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SubjectRe: [PATCH] x86: bpf_jit_comp: simplify trivial boolean return
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On Wed, 2014-11-26 at 10:34 -0800, Alexei Starovoitov wrote:
> On Wed, Nov 26, 2014 at 10:02 AM, Joe Perches <joe@perches.com> wrote:
> > On Wed, 2014-11-26 at 09:23 -0800, Alexei Starovoitov wrote:
> >> On Wed, Nov 26, 2014 at 8:58 AM, Joe Perches <joe@perches.com> 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
>
> interesting. Compiler obviously thinks that 178 byte increase
> with -O2 is the right trade off. Which I agree with :)
>
> If I think dropping 'inline' and using -Os will give bigger savings...

This was allyesconfig which already uses -Os

Using -O2, there is no difference using inline
or not, but the size delta with the bitmask is
much larger

$ size arch/x86/net/built-in.o* (allyesconfig, but not -Os)
text data bss dec hex filename
13410 820 3624 17854 45be arch/x86/net/built-in.o.new
16130 884 4200 21214 52de arch/x86/net/built-in.o.old
16130 884 4200 21214 52de arch/x86/net/built-in.o.static




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