Re: sub-optimal code for packed boolean arrays -- bug or inherent limitation

["Jeffrey R. Carter" <spam.jrcarter.not@acm.nospam.org>]

Alinabi wrote:
> 
> The only notable exception is procedure Flip, which becomes
> 
> procedure Flip(B : in out Bitboard_T; I : in Index_T) is
>   30:   89 f1                   mov    %esi,%ecx
>     begin
>         B(i) := not B(i);
>   32:   48 c7 c0 fe ff ff ff    mov    $0xfffffffffffffffe,%rax
>   39:   48 d3 c0                   rol    %cl,%rax
>   3c:   48 21 f8                   and   %rdi,%rax
>   3f:   48 d3 ef                    shr    %cl,%rdi
>   42:   83 f7 01                   xor    $0x1,%edi
>   45:   83 e7 01                  and    $0x1,%edi
>   48:   48 d3 e7                  shl     %cl,%rdi
>   4b:   48 09 f8                   or      %rdi,%rax
>     end;
>   4e:   c3                             retq
> 
> instead of the shorter
> 
> mov  %esi,  %ecx
> mov  0x1,   %rax
> shl    %cl,    %rax
> xor    % rax, %rdx
> retq
> 
> I don't know much (if anything) about compiler internals, so I was
> wondering if I should file a bug report. Is there some good
> theoretical justification for all that extraneous shifts and
> rotations? I would think that if the compiler can figure out the best
> way to set or clear a bit in a register using shift and logical ops,
> than it should also be able to negate it efficiently.

The code gives the correct results, so it can't be a compiler bug. 
Sometimes a compiler will do better than hand-coded assembler; sometimes 
it will do worse. I doubt the compiler's code will make your chess 
program fail to meet its timing requirements; I doubt if you'll notice 
the difference at all, so I don't see why you care. If you must have a 
specific sequence of machine code operations, you should use a 
machine-code insertion.

-- 
Jeff Carter
"Strange women lying in ponds distributing swords
is no basis for a system of government."
Monty Python & the Holy Grail
66