从栈分配内存

alloca() allocate space in the stack frame	Function

SYNOPSIS

#include <alloca.h>

void *alloca(size_t size);

DESCRIPTION

The alloca() functionallocates space in the stack frame of the caller, and returns apointer to the allocated block. This temporary space isautomatically freed when the function from which alloca() is called returns.

PARAMETERS

size 

Specifies the size of the allocated block of memory.

RETURN VALUES

The alloca() functionreturns a null pointer if there is insufficient memory.

delphi 中结构，对象都是从堆上分配的。从栈上分配结构更快，不必担心内存碎片。

procedure FillDWord(var Dest; Count, Value: Integer); register;
asm
  XCHG  EDX, ECX
  PUSH  EDI
  MOV   EDI, EAX
  MOV   EAX, EDX
  REP   STOSD
  POP   EDI
end;

{ StackAlloc allocates a 'small' block of memory from the stack by
  decrementing SP.  This provides the allocation speed of a local variable,
  but the runtime size flexibility of heap allocated memory.  }
function StackAlloc(Size: Integer): Pointer; register;
asm
  POP   ECX          { return address }
  MOV   EDX, ESP
  ADD   EAX, 3
  AND   EAX, not 3   // round up to keep ESP dword aligned
  CMP   EAX, 4092
  JLE   @@2
@@1:
  SUB   ESP, 4092
  PUSH  EAX          { make sure we touch guard page, to grow stack }
  SUB   EAX, 4096
  JNS   @@1
  ADD   EAX, 4096
@@2:
  SUB   ESP, EAX
  MOV   EAX, ESP     { function result = low memory address of block }
  PUSH  EDX          { save original SP, for cleanup }
  MOV   EDX, ESP
  SUB   EDX, 4
  PUSH  EDX          { save current SP, for sanity check  (sp = [sp]) }
  PUSH  ECX          { return to caller }
end;

{ StackFree pops the memory allocated by StackAlloc off the stack.
- Calling StackFree is optional - SP will be restored when the calling routine
  exits, but it's a good idea to free the stack allocated memory ASAP anyway.
- StackFree must be called in the same stack context as StackAlloc - not in
  a subroutine or finally block.
- Multiple StackFree calls must occur in reverse order of their corresponding
  StackAlloc calls.
- Built-in sanity checks guarantee that an improper call to StackFree will not
  corrupt the stack. Worst case is that the stack block is not released until
  the calling routine exits. }
procedure StackFree(P: Pointer); register;
asm
  POP   ECX                     { return address }
  MOV   EDX, DWORD PTR [ESP]
  SUB   EAX, 8
  CMP   EDX, ESP                { sanity check #1 (SP = [SP]) }
  JNE   @@1
  CMP   EDX, EAX                { sanity check #2 (P = this stack block) }
  JNE   @@1
  MOV   ESP, DWORD PTR [ESP+4]  { restore previous SP  }
@@1:
  PUSH  ECX                     { return to caller }
end;