GCC generate Canary or not?

Question

my gcc version is 4.8.2 and operating system is ubuntu 14.04 (64 bit). I found that sometimes gcc auto generate the canary to do buffer overflow protection sometimes not, why?

case to generate canary: when SIZE is multiple of four

#include<stdio.h>
#define SIZE 4

int main()
{
    char s[SIZE];
    scanf("%s", s);
    return 0;
}

asm after gcc -c -g -Wa,-a,-ad

...
   4:a.c           **** int main()
   5:a.c           **** {
  13                    .loc 1 5 0
  14                    .cfi_startproc
  15 0000 55            pushq   %rbp
  16                    .cfi_def_cfa_offset 16
  17                    .cfi_offset 6, -16
  18 0001 4889E5        movq    %rsp, %rbp
  19                    .cfi_def_cfa_register 6
  20 0004 4883EC10      subq    $16, %rsp
  21                    .loc 1 5 0
  22 0008 64488B04      movq    %fs:40, %rax
  22      25280000 
  22      00
  23 0011 488945F8      movq    %rax, -8(%rbp)
  24 0015 31C0          xorl    %eax, %eax
   6:a.c           ****     char s[SIZE];
   7:a.c           ****     scanf("%s", s);
...

case not to generate canary : not the multiple of four

#include<stdio.h>
#define SIZE 2

int main()
{
    char s[SIZE];
    scanf("%s", s);
    return 0;
}

asm after gcc -c -g -Wa,-a,-ad

...
   4:a.c           **** int main()
   5:a.c           **** {
  13                    .loc 1 5 0
  14                    .cfi_startproc
  15 0000 55            pushq   %rbp
  16                    .cfi_def_cfa_offset 16
  17                    .cfi_offset 6, -16
  18 0001 4889E5        movq    %rsp, %rbp
  19                    .cfi_def_cfa_register 6
  20 0004 4883EC10      subq    $16, %rsp
   6:a.c           ****     char s[SIZE];
   7:a.c           ****     scanf("%s", s);
...

Answer 1

OK, I guess we know the answer from comments, so I'll post it here to state it explicitly.

Putting canaries in a lot of functions can result in performance degradation. That's why there are several ways to tell GCC we want to use them, which are described well here. Main ideas:

1. Canaries are not used by default, one needs to pass one of flags that enable them.
2. To save execution time, GCC uses simple heuristic with -fstack-protector flag: add canaries for functions that use alloca or local buffers larger than 8 bytes (by default).
3. The heuristic can be tweaked with ssp-buffer-size parameter: --param ssp-buffer-size=4.

Apparently Ubuntu ships version of GCC with size of buffer changed to 4, so buffers less than that don't trigger generation of a canary. I confirm (and anyone else should be able to repeat) that by compiling two examples with --param ssp-buffer-size=4, which produces assembly with canaries for only one of them.