ObjC Bool

Objective-C

Source code

main.m

objc

#import <Foundation/Foundation.h>

@interface BoolTest: NSObject
  -(void) testcbool: (_Bool) avalue;
  -(void) testobjcbool: (BOOL) avalue;
@end

@implementation BoolTest
-(void) testcbool: (_Bool) avalue
{
    NSLog(@" cbool %s", avalue ? "true" : "false");
}

-(void) testobjcbool: (BOOL) avalue
{
    NSLog(@" objcbool %s", avalue ? "true" : "false");
}
@end

void simpleC(_Bool avalue)
{
    NSLog(@" plainc %s", avalue ? "true" : "false");
    return;
}

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // insert code here...
        int a = 10;
        int b = 10;
        BoolTest* tt = [[BoolTest alloc] init];
        [tt testcbool:(a==b)];
        [tt testobjcbool:(a==b)];
        simpleC(a==b);
   }
    return 0;
}

Compile

You need to have Xcode command-line tools installed

text

clang  -framework Foundation -arch x86_64 main.m -o main

produces the executable. (The executable can be debugged and lldb can show the disassebly)

In order to get assembly file from the compiler the following command-line could be used

text

clang  -framework Foundation -arch x86_64 main.m --assemble -o main.as

Disassembly x86_64

Boolean parameters, no matter BOOL (signed char) or _Bool are always passed in the entire register

No optimization

asm

    0x100000e5d <+93>:  movl   -0x14(%rbp), %edx
    0x100000e60 <+96>:  cmpl   -0x18(%rbp), %edx
    0x100000e63 <+99>:  sete   %r8b
    0x100000e67 <+103>: movq   0x31a(%rip), %rsi         ; "testcbool:"
    0x100000e6e <+110>: andb   $0x1, %r8b

    0x100000e72 <+114>: movzbl %r8b, %edx

    0x100000e76 <+118>: movq   0x183(%rip), %rcx         ; (void *)0x00007fff7e936e80: objc_msgSend
    0x100000e7d <+125>: movq   %rax, %rdi
    0x100000e80 <+128>: callq  *%rcx
    0x100000e82 <+130>: movq   -0x20(%rbp), %rax

    0x100000e86 <+134>: movl   -0x14(%rbp), %edx
    0x100000e89 <+137>: cmpl   -0x18(%rbp), %edx
    0x100000e8c <+140>: sete   %r8b
    0x100000e90 <+144>: andb   $0x1, %r8b

    0x100000e94 <+148>: movzbl %r8b, %edx

    0x100000e98 <+152>: movb   %dl, %r8b
    0x100000e9b <+155>: movq   0x2ee(%rip), %rsi         ; "testobjcbool:"
    0x100000ea2 <+162>: movq   %rax, %rdi
    0x100000ea5 <+165>: movsbl %r8b, %edx
    0x100000ea9 <+169>: callq  0x100000ef8               ; symbol stub for: objc_msgSend

    0x100000eae <+174>: movl   -0x14(%rbp), %edx
    0x100000eb1 <+177>: cmpl   -0x18(%rbp), %edx
    0x100000eb4 <+180>: sete   %r8b
    0x100000eb8 <+184>: andb   $0x1, %r8b

    0x100000ebc <+188>: movzbl %r8b, %edi

    0x100000ec0 <+192>: callq  0x100000db0               ; simpleC at main.m:29

O3

As expected, O3 evaluates constant expressions and puts the end result into the code. Still boolean parameters are being written as long values into registers.

asm

    0x100000e97 <+55>:  movq   %rax, %rbx
    0x100000e9a <+58>:  movq   0x2df(%rip), %rsi         ; "testcbool:"
    0x100000ea1 <+65>:  movl   $0x1, %edx
    0x100000ea6 <+70>:  movq   %rbx, %rdi
    0x100000ea9 <+73>:  callq  *%r15
    0x100000eac <+76>:  movq   0x2d5(%rip), %rsi         ; "testobjcbool:"
    0x100000eb3 <+83>:  movl   $0x1, %edx
    0x100000eb8 <+88>:  movq   %rbx, %rdi
    0x100000ebb <+91>:  callq  *%r15
    0x100000ebe <+94>:  leaq   0x1b3(%rip), %rdi         ; @" plainc %s"
    0x100000ec5 <+101>: leaq   0x70(%rip), %rsi          ; "true"
    0x100000ecc <+108>: xorl   %eax, %eax
    0x100000ece <+110>: callq  0x100000ef2               ; symbol stub for: NSLog
    0x100000ed3 <+115>: movq   %rbx, %rdi

From the Free Pascal wiki, page "ObjC Bool", as it stood on 2022-05-04 after a change by Trev, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's Licensing page. If you have concerns about this material, message the admin.

Revision 2, last changed by WikiArchive 11-6-2026.

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