[PATCH] x86/asm/irq: Don't use POPF but STI

[Ingo Molnar]

So because the POPF instruction is slow and STI is faster on 
essentially all x86 CPUs that matter, instead of:

  ffffffff81891848:       9d                      popfq

we can do:

  ffffffff81661a2e:       41 f7 c4 00 02 00 00    test   $0x200,%r12d
  ffffffff81661a35:       74 01                   je     ffffffff81661a38 <snd_pcm_stream_unlock_irqrestore+0x28>
  ffffffff81661a37:       fb                      sti
  ffffffff81661a38:

This bloats the kernel a bit, by about 1K on the 64-bit defconfig:

   text    data     bss     dec     hex filename
   12258634        1812120 1085440 15156194         e743e2 vmlinux.before
   12259582        1812120 1085440 15157142         e74796 vmlinux.after

the other cost is the extra branching, adding extra pressure to the
branch prediction hardware and also potential branch misses.
---
 arch/x86/include/asm/irqflags.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/irqflags.h b/arch/x86/include/asm/irqflags.h
index b77f5edb03b0..8bc2a9bc7a06 100644
--- a/arch/x86/include/asm/irqflags.h
+++ b/arch/x86/include/asm/irqflags.h
@@ -69,7 +69,8 @@ static inline notrace unsigned long arch_local_save_flags(void)
 
 static inline notrace void arch_local_irq_restore(unsigned long flags)
 {
-	native_restore_fl(flags);
+	if (likely(flags & X86_EFLAGS_IF))
+		native_irq_enable();
 }
 
 static inline notrace void arch_local_irq_disable(void)

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