From: Andrey Neporada <neporada@gmail.com>
Date: Sat, 3 Dec 2016 08:37:07 +0000 (+0400)
Subject: Help GCC to optimize msb() to single instruction
X-Git-Url: https://git.sesse.net/?p=stockfish;a=commitdiff_plain;h=bf8b45fe6305c89a9a30f660de34164cf3bdcdf2;hp=bf8b45fe6305c89a9a30f660de34164cf3bdcdf2

Help GCC to optimize msb() to single instruction

GCC compiles builtin_clzll to “63 ^ BSR”. BSR is processor instruction "Bit Scan Reverse".
So old msb() function is basically 63 - 63 ^ BSR.
Unfortunately, GCC fails to simplify this expression.

Old function compiles to

    bsrq    %rdi, %rdi
    movl    $63, %eax
    xorq    $63, %rdi
    subl    %edi, %eax
    ret

New function compiles to

    bsrq    %rdi, %rax
    ret

BTW, Clang compiles both function to the same (optimal) code.

No functional change.
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