Does variable declaration mean memory allocation?

Question

Will it be precise to say that in

void f() {
    int x;
    ...
}

"int x;" means allocating sizeof(int) bytes on the stack?

Are there any specifications for that?

Answer 1

Nothing in the standard mandates that there is a stack. And nothing in the standard mandates that a local variable needs memory allocated for it. The variable could be placed in a register, or even removed altogether as an optimization.

Answer 2

There are no specification about that and your assumption is often (but not always) false. Consider some code like

void f() {
   int x;
   for (x=0; x<1000; x++) 
     { // do something with x 
     }
   // x is no more used here
}

First, an optimizing compiler would put x inside some register of the machine and not consume any stack location (unless e.g. you do something with the address &x like storing it in a global).

Also the compiler could unroll that loop, and remove x from the generated code. For example, many compilers would replace

for (x=0; x<5; x++) g(x);

with the equivalent of

g(0); g(1); g(2); g(3); g(4);

and perhaps replace

for (x=0; x<10000; x++) t[x]=x;

with something like

for (α = 0;  α < 10000;  α += 4) 
  { t[α] =  α; t[α+1] =  α+1; t[α+2] =  α+2; t[α+3] =  α+3; };

where α is a fresh variable (or perhaps x itself).

Also, there might be no stack. For C it is uncommon, but some other languages did not have any stack (see e.g. old A.Appel's book compiling with continuations).

BTW, if using GCC you could inspect its intermediate (Gimple) representations with e.g. the MELT probe (or using gcc -fdump-tree-all which produces hundreds of dump files!).