Using sizeof(unsigned int) is not equal to using a constant in some cases

[A]

Hello,

during the use of gcc I noticed the following thing: sometimes when I use
sizeof(<type>) the compiler might generate other assembler code than
when using just a number instead.

The example of this: ( online version: https://gcc.godbolt.org/z/-KCHqM )

    return val >> n | val << (32 - n);

unrolls to

    mov     eax, DWORD PTR [rbp-8]
    mov     edx, DWORD PTR [rbp-4]
    mov     ecx, eax
    ror     edx, cl                 ← fast ror here
    mov     eax, edx

but

    return val >> n | val << (sizeof(val) * 8 - n);

unrolls to

    mov     eax, DWORD PTR [rbp-8] ─┐
    mov     edx, DWORD PTR [rbp-4]  │
    mov     esi, edx                │
    mov     ecx, eax                │
    shr     esi, cl                 │
    mov     eax, 32                 ├─ no ror
    sub     eax, DWORD PTR [rbp-8]  │
    mov     edx, DWORD PTR [rbp-4]  │
    mov     ecx, eax                │
    sal     edx, cl                 │
    mov     eax, edx                │
    or      eax, esi               ─┘

Is it a bug or a feature? I see this problem at least
from 5.3, it also is present at the current version.

The problem is noticeable at -O0 level, unfortunately we have to use -O0
for smoother debug.

With best regards,
Alex.