Author: Dezhi Zhao
Date: 08:49:58 01/16/99
Go up one level in this thread
On January 16, 1999 at 10:00:51, Bo Persson wrote:
>This is my solution for VC6, not perfect but working!
>
>// warning C4035: 'BSF' : no return value
>#pragma warning(disable : 4035)
>
> inline unsigned BSF(unsigned Bits)
> {
> __asm mov eax,[Bits]
> __asm bsf eax,eax
> }
>
>#pragma warning(default : 4035)
I'm glad we share the same experience. This inline BSF func is equivalent
to my second approach in my post. However inlining your BSF() into
a C function will affect the optimization. BSF() uses eax and the VC
compiler will generate the 2 lines as you write verbatim. If eax has been
taken by another varible, which is very likely to happen, the complier
has to generate extra save/reload instructions. It would be better if the
compiler has a directive (hint to compiler), saying eax can be renamed
during compiling time.
>
> inline SQUARE BitBoard::GetLowMember()
> {
> if (Half[0] != 0)
> {
> const unsigned BitFound = BSF(Half[0]);
> Half[0] ^= (1 << BitFound);
> return static_cast<SQUARE>(BitFound);
> }
> else
> {
> const unsigned BitFound = BSF(Half[1]);
> Half[1] ^= (1 << BitFound);
> return static_cast<SQUARE>(BitFound + 4 * FILES);
> }
>
> } /* GetLowMember */
>
>
>VC6 still puts the argument 'Bits' on the stack and reloads it, so you loose a
>few instructions but still come out faster *on a PII*.
>
>Compared to a 16-bit table lookup, the BSF is faster on a PII, makes no
>difference on an AMD K6 and is actually *slower* on a Pentium 133!
>
>
>
>BTW, you don't have to invent new operators for C/C++ to improve inlining. Many
>years ago I used the TopSpeed C++ that solved regsiter assignment with special
>pragmas, like:
>
>#pragma call(reg_param=>(cx,dx), reg_return=>ax)
>
>Worked fine!
>
>
>
>Bo Persson
>bop@malmo.mail.telia.com
I agree that a directive will also do that. But a new operator can even
save the asm stuff.
is TopSpeed C++ a 32 bit one? How about code quality generated by it,
especially compared with VC6?
Dezhi Zhao
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