How to force an ADD asm instruction (x64)?

[Jeffrey Walton <noloader at gmail dot com>]

Hi All,

I observed unexpected results while testing an an optimized x64
routine (using -g and -O2). It seems on x86_64, adds can be optimized
as follows (using LEA):

# Adding 10 + 10
...
0x0000000100000d44 <main+20>:	mov    esi,0xa
0x0000000100000d49 <main+25>:	mov    edi,0xa
0x0000000100000d4e <main+30>:	call   0x100000ce0 <sadd>

#    int sadd(int a, int b, int* r);;
#    result is really a bool, {pro|epi}logue omitted
0x0000000100000ce4 <sadd+4>:	jo     0x100000ce8 <sadd+8>
0x0000000100000ce6 <sadd+6>:	jmp    0x100000ce8 <sadd+8>
0x0000000100000ce8 <sadd+8>:	test   rdx,rdx
0x0000000100000ceb <sadd+11>:	je     0x100000cf2 <sadd+18>
0x0000000100000ced <sadd+13>:	lea    eax,[rsi+rdi]
0x0000000100000cf0 <sadd+16>:	mov    DWORD PTR [rdx],eax
0x0000000100000cf2 <sadd+18>:	xor    eax,eax

It seems an add is performed via the load effective address:
    lea    eax,[rsi+rdi]

Worse, the test for overflow is performed before the add:
    jo     0x100000ce8 <sadd+8>
    ....
    lea    eax,[rsi+rdi]

Any ideas on what knob turning I should perform? The C/C++ source for
`sadd` is below. There's not much to it - just a test on an x86 flag.

Jeff

int sadd(int a, int b, int* r)
{	
	int overflow = 0;
	int result = a + b;
	
	asm volatile("jo 1f");
	asm volatile("jmp 2f");
	
	asm volatile("1:");
	overflow = 1;
	
	asm volatile("2:");
	if(r)
		*r = result;
	
	// TRUE (1) if safe to use
	return !overflow;
}