One danger with hand optimising ASM code is illustrated by your example. In
the generated code:
> mov byte ptr [eax+$304], $0
writes 0 to BYTE location [eax+$304], whereas you
mov [eax+$304], edx
writing a DWORD into location [eax+$304], possibly overwriting memory.
This sort of bugs can be vary hard to track down. The software would seem
to work for some time, then a bug would appear in a complete unrelated area.
You should therefore write your code like:
xor edx, edx
mov [eax+$2fc], edx
mov [eax+$300], edx
mov byte ptr [eax+$304], $0
which is only a saving of one instruction!
On a Pentium CPU (and the AMD CPU's as well), basic instructions execute in
1 CPU clock cycle. This means your hand optimised assembler only runs 1
nano second faster than that generated by Delphi. I won't bother if I were
you.
Dennis.
----- Original Message -----
From: "Leigh Wanstead" <[EMAIL PROTECTED]>
To: "Multiple recipients of list delphi" <[EMAIL PROTECTED]>
Sent: Tuesday, June 17, 2003 11:43 AM
Subject: [DUG]: Delphi optimizer
> Hello everyone,
>
> I was surprised that Delphi 7 generate following asmbler code for this
> pascal code. I have turned on Code generation/Optimization.
>
> fMyRecord.fLength := 0;
> fMyRecord.fCount := 0;
> fMyRecord.fReadOnly := False;
>
> xor edx, edx
> mov [eax+$2fc], edx
>
> xor edx, edx
> mov [eax+$300], edx
>
> mov byte ptr [eax+$304], $0
>
> I think the optimized asm code will look like this.
>
> xor edx, edx
> mov [eax+$2fc], edx
> mov [eax+$300], edx
> mov [eax+$304], edx
>
> I tested the hand written asm code, it is 5 cpu clock cycle faster. I hate
> to put asm in my source code. So the question is how to write delphi code
to
> generate optimize asm code like this one.
>
> I know this slow down may not sound that much, just for research purpose.
>
> I have enclosed delphi sample code for your reference. I look forward to
> hearing from you.
>
> TIA
>
> Best Regards
> Leigh
>
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