Author: Dezhi Zhao
Date: 09:34:42 01/16/99
Go up one level in this thread
On January 16, 1999 at 11:13:43, Robert Hyatt wrote:
>On January 16, 1999 at 07:27:53, Dezhi Zhao wrote:
[snip]
>>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
Dr. Hyatt:
sorry! I did not express myself clearly here. I mean VC the compiler does
not make the asm function inlined in the way it should be. Yes, an asm
function can behave as a micro and save a function calls. But the inlined
code do have some unwanted overhead, extra save/reload instructions.
I did read Mr. Nalimov's asm functions in Crafty mailing list.
However if you inline them in to your existing C functions, I think
you will produce the same effect and the same side effect as I described in the
previous post as follows:
>>
>>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.
>>
It seems to me that the asm function should not be inlined if the C caller
is complex enough. The compiler just pastes the asm lines into the generated
code. The asm instructions use fixed registers , that affects the complier's
decision on register useage.
Dezhi Zhao
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