Data Type Conversion

[VladimirF]

OK, here are new results.
I have used Joeman’s adoptation of ninja9578’s asm. I don’t speak GCC’s asm, but the code looks very similar (to a naked eye).
Anyway, here is machine code, see if there is anything you don’t like about it:

	Asm();
004013F1  push        edx  
004013F2  push        ebx  
004013F3  push        ecx  
004013F4  mov         edx,offset src (6362480h) 
004013F9  mov         ebx,offset dst (404380h) 
004013FE  mov         ecx,dword ptr ds:[403158h] 
00401404  mov         ah,byte ptr [edx+0Ch] 
00401407  mov         al,byte ptr [edx+8] 
0040140A  shl         eax,10h 
0040140D  mov         ah,byte ptr [edx+4] 
00401410  mov         al,byte ptr [edx] 
00401412  mov         dword ptr [ebx],eax 
00401414  add         ebx,4 
00401417  add         edx,10h 
0040141A  loop        main+374h (401404h) 
0040141C  pop         edx  
0040141D  pop         ebx  
0040141E  pop         ecx

And results:

Init array
291.013

Simple loop
88.0397

4-in-1 loop
85.2146

ninja9578's Asm
105.585

Chris F's Asm
90.5299

Lindley
90.5767

Vlad's SSE
81.3531

For lucky owners of Visual Studio 2005 (or above) I am attaching zipped solution so that you can verify my findings.
My conclusion: it is VERY HARD (if not impossible) to beat optimizing compiler with hand-coded asm. I bet it (at least the good one) knows everything about cache, pipeline, prefetch, etc.
So the bottom line (as I see it) is: in a fight for performance choose the best algorithm and implement it in as simple as possible way, so the compiler doesn’t get confused and is able to optimize it nicely.