Re: [PATCH] bitops/32: Convert variable_ffs() and fls() zero-case handling to C

[Ingo Molnar <mingo@xxxxxxxxxx>]

* Ingo Molnar <mingo@xxxxxxxxxx> wrote:

> And once we remove 486, I think we can do the optimization below to 
> just assume the output doesn't get clobbered by BS*L in the 
> zero-case, right?
> 
> In the text size space it's a substantial optimization on x86-32 
> defconfig:
> 
>         text     data        bss           dec            hex filename
>   16,577,728    7598826    1744896      25921450        18b87aa 
> vmlinux.vanilla      # CMOV+BS*L
>   16,577,908  7598838    1744896      25921642        18b886a 
> vmlinux.linus_patch  # if()+BS*L
>   16,573,568  7602922    1744896      25921386        18b876a 
> vmlinux.noclobber    # BS*L

And BTW, *that* is a price that all of non-486 x86-32 was paying for 
486 support...

And, just out of intellectual curiosity, I also tried to measure the 
code generation price of the +1 standards-quirk in the fls()/ffs() 
interface as well:

         text      data        bss           dec            hex filename
   16,577,728   7598826    1744896      25921450        18b87aa vmlinux.vanilla 
     # CMOV+BS*L
   16,577,908   7598838    1744896      25921642        18b886a 
vmlinux.linus_patch  # if()+BS*L
   16,573,568   7602922    1744896      25921386        18b876a 
vmlinux.noclobber    # BS*L
   ..........
   16,573,552   7602922    1744896      25921370        18b875a vmlinux.broken  
     # BROKEN: 0 baseline instead of 1

... and unless I messed up the patch, it seems to have a surprisingly 
low impact - maybe because the compiler can amortize its cost by 
adjusting all dependent code mostly at build time, so the +1 doesn't 
end up being generated most of the time?

Thanks,

        Ingo

===============================>

This broken patch is broken: it intentionally breaks the ffs()/fls() 
interface in an attempt to measure the code generation effects of 
interface details.

NOT-Signed-off-by: <anyone@anywhere.anytime>
---
 arch/x86/include/asm/bitops.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index e3e94a806656..21707696bafe 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -318,7 +318,7 @@ static __always_inline int variable_ffs(int x)
            : "=r" (r)
            : ASM_INPUT_RM (x), "0" (-1));
 
-       return r + 1;
+       return r;
 }
 
 /**
@@ -362,7 +362,7 @@ static __always_inline int fls(unsigned int x)
            : "=r" (r)
            : ASM_INPUT_RM (x), "0" (-1));
 
-       return r + 1;
+       return r;
 }
 
 /**