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;ds=sidebyside

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.
---

diff --git a/src/bitboard.h b/src/bitboard.h
index 715f6c4a..f41abeb2 100644
--- a/src/bitboard.h
+++ b/src/bitboard.h
@@ -291,7 +291,7 @@ inline Square lsb(Bitboard b) {
 
 inline Square msb(Bitboard b) {
   assert(b);
-  return Square(63 - __builtin_clzll(b));
+  return Square(63 ^ __builtin_clzll(b));
 }
 
 #elif defined(_WIN64) && defined(_MSC_VER)