Apple clang -O1 not optimizing enough?

I have this code in C:

int main(void){    int a = 1 + 2;    return 0;}

When I objdump -x86-asm-syntax=intel -d a.out which is compiled with -O0 flag with GCC 9.3.0_1, I get:

0000000100000f9e _main:100000f9e: 55                           push    rbp100000f9f: 48 89 e5                     mov rbp, rsp100000fa2: c7 45 fc 03 00 00 00         mov dword ptr [rbp - 4], 3100000fa9: b8 00 00 00 00               mov eax, 0100000fae: 5d                           pop rbp100000faf: c3                           ret

and with -O1 flag:

0000000100000fc2 _main:100000fc2: b8 00 00 00 00               mov eax, 0100000fc7: c3                           ret

which removes the unused variable a and stack managing altogether.

However, when I use Apple clang version 11.0.3 with -O0 and -O1, I get

0000000100000fa0 _main:100000fa0: 55                           push    rbp100000fa1: 48 89 e5                     mov rbp, rsp100000fa4: 31 c0                        xor eax, eax100000fa6: c7 45 fc 00 00 00 00         mov dword ptr [rbp - 4], 0100000fad: c7 45 f8 03 00 00 00         mov dword ptr [rbp - 8], 3100000fb4: 5d                           pop rbp100000fb5: c3                           ret

and

0000000100000fb0 _main:100000fb0: 55                           push    rbp100000fb1: 48 89 e5                     mov rbp, rsp100000fb4: 31 c0                        xor eax, eax100000fb6: 5d                           pop rbp100000fb7: c3                           ret

respectively.I never get the stack managing part stripped off as in GCC.Why does (Apple) Clang keep unnecessary push and pop?

This may or may not be a separate question, but with the following code:

int main(void){    // return 0;}

GCC creates a same ASM with or without the return 0;.However, Clang -O0 leaves this extra

100000fa6: c7 45 fc 00 00 00 00         mov dword ptr [rbp - 4], 0

when there is return 0;.

Why does Clang keep these (probably) redundant ASM codes?