Subject: Re: [PATCH] x86: bpf_jit_comp: simplify trivial boolean return

From: Joe Perches <[email-masked]>
Date: Wed, 26 Nov 2014 10:02:45 -0800

On Wed, 2014-11-26 at 09:23 -0800, Alexei Starovoitov wrote:
> On Wed, Nov 26, 2014 at 8:58 AM, Joe Perches <[email-masked]> wrote:

> > Is there any value in reordering these tests for frequency
> > or maybe using | instead of || to avoid multiple jumps?
> 
> probably not. It's not a critical path.
> compiler may fuse conditions depending on values anyway.
> If it was a critical path, we could have used
> (1 << reg) & mask trick.
> I picked explicit 'return true' else 'return false' here,
> because it felt easier to read. Just a matter of taste.

There is a size difference though: (allyesconfig)

$ size arch/x86/net/built-in.o*
   text	   data	    bss	    dec	    hex	filename
  12999	   1012	   4336	  18347	   47ab	arch/x86/net/built-in.o.new
  13177	   1076	   4592	  18845	   499d	arch/x86/net/built-in.o.old
---
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 3f62734..09e2cea 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -135,11 +135,11 @@ static const int reg2hex[] = {
  */
 static inline bool is_ereg(u32 reg)
 {
-	if (reg == BPF_REG_5 || reg == AUX_REG ||
-	    (reg >= BPF_REG_7 && reg <= BPF_REG_9))
-		return true;
-	else
-		return false;
+	return (1 << reg) & (BIT(BPF_REG_5) |
+			     BIT(AUX_REG) |
+			     BIT(BPF_REG_7) |
+			     BIT(BPF_REG_8) |
+			     BIT(BPF_REG_9));
 }
 
 /* add modifiers if 'reg' maps to x64 registers r8..r15 */