Can a C compiler rearrange stack variables?

Question

I have worked on projects for embedded systems in the past where we have rearranged the order of declaration of stack variables to decrease the size of the resulting executa

Answer 1

This does not answer your question but here is my 2 cents about a related issue...

I did not have the problem of stack space optimization but I had the problem of mis-alignment of double variables on the stack. A function may be called from any other function and the stack pointer value may have any un-aligned value. So I have come up with the idea below. This is not the original code, I just wrote it...

#pragma pack(push, 16)

typedef struct _S_speedy_struct{

 double fval[4];
 int64  lval[4];
 int32  ival[8];

}S_speedy_struct;

#pragma pack(pop)

int function(...)
{
  int i, t, rv;
  S_speedy_struct *ptr;
  char buff[112]; // sizeof(struct) + alignment

  // ugly , I know , but it works...
  t = (int)buff;
  t +=  15; // alignment - 1
  t &= -16; // alignment
  ptr = (S_speedy_struct *)t;

  // speedy code goes on...
}