unexpected value when assign unsigned int(by negtive it) to signed long long

I happened run into a strange problem, see the code below:

#include <stdio.h>

int main() {
    unsigned char a = 10;
    unsigned int b = 10;

    long long x = -a;
    long long y = -b;

    printf("x = %lld, y = %lld\n", x, y);
    return 0;
}

Output:

x = -10, y = 4294967286

As you can see, when I assign -b to y (long long), it gives the wrong result, but the value of x is as expected.

After reading the asm code, I found that when assign negtive unsigned char to long long, the compiler generates a cdqe instruction(see line +25) to extend the sign to rax, while it doesn't do the same thing when unsigned int to long long.

I know the reason we got the value 4294967286 is that the high 32 bit of rax are all zero, rax = 0x00000000fffffff6.

So my question is why the compiler missing the cdqe instruction in the unsigned int case?

Dump of assembler code for function main:
   0x000000000040052d <+0>:     push   rbp
   0x000000000040052e <+1>:     mov    rbp,rsp
   0x0000000000400531 <+4>:     sub    rsp,0x20
=> 0x0000000000400535 <+8>:     mov    BYTE PTR [rbp-0x1],0xa
   0x0000000000400539 <+12>:    mov    DWORD PTR [rbp-0x8],0xa
   0x0000000000400540 <+19>:    movzx  eax,BYTE PTR [rbp-0x1]
   0x0000000000400544 <+23>:    neg    eax
   0x0000000000400546 <+25>:    cdqe   
   0x0000000000400548 <+27>:    mov    QWORD PTR [rbp-0x10],rax
   0x000000000040054c <+31>:    mov    eax,DWORD PTR [rbp-0x8]
   0x000000000040054f <+34>:    neg    eax
   0x0000000000400551 <+36>:    mov    eax,eax
   0x0000000000400553 <+38>:    mov    QWORD PTR [rbp-0x18],rax
   0x0000000000400557 <+42>:    mov    rdx,QWORD PTR [rbp-0x18]
   0x000000000040055b <+46>:    mov    rax,QWORD PTR [rbp-0x10]
   0x000000000040055f <+50>:    mov    rsi,rax
   0x0000000000400562 <+53>:    mov    edi,0x400610
   0x0000000000400567 <+58>:    mov    eax,0x0
   0x000000000040056c <+63>:    call   0x400410 <printf@plt>
   0x0000000000400571 <+68>:    mov    eax,0x0
   0x0000000000400576 <+73>:    leave  
   0x0000000000400577 <+74>:    ret    

Env:

OS: CentOS Linux release 7.8.2003 (Core), Linux 3.10.0-327.el7.x86_64 #1 SMP Thu Nov 19 22:10:57 UTC 2015 x86_64 x86_64 GNU/Linux
Gcc: gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-39)