Calling conventions

[Steve Hutchesson]

An issue that was raised recently in this forum was the "need" for an old
Microsoft standard calling convention called FASTCALL. Here is the viable
alternative that can be written directly in PowerBASIC when an external
DLL is being called, for lack of another name, this is DIRTYCALL calling
convention.

It works by making a standard DLL function call to retrieve the address
of a label in the DLL function, then it directly calls that address with
a value in the EAX register which is used in the DLL internal function
to perform a simple calculation.

On a large DLL with many function, this would be done first so that you
have the addresses in an array or a global variable and then call the
internal function in the normal DLL function as the program requires.

This method benchmarks on my PIII 600 at about 1100 milliseconds for
100 million iterations of the direct call to the address in the DLL
including the simple calculation in the DLL.

Depending on how familiar you are with PowerBASIC's inline assembler, you
can pass 6 DWORD size parameters in the available integer registers and 8
floating point values in the FP registers. If you have the need to pass
more parameters, you can pass the address of an array or structure in one
integer register.

FASTCALL, ho hum

Regards,

[email protected]

  Test code in calling EXE file.
  
      TheAddress& = TestFunction
  
      tc& = GetTickCount()
  
      ! mov ecx, 100000000    ' 100 million
    tst:
      ! mov eax, ecx          ' pass directly in EAX
      ! call TheAddress&
      ! dec ecx
      ! jnz tst
  
      tc2& = GetTickCount() - tc&
  
      MessageBox hWin,ByCopy str$(tc2&),"Test", _
                 %MB_OK or %MB_ICONINFORMATION
  
  
  
  Test DLL code
  
  '##########################################################################
  
      #COMPILE DLL
  
  '##########################################################################
  
  FUNCTION TestFunction ALIAS "TestFunction" () EXPORT as LONG
  
      ! jmp past_It
  
    ' --------------------------
    ' The REAL function is here
    ' --------------------------
  
      LabelAddress:
  
      ! shl eax, 1
  
      ! ret
  
    ' --------------------------
  
      past_It:
  
      FUNCTION = CodePtr(LabelAddress)
  
  END FUNCTION
  
  '##########################################################################

[This message has been edited by Steve Hutchesson (edited January 27, 2001).]

[Peter Manders]

Steve,

One word: brilliant!


Peter.


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[email protected]

[Daniel Corbier]

I haven't tried this yet. But it sounds great. Steve, you
mentioned the benchmark for your direct call. But how did it
compare with the speed of a conventional PB call? (I'm assuming
that it was much faster, right?)

My concern now would be whether the next version of the compiler
would break my code if I decided to rely on something like
this. I had once made an unconventional use of label addresses
(after checking with tech support first). It worked smoothly
with PB 5. But I had to undo it with PB 6.0.

------------------
Daniel Corbier
UCalc Fast Math Parser
http://www.ucalc.com

[Steve Hutchesson]

Daniel,

It should not be a problem because it uses a documented function
to get the address and normal assembler instructions to do the
direct call to that label. If you are interested in trying this
technique, note that you cannot use LOCAL variable at all in
area between the label and the RET instruction, what you can do
is use GLOBAL variables in that area as PB is a 2 pass compiler
which will handle them within the code without problems.

You can pass data in EAX, ECX & EDX easily, you will need to
be careful with EBX, ESI & EDI if you wish to use them for
passing parameters but I think from memory that PB tidies this
up for you. If you have problems there, just preserve EBX, ESI & EDI
at the beginning of the proc that calls the code in the DLL and
it should work OK.

I don't think there are any limits on how you use the 8 floating
point registers but you will have to work out how to get the data
you want to pass into that format.

Let us know how you go with the idea as I am sure many are interested
in the type of code you are writing.

Regards,

[email protected]

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