31f563c36bca 100644 (file)
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -767,6 +767,7 @@ static cycle_t read_tsc(struct clocksource *cs)
 static cycle_t __vsyscall_fn vread_tsc(void)
 {
        cycle_t ret;
+       u64 last;
 
        /*
         * Empirically, a fence (of type that depends on the CPU)
@@ -778,8 +779,21 @@ static cycle_t __vsyscall_fn vread_tsc(void)
        rdtsc_barrier();
        ret = (cycle_t)vget_cycles();
 
-       return ret >= VVAR(vsyscall_gtod_data).clock.cycle_last ?
-               ret : VVAR(vsyscall_gtod_data).clock.cycle_last;
+       last = VVAR(vsyscall_gtod_data).clock.cycle_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