Author: Robert Hyatt
Date: 08:13:43 01/16/99
Go up one level in this thread
On January 16, 1999 at 07:27:53, Dezhi Zhao wrote:
>Hi!
>
>I used bsf and bsr extensively in my program writen with VC.
>As I read your FisrtOne()/LastOne() asm functions, they reminded me of
>the difficulties in using them. I would say your FisrtOne()/LastOne()
>are perfect x86 asm programming. However it is perfect at function level.
>As you know these functions are really short in code size, it would be
>ideal to make them inline. I tried one year ago, but failed because
>VC does not allow a function containing asm statement inlined.
>The documentation says the compiler copy the asm code as it is.
>Apparently there is no register renaming facility for VC compiler to
>make the asm function to be inlined.
get the crafty source and look at x86.c... you can inline asm routines
quite easily. That file has the stuff needed to do it...
Bob
>And I still leave the bsf/bsr stuff in a fuction, which really makes me
>unhappy. Here is my FisrtOne() for int32:
>
>int _fastcall FisrtOne(int map)
>{
> _asm bsf eax, ecx
>}
>
>You will certainly agree with me that the function call overhead
>is much bigger than the function body, an asm instruction that takes
>only 1/2 clocks in P6.
>
>I also tried the following to embed it into a big user:
>
>int big_bsf_caller()
>{
> // ..... many stuff before we need bsf/ bsr
>
> int bitstring;
>
> //... map is set after some calculations
>
> int first_one;
> _asm bsf first_one , bitstring
>
>
> // ..... many stuff after we used bsf/ bsr
>}
>
>It works but is even worse than the function approach. After reading the
>VC-generated asm code, I found that bitstring is treated as a mem operand
>in the bsf operation even though bitstring was in fact in a register before
>the operation. The complier moves the value of the register representing
>bitstring back into stack and generates bsf reg, [esp + 8] alike instruction!
>And I found the embedded instruction does harm the caller in terms of
>optimizations (bad register uasage), as the documation suggests.
>This experiment was done 1 year ago with VC5. This is probably still true
>for VC6.
>
>Sure, one could write big_bsf_caller() all in inline asm, but that only
>hardens the design too early.
>
>Life would be easier if your Microsoft would invent new c operators for
>bsf/bsr as language extensions. This is quite easy for the complier writers
>because bsf/bsr is just like other ordinary binary instructions (eg. xor).
>I would like to suggest they use <% and >% as operators for bsr and bsf
>instructions respectively.
>
>After the lengthy problem description, my question is:
>Do you have any practical work-around solutions?
>
>Thanks and
>Best Regards
>
>Dezhi Zhao
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