Commit-ID: 3729db5ca2b2000c660e5a5d0eb68b1053212cab
Gitweb: http://git.kernel.org/tip/3729db5ca2b2000c660e5a5d0eb68b1053212cab
Author: Andy Lutomirski <luto@xxxxxxx>
AuthorDate: Mon, 23 May 2011 09:31:26 -0400
Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Tue, 24 May 2011 14:51:28 +0200
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@xxxxxxx>
Cc: Andi Kleen <andi@xxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: Eric Dumazet <eric.dumazet@xxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Link: http://lkml.kernel.org/r/%3C561280649519de41352fcb620684dfb22bad6bac.1306156808.git.luto%40mit.edu%3E
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
arch/x86/kernel/tsc.c | 18 ++++++++++++++++--
1 files changed, 16 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 1e62442..24249a5 100644
--- 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
--
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