Re: [Minios-devel] [UNIKRAFT PATCH v6 7/7] arch/arm64: Reimplement ukarch_ffsl with gcc builtin

[Santiago Pagani <Santiago.Pagani@xxxxxxxxx>]

Reviewed-by: Santiago Pagani <santiago.pagani@xxxxxxxxx>

On 22.10.19, 09:45, "Minios-devel on behalf of Jia He" 
<minios-devel-bounces@xxxxxxxxxxxxxxxxxxxx on behalf of justin.he@xxxxxxx> 
wrote:

    Using builtin is better as you can let the GCC folks to maintain the
    function.
    
    I've compared the performance between original ffsl and gcc builtin
    on a ThunderX2 host(gcc version 7.4.0), default gcc optimization
    options.
    
     for (x=0; x<0xfffffff; x++)
    
     original:
     real    0m1.723s
     user    0m1.723s
     sys     0m0.000s
    
     gcc builtin:
     real    0m1.550s
     user    0m1.546s
     sys     0m0.004s
    
    Signed-off-by: Jia He <justin.he@xxxxxxx>
    ---
    V6: change the returned bit postion from [1...32] to [0...31]
    
     arch/arm/arm64/include/uk/asm/atomic.h | 25 ++-----------------------
     1 file changed, 2 insertions(+), 23 deletions(-)
    
    diff --git a/arch/arm/arm64/include/uk/asm/atomic.h 
b/arch/arm/arm64/include/uk/asm/atomic.h
    index 431ac84..7319d66 100644
    --- a/arch/arm/arm64/include/uk/asm/atomic.h
    +++ b/arch/arm/arm64/include/uk/asm/atomic.h
    @@ -68,30 +68,9 @@ static inline unsigned int ukarch_fls(unsigned int x)
      *
      * Undefined if no bit exists, so code should check against 0 first.
      */
    -static inline unsigned long ukarch_ffsl(unsigned long word)
    +static inline unsigned long ukarch_ffsl(unsigned long x)
     {
    -   int clz;
    -
    -   /* xxxxx10000 = word
    -    * xxxxx01111 = word - 1
    -    * 0000011111 = word ^ (word - 1)
    -    *      4     = 63 - clz(word ^ (word - 1))
    -    */
    -
    -   __asm__("sub x0, %[word], #1\n"
    -           "eor x0, x0, %[word]\n"
    -           "clz %[clz], x0\n"
    -           :
    -           /* Outputs: */
    -           [clz] "=r"(clz)
    -           :
    -           /* Inputs: */
    -           [word] "r"(word)
    -           :
    -           /* Clobbers: */
    -           "x0");
    -
    -   return 63 - clz;
    +   return __builtin_ffsl(x) - 1;
     }
     
     /**
    -- 
    2.17.1
    
    
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