[issue1770] Patch for i386 to use bsrl for libc fls().

Venkatesh Srinivas (via DragonFly issue tracker) sinknull at leaf.dragonflybsd.org
Tue May 25 22:14:57 PDT 2010

Venkatesh Srinivas <me at acm.jhu.edu> added the comment:


This is an improved version of the patch that corrects my silly jz mistake and 
adds ffsl and flsl. It also implements both for x86_64. The x86-64 version is 
branchless, using cmov.

This alternate snippet may be better and would allow for a branchless f?f(l) on 
i386, except bsr and bsf are defined differently on Intel and AMD x86 machines. 
It may be better on x86_64 than the cmov-based version:
        xorl    %eax, %eax
        xorl    %ecx, %ecx
        bsrl    4(%esp), %eax
        setnz   %cl
        addl    %ecx, %eax

-- vs

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