Trust in the Optimizer ???

test.c

// test available here
//  https://godbolt.org/z/EiXxwW


// This is the basic function
int upTree( int N ) {
	int n;
	for( n = 0; n < 32; n++ ) if( !(N&(1<<n)) ) break;
  return (N & ~((0x1)<<(n+1))) | ( 0x01 << n );
}
  
// THese are alternative endings...
#if _ALTERNATE_ENDINGS_
	int val = (0x2)<<(n);
   return (N & ~(val<<1)) | (val);
	 return (N & ~val) | ( val>>1 );
	 return (N & ~((0x2)<<(n))) | ( 0x01 << n );
	 return (N & ~((0x1)<<(n+1))) | ( 0x01 << n );
#endif
  

// These are optimized code generated on godbolt.org, with GCC 10.1 -O3...

#if 0
return (N & ~((0x2)<<(n))) | ( 0x01 << n );
        not     eax
        and     edi, eax
        mov     eax, edi
        bts     eax, ecx
        ret

return (N & ~((0x1)<<(n+1))) | ( 0x01 << n );
        btr     edi, ecx
        mov     eax, edi
        bts     eax, edx
        ret


	mov     eax, 2
        sal     eax, cl
        mov     edx, eax
return (N & ~(val<<1)) | (val);
	add     eax, eax
        not     eax
        and     edi, eax
        mov     eax, edi
        or      eax, edx
        ret

	mov     edx, 2
        sal     edx, cl
return (N & ~val) | ( val>>1 );
	mov     eax, edx
        not     eax
        and     edi, eax
        mov     eax, edx
        sar     eax
        or      eax, edi
        ret
#endif


// These are What was generated by MinGW 7.2.0 -O3


	//return (N & ~((0x1)<<(n+1))) | ( 0x01 << n );

#if 0
   0x000000000040158f <+31>:    mov    r8d,0x1
   0x0000000000401595 <+37>:    mov    r9d,r8d
   0x0000000000401598 <+40>:    shl    r9d,cl
   0x000000000040159b <+43>:    mov    ecx,r9d
   0x000000000040159e <+46>:    not    ecx
   0x00000000004015a0 <+48>:    and    eax,ecx
   0x00000000004015a2 <+50>:    mov    ecx,edx
   0x00000000004015a4 <+52>:    shl    r8d,cl
   0x00000000004015a7 <+55>:    or     eax,r8d
   0x00000000004015aa <+58>:    ret
#endif

	int const val = (0x1)<<(n);
	return (N & ~val) | ( val>>1 );
#if 0
   0x000000000040158d <+29>:    mov    $0x1,%edx
   0x0000000000401592 <+34>:    shl    %cl,%edx
   0x0000000000401594 <+36>:    mov    %edx,%ecx
   0x0000000000401596 <+38>:    sar    %edx
   0x0000000000401598 <+40>:    not    %ecx
   0x000000000040159a <+42>:    and    %ecx,%eax
   0x000000000040159c <+44>:    or     %edx,%eax
   0x000000000040159e <+46>:    retq
#endif