Uninitialized bool mischief

bool1.asm

        .file   "bool1.c"
        .intel_syntax noprefix
        .section        .rodata
.LC0:
        .string "p is true"
.LC1:
        .string "p is not true"
.LC2:
        .string "p is false"
.LC3:
        .string "p is not false"
        .text
        .globl  main
        .type   main, @function
main:
.LFB0:
        push    rbp
.LCFI0:
        mov     rbp, rsp
.LCFI1:
        sub     rsp, 32
.LCFI2:
        mov     DWORD PTR [rbp-20], edi
        mov     QWORD PTR [rbp-32], rsi
        movzx   eax, BYTE PTR [rbp-1]
        test    al, al
        je      .L2
        mov     edi, OFFSET FLAT:.LC0
        call    puts
        jmp     .L3
.L2:
        mov     edi, OFFSET FLAT:.LC1
        call    puts
.L3:
        movzx   eax, BYTE PTR [rbp-1]
        xor     eax, 1
        test    al, al
        je      .L4
        mov     edi, OFFSET FLAT:.LC2
        call    puts
        jmp     .L5
.L4:
        mov     edi, OFFSET FLAT:.LC3
        call    puts
.L5:
        mov     eax, 0
        leave
.LCFI3:
        ret

bool1.c

#include <stdio.h>
#include <stdbool.h>

int main(int argc, char *argv[])
{
    volatile bool p;

    if ( p )
        puts("p is true");
    else
        puts("p is not true");

    if ( ! p )
        puts("p is false");
    else
        puts("p is not false");

    return 0;
}

bool1_O2.asm

        .file   "bool1.c"
        .intel_syntax noprefix
        .section        .rodata.str1.1,"aMS",@progbits,1
.LC0:
        .string "p is true"
.LC1:
        .string "p is not true"
.LC2:
        .string "p is false"
.LC3:
        .string "p is not false"
        .section        .text.startup,"ax",@progbits
        .p2align 4,,15
        .globl  main
        .type   main, @function
main:
.LFB22:
        sub     rsp, 24
.LCFI0:
        movzx   eax, BYTE PTR [rsp+15]
        test    al, al
        je      .L2
        mov     edi, OFFSET FLAT:.LC0
        call    puts
.L3:
        movzx   eax, BYTE PTR [rsp+15]
        test    al, al
        je      .L7
        mov     edi, OFFSET FLAT:.LC3
        call    puts
.L5:
        xor     eax, eax
        add     rsp, 24
.LCFI1:
        ret

bool2.c

#include <stdio.h>
#include <stdbool.h>

void fuzz()
{
    volatile unsigned char x = 0x55;
}

void bool_test()
{
    volatile bool p;

    if ( p )
        puts("p is true");
    else
        puts("p is not true");

    if ( ! p )
        puts("p is false");
    else
        puts("p is not false");
}

int main(int argc, char *argv[])
{
    fuzz();
    bool_test();

    return 0;
}

cast.asm

        movzx   eax, BYTE PTR c[rip]
        test    al, al
        setne   al
        mov     BYTE PTR p[rip], al

cast.c

#include <stdbool.h>

volatile char c = 0xff;
volatile bool p;

int main(int argc, char* argv[])
{
    p = (bool)c;
    return 0;
}

snippet.c

bool p;

/* ... */

if ( p )
    puts("p is true");

if ( ! p )
    puts("p is false");