How can gcc generate assembly which can set up stack frame with ENTER instruction

Question

I generate assembly code like this with gcc, Can gcc generate code which has ENTER for stack frame?

.file   "temp.c"
.text
.globl  main
.type   main, @function
main:
pushq   %rbp
movq    %rsp, %rbp
movl    $0, -12(%rbp)
movl    $0, -8(%rbp)
movl    $0, -4(%rbp)
movl    $0, %eax
popq    %rbp
ret
.size   main, .-main
.ident  "GCC: (Ubuntu 7.4.0-1ubuntu1~18.04) 7.4.0"
.section    .note.GNU-stack,"",@progbits

Here 's original code:

 #include <stdio.h>
  int main(){
   int a;
   int b;
   int c;
   a = 0;
   b = 0;
   c = 0;
}

Answer 1

GCC will never emit enter because it's super-slow compared to its normal frame-pointer setup of 2 or 3 single-uop instructions.

(If it makes a frame pointer at all; gcc -O1 and higher enables -fomit-frame-pointer . Except when optimizing for size, because x(%rsp) addressing modes use an extra byte vs. x(%rbp) modes.)

# equivalent to  enter $24, $0  (4 bytes)
    push   %rbp               # 1 byte
    mov    %rsp, %rbp         # 3 bytes
    sub    $24, %rsp          # 4 bytes only for a non-zero immediate

Specifically, on Skylake enter is 12 uops, and one per 8-cycle throughput for enter a, 0 ( Agner Fog's instruction tables ). With a non-zero nesting level, it's insanely slow, like 87 cycles + 7 * nesting level .

On Ryzen, enter is 12 uops, with one per 16-cycle throughput.

leave is fine, though: it's only 3 uops on Intel CPUs. (That's still one more than mov %rbp, %rsp / pop %rbp though. The 3 uops doesn't include a stack-sync uop; it's 3 even if the stack engine was in sync before leave .)

---

The only reason to use enter would be optimizing for code-size at the expense of speed. But even gcc -Os doesn't care enough about code-size to have an option for that.

Even clang -Oz (which will use push $1 / pop %rax to save 2 bytes vs. mov $1, %eax ) doesn't use enter . ( Godbolt compiler explorer )

But enter 0,0 doesn't even save code-size so it's just pure bad.

And I follow a manual that said program start entry with ENTER

That's one (obsolete and not recommended) option.

If you want to write your own compiler that makes slow code, go ahead.