Read or Write aligment, what is best? - Forums

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When its not possible to always have both read and write DWORD aligned data (like in the case of the strcat function). Whats is better to have READ or WRITE aligment or is there no difference? READ aligment example:


;ESI -> Source
;EDI -> Destination
;ECX -> Number of bytes > 0 for this example
  mov eax, esi  ; Used for
  and eax, 3    ; Aligning source to DWORD boundaries
  jz @@L0
@@:
  movsb
  dec ecx
  jz @@Quit
  dec eax
  jnz @B
@@L0:
  mov eax, ecx
  shr ecx, 2
  and ecx, ecx
  jz @F
  rep movsd
@@:
  and eax, 3
  jz @@Quit
  mov ecx, eax
  rep movsb
@@Quit:

Write Aligment example:


;ESI -> Source
;EDI -> Destination
;ECX -> Number of bytes > 0 for this example
  mov eax, edi  ; Used for
  and eax, 3    ; Aligning DESTINATION to DWORD boundaries
  jz @@L0
@@:
  movsb
  dec ecx
  jz @@Quit
  dec eax
  jnz @B
@@L0:
  mov eax, ecx
  shr ecx, 2
  and ecx, ecx
  jz @F
  rep movsd
@@:
  and eax, 3
  jz @@Quit
  mov ecx, eax
  rep movsb
@@Quit:

Also, whats is the best way to test the speed of code?

Posted on 2001-07-05 00:57:00 by dxantos

Read or Write aligment, what is best?

Use the rdtsc instruction to count the number of clock cycles. Here is a macro that will display the number clock cycles in a message box :


PERF_ON   MACRO
IFNDEF __PERF
  .DATA
  __perfBuffer    BYTE 21 DUP (0)
  __perfFmt       BYTE "%ld%ld", 0
  __perfTitle     BYTE "Performance counter", 0
  __PERF = 1
  sprintf PROTO C :DWORD, :DWORD, :DWORD, :VARARG
  memset  PROTO :DWORD, :DWORD, :DWORD
  .CODE
ENDIF
  rdtsc
  push    eax
  push    edx
ENDM

PERF_OFF   MACRO
  rdtsc
  pop     esi
  pop     edi
  sub     eax, edi
  sbb     edx, esi
  
  mov     DWORD PTR __perfBuffer[0], 0
  mov     DWORD PTR __perfBuffer[4], 0
  mov     DWORD PTR __perfBuffer[8], 0
  mov     DWORD PTR __perfBuffer[12], 0
  mov     DWORD PTR __perfBuffer[16], 0
  mov     __perfBuffer[20], 0

  pushad
  INVOKE  sprintf, ADDR __perfBuffer, ADDR __perfFmt, edx, eax 
  INVOKE  MessageBox, NULL, ADDR __perfBuffer, ADDR __perfTitle, MB_OK
  popad
ENDM

To use it, link your prog with msvcrt.lib. To test your code, use :


PERF_ON
; code to test
PERF_OFF

To test memory alignement, use this code :


.DATA
ALIGN 4
   dummy BYTE 0            
   a     DWORD  0         ; a is not aligned
ALIGN 4
   b    DWORD 0           ; b is aligned
.CODE
start :

   PERF_ON
   mov  a, eax         ; read
   PERF_OFF
   
   PERF_ON
   mov  eax, a         ; write 
   PERF_OFF
   
   PERF_ON
   mov  a, eax         ; read
   PERF_OFF
   
   PERF_ON
   mov  eax, a         ; write
   PERF_OFF

   PERF_ON
   mov  b, eax         ; read
   PERF_OFF
   
   PERF_ON
   mov  eax, b         ; write
   PERF_OFF

   PERF_ON
   mov  b, eax         ; read
   PERF_OFF
   
   PERF_ON
   mov  eax, b         ; write
   PERF_OFF
   INVOKE   ExitProcess, 0
END start

The results shows that writing is faster than reading, but there is not a big difference between aligned and misaligned access. It's strange. The Intel optimization manuals says :

A misaligned data access that causes an access request for data already in the L1 cache can cost six to nine cycles. A misaligned access that causes an access request from L2 cache or from memory, however, incurs a penalty that is processor-dependent. Align the data as follows: � Align 8-bit data at any address. � Align 16-bit data to be contained within an aligned four byte word. � Align 32-bit data so that its base address is a multiple of four. � Align 64-bit data so that its base address is a multiple of eight. � Align 80-bit data so that its base address is a multiple of sixteen. A 32-byte or greater data structure or array should be aligned so that the beginning of each structure or array element is aligned in a way that its base address is a multiple of thirty-two.

Posted on 2001-07-05 03:21:00 by karim

Read or Write aligment, what is best?

thanks for the prompt reply. BTW: If you use wsprintfA instead of sprintf yo can avoid to include msvcrt.lib.

Posted on 2001-07-05 09:24:00 by dxantos

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