Author Topic: Fill memory  (Read 5035 times)

Offline TightCoderEx

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Fill memory
« on: May 11, 2012, 11:45:15 PM »
In essence this procedure has but one purpose and that is to fill a block of memory with a single byte. To facilitate different aspects, it has 3 entry points

InitFrame: Calculates the area by the difference between RSP & RBP.  You can call this procedure anywhere in the caller so long as there is a stack frame and no other registers have been pushed on the stack.

Code: [Select]
; =========================================================================================================
; This procedure has three entry points each of which finally falls into routine that fills memory.
; Interation count is reduced by writing as many QWORDs as possible, but region no matter size will be
; filled correctly.

; ENTRY: AL = Fill pattern (Bits 4 - 63 are irrelevant as they will be shifted out)
; RCX = Size of area in bytes
; RDI = Pointer to area to be written.

; LEAVE: RAX = Pattern extended through all 64 bits
; RCX = Unchanged except when InitFrame is called then sizeof fill area.
; RDI = Pointer to fill area.

; ---------------------------------------------------------------------------------------------------------

; InitFrame only requires AL be set, RCX & RDI are calculated

  InitFrame: mov rdi, rsp ; Get pointer to base of fill area
    add rdi, 8 ; Bump past callers return
    mov rcx, rbp
    sub rcx, rdi ; Get actual number of bytes to fill

FillMem has three parts:
  1: AL = 0, simply bounce to ZeroMem and RAX will be set accordingly
  2: AL = -1 simply set RAX to zero and decrement once
  3: AL = -2 through 2 or 1 - FE.  Little more involved copying pattern.

Falling into this is how InitFrame determines what its pattern is going to be

Code: [Select]
    ; Test if we are supposed to be filling with nulls
     FillMem: or al, al ; Are we going to fill with nulls
    jz ZeroMem
    ; Test if we are supposed to be filling with -1's
    inc al ; if AL = FF, bump to NULL to set ZF
    jnz .Shift ; ZR = 0, means we have to extend pattern
    ; More time effective that using .Shift and save a few bytes over move rax, -1.
    xor rax, rax
    dec rax
    jmp ZeroMem + 3

Is there a better way of doing this?

Code: [Select]

; Shift contents of AL through RAX
      .Shift: push rcx ; Save size of fill area
    xor ecx, ecx ; Trash bits
    mov cl, 7
    dec al ; Adjust back to original value
    mov dl, al ; Save a copy of fill byte
      .L0: shl rax, 8 ; Shift in 8 zero bits
      mov al, dl ; and copy fill byte into nullified space
      loop .L0
    pop rcx ; Retrive buffer size
    jmp $ + 5 ; Bounce over next instruction

and finally finish by filling buffer

Code: [Select]
; This entry point just simply nullify's RAX as more often that not, calling code would need to
; do this
     ZeroMem: xor rax, rax ; Set Fill pattern
; RAX = Fill pattern
; RCX = Size of buffer
; RDI = Pointer of area to be filled
; As area may not be quadword aligned, preamble tests bits 0 - 2 as each of those indicates the
; number of bytes, words, dwords that need to be written to align buffer on 64 bits
      push rcx ; Preserve
      push rdi
      ; If bit 0 is on, fill one byte
      sar rcx, 1 ; Shift bit 0 into CY
      jnc $ + 3
      ; Now we are word aligned and if bit 1 was on fill another word
      sar rcx, 1 ; Shift bit 1 into CY
      jnc $ + 4
      ; Now we are dword aligned and if bit 2 was on fill another dword
      sar rcx, 1 ; Shift bit 2 into CY
      jnc $ + 3
      ; RCX now equals the number of qwords to fill
      repnz stosq ; Finish by writing RCX qwords.
      pop rdi
      pop rcx ; Recover

I've tested this fairly extensively, but for buffers smaller than 8 bytes or null for that matter I haven't. Doesn't seem reasonable this would be used for an area that small.

Offline Frank Kotler

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Re: Fill memory
« Reply #1 on: May 12, 2012, 06:02:04 PM »
Code: [Select]
or rax, -1
... might be a shorter way to fill rax with -1. Dunno how it would compare for speed...


Offline TightCoderEx

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Re: Fill memory
« Reply #2 on: May 12, 2012, 07:23:04 PM »
Code: [Select]
or rax, -1

... might be a shorter way to fill rax with -1. Dunno how it would compare for speed...

Good catch Frank and it is shorter by 2 bytes.  I don't generally concern myself too much with cycles unless they are in a high iteration count loop.  I try to get from point A to point B as efficiently as possible algorithm wise using the least amount of code and at a glance, the logic stands right out. At least, that's the objective anyway