Testing a myth that an integer, casted from a pointer, can be used to determine its alignment.

conf.h

#if __STDC__

#else
# error A standard C compiler is required!
#endif

myth.c

#include "conf.h"

#include <stddef.h> /* size_t; offsetof. */

extern int uMyth_( void )
{
	typedef const struct { const char Trap; const size_t Gold; } BOX;
	auto const size_t Rule[ 2 ] = { offsetof( BOX, Gold ), sizeof(size_t) };

	register const size_t
		ConA = ( Rule[ 0 ] - 1 ) & ( Rule[ 0 ] | Rule[ 1 ] | (size_t) ( Rule + 0 ) ),
		ConS = ( Rule[ 1 ] - 0 ) ^ ( (size_t) ( Rule + 1 ) - (size_t) ( Rule + 0 ) );

	return !( ConA | ConS );
}

test.c

#include "conf.h"

#include <stdlib.h> /* EXIT_FAILURE, EXIT_SUCCESS. */

extern int main( void )
{
	extern int uMyth_( void );
	return uMyth_( )? EXIT_SUCCESS: EXIT_FAILURE;
}

Compiler Tests

Special thanks to Compiler Explorer.

---

GCC

-Wall -Wextra -pedantic-errors -O2 -std=c89

x86-64 gcc 10.2

uMyth_:
        mov     eax, 1
        ret

ARM64 gcc 8.2

uMyth_:
        mov     w0, 1
        ret

RISC-V rv64gc gcc 8.2.0

uMyth_:
        li      a0,1
        ret

PowerPC64 gcc 6.3.0

uMyth_:
        li 3,1
        blr
        .long 0
        .byte 0,0,0,0,0,0,0,0

MIPS64 gcc 5.4

uMyth_:
        j       $31
        li      $2,1                        # 0x1

MSP430 gcc 6.2.1

uMyth_:
        MOV.B   #1, R12
        RET

---

Clang

-Wall -Wextra -pedantic-errors -O2 -std=c89

x86-64 clang 11.0.1

uMyth_:                                 # @uMyth_
        mov     eax, 1
        ret

armv8-a clang 11.0.1

uMyth_:                                 // @uMyth_
        mov     w0, #1
        ret

RISC-V rv64gc clang 11.0.1

uMyth_:                                 # @uMyth_
        addi    a0, zero, 1
        ret

---

MSVC

-WX -Wall -wd4820 -O2 -TC -Za

x64 msvc v19.14 (WINE)

Rule$ = 0
uMyth_  PROC                                                ; COMDAT
$LN4:
        sub     rsp, 24
        lea     rax, QWORD PTR Rule$[rsp]
        test    al, 7
        mov     eax, 0
        sete    al
        add     rsp, 24
        ret     0
uMyth_  ENDP

ARM64 msvc v19.14 (WINE)

        ;Flags[SingleProEpi] functionLength[24] RegF[0] RegI[0] H[0] frameChainReturn[UnChained] frameSize[16]

|uMyth_| PROC
|$LN4|
        sub         sp,sp,#0x10
        mov         x8,sp
        tst         x8,#7
        cseteq      w0
        add         sp,sp,#0x10
        ret

        ENDP  ; |uMyth_|

---

ICC

-Wall -Wextra -pedantic-errors -O2 -std=c89

x86-64 icc 19.0.1

uMyth_:
        mov       eax, 1                                        #20.19
        ret                                                     #20.19