local variable storage allocation in assembly

the following snippet is often used when said to be allocate storage for local variables

addl $8,%esp  //allocate 8-byte storage

push %ebx     //store some value onto the stack

why not simply push the value onto the stack,but rather allocated some space in advance?

From stackoverflow

• So that the allocated space is a fixed size, which is simplest for the compiler as the memory can be accessed with "DWORD PTR [ebp+(offset)]". The usual function prologue is something like:

  (Intel syntax) push ebp add ebp, 8 mov ebp, esp

  for a function with 8 bytes of local variables.

• If you're asking why doesn't a compiler generate push instructions for local storage rather than direct manipulation of the stack pointer, it's a matter of efficiency.

  Automatic variables (in C anyway) are uninitialized, so the code sequence would be (I'm going to use subl since I'm used to stacks growing down in memory):

  C Code            Actual assembly        Your suggestion
  void x(void) {
      int a;        subl 8,%esp            push 0
      int b;                               push 0
      : : :         blah blah              blah blah
  }
  

  My answer is that it's unnecessary and inefficient in cases such as:

  C Code            Actual assembly        Your suggestion
  void x(void) {
      int a[100];   subl 400,%esp          push 0
                                           push 0
                                           push 0
                                           : : :
                                           push 0
      : : :         blah blah              blah blah
  

  What you propose may make sense for something like:

  C Code            Your suggestion
  void x(void) {
      int a = 7;    push 7
      int b = 9;    push 9
      int c[4];     subl 16,%esp
      : : :         blah blah
  }
  

  yfel : got it!thanks!!!