How do nested scopes affect stack depth?

Question

I tried compiling the following c-code using MSVC into assembly both with (CL TestFile.c /Fa /Ot) and without optimizations (CL TestFile.c /Fa) and the result is they produce the same stack-depth.

Why does the compiler use 8 bytes for each of the 3 varibles x, y, and z when it knows it will use a maximum of 16 bytes? Instead of y$1 = 4 and z$2 = 8 could it not use y$1 = 4 and z$2 = 4 so y and z use the same memory on the stack without any problems?

int main() {
  int x = 123;
  if (x == 123) {
    int y = 321;
  }
  else {
    int z = 234;
  }
}

; Parts of the assembly code
x$ = 0
y$1 = 4
z$2 = 8
main PROC
$LN5:
  sub rsp, 24
; And so on...

user_298225 · Answer

Nested scopes do not affect stack depth. Per the C standard, nested scopes affect visibility of identifiers and do not impose any requirements on how a C implementation uses the stack, if it has one. A C compiler is permitted by the C standard generate any code that gets the same observable behavior.

For the program shown in the question, the only observable behavior is to exit with a success status, so a good compiler should, when optimizing, generate a minimal program. For example, GCC 10.2 for x86-64 generates just an xor and a ret:

main:
        xor     eax, eax
        ret

So does Clang 11.0.1. If MSVC does not, that is a deficiency in it. (However, it may be that the switches /Os and /Ot do not request optimization or do not request much optimization; they may just express a preference for speed or time when used in conjunction with other optimization switches.)

Further, a good compiler should perform lifetime analysis of the use of objects, constructing a graph representing where nodes are places in code and are labeled with creations or uses of values and directed edges are potential program control flows (or some equivalent representation of the source code). Then assembler (or intermediate code) should be generated to implement the semantics required by the graph. If two sets of source code have equivalent graphs, the compiler should generate equivalent assembly (or intermediate code) for them (up to some reasonable ability to process complicated graphs) regardless of whether definitions in nested scopes were used or not.

How do nested scopes affect stack depth?

1 Answers