Re: Option to make unsigned->signed conversion always well-defined?

[Andrew Haley <aph at redhat dot com>]

On 10/06/2011 07:24 PM, Ulf Magnusson wrote:
> bool overflow_unsafe(unsigned int a, unsigned int b) {
>     const unsigned int sum = (int8_t)a + (int8_t)b;
>     return (int8_t)sum != sum;
> }
> 
> bool overflow_safe(unsigned int a, unsigned int b) {
>     const int sum = as_signed_8(a) + as_signed_8(b);
>     return as_signed_8(sum) != sum;
> }

I get this:

overflow:
	sxtb	r1, r1
	sxtab	r1, r1, r0
	sxtb	r0, r1
	subs	r0, r0, r1
	it	ne
	movne	r0, #1
	bx	lr

It's not obvious to me how you'd improve on it.

Andrew.