[Bug libc/24268] bench-ffsll performance degradation on glibc 2.29

[kangshan0910 at hotmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=24268

--- Comment #2 from Shan <kangshan0910 at hotmail dot com> ---
Thanks for your quick response. I tried perf to collect the performance data,
the cache miss is very little and the cycle counts are like the following.

With the commit
Samples: 40K of event 'cycles', Event count (approx.): 46652464962
Children Samples  Command   Shared Object Symbol 
42.00%    17166 bench-ffsll libc-2.28.so [.] ffsll
23.90%    9766  bench-ffsll  bench-ffsll [.] main
Samples: 40K of event 'cycles', 4000 Hz, Event count (approx.): 46652464962
ffsll  /usr/lib64/haswell/libc-2.28.so [Percent: local period]
Percent│
       │
       │
       │    Disassembly of section .text:
       │
       │    00000000000a07f0 <ffsll@@GLIBC_2.2.5>:
 24.81 │      endbr64
       │      mov    $0xffffffff,%edx
       │      bsf    %rdi,%rax
 50.39 │      cmove  %rdx,%rax
  0.36 │      inc    %eax
 24.45 │    ← retq 

Witout the commit
Samples: 40K of event 'cycles', Event count (approx.): 46691907747
Children Samples  Command   Shared Object Symbol
34.97%   14297  bench-ffsll libc-2.28.so [.] ffsll
26.78%   10932  bench-ffsll bench-ffsll  [.] main
Samples: 40K of event 'cycles', 4000 Hz, Event count (approx.): 46691907747
ffsll  /usr/lib64/haswell/libc-2.28.so [Percent: local period]
Percent│
       │
       │
       │    Disassembly of section .text:
       │
       │    00000000000a0310 <ffsll@@GLIBC_2.2.5>:
 33.00 │      endbr64
  0.04 │      mov    $0xffffffff,%edx
  0.10 │      bsf    %rdi,%rax
  0.12 │      cmove  %rdx,%rax
 33.93 │      inc    %eax
 32.81 │    ← retq

I also suspected that it was caused by the libc binary layout change. But I
failed to prove that. Do you have any suggestions?

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