Pointer(s)^ versus s[1]

Question

In a function that reads data (data meaning exclusively strings) from disk, which should I prefer? Which is better?

A) DiskStream.Read(Pointer(s         


        

Answer 1

If you care about optimization you should prefer the first variant. Just look at the code generated by compiler:

Unit7.pas.98: Stream.Read(Pointer(S)^, 10);
00470EA9 8B55FC           mov edx,[ebp-$04]
00470EAC B90A000000       mov ecx,$0000000a
00470EB1 8BC6             mov eax,esi
00470EB3 8B18             mov ebx,[eax]
00470EB5 FF530C           call dword ptr [ebx+$0c]

Unit7.pas.99: Stream.Read(s[1], 10);
00470EB8 8B5DFC           mov ebx,[ebp-$04]
00470EBB 85DB             test ebx,ebx
00470EBD 7418             jz $00470ed7
00470EBF 8BC3             mov eax,ebx
00470EC1 83E80A           sub eax,$0a
00470EC4 66833802         cmp word ptr [eax],$02
00470EC8 740D             jz $00470ed7
00470ECA 8D45FC           lea eax,[ebp-$04]
00470ECD 8B55FC           mov edx,[ebp-$04]
00470ED0 E8CB3FF9FF       call @InternalUStrFromLStr
00470ED5 8BD8             mov ebx,eax
00470ED7 8D45FC           lea eax,[ebp-$04]
00470EDA E89950F9FF       call @UniqueStringU
00470EDF 8BD0             mov edx,eax
00470EE1 B90A000000       mov ecx,$0000000a
00470EE6 8BC6             mov eax,esi
00470EE8 8B18             mov ebx,[eax]
00470EEA FF530C           call dword ptr [ebx+$0c]

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UPDATE

The above code is generated by Delphi 2009 compiler. You can improve the code by using {$STRINGCHECKS OFF} directive, but you still have UniqueStringU function call overhead:

Unit7.pas.100: Stream.Read(s[1], 10);
00470EB8 8D45FC           lea eax,[ebp-$04]
00470EBB E8B850F9FF       call @UniqueStringU
00470EC0 8BD0             mov edx,eax
00470EC2 B90A000000       mov ecx,$0000000a
00470EC7 8BC3             mov eax,ebx
00470EC9 8B18             mov ebx,[eax]
00470ECB FF530C           call dword ptr [ebx+$0c]