Re: New no-undefined-overflow branch

[Richard Guenther <richard dot guenther at gmail dot com>]

On Fri, Mar 6, 2009 at 3:29 PM, Paolo Bonzini <bonzini@gnu.org> wrote:
>
>> So while trapping variants can certainly be introduced it looks like
>> this task may be more difficult.
>
> I don't think you need to introduce trapping tree codes. ?You can
> introduce them directly in the front-end as
>
> ? s = x +nv y

I think this should be

  s = x + y

otherwise the compiler can assume that for the following check
the addition did not overflow.

> ? (((s ^ x) & (s ^ y)) < 0) ? trap () : s
>
> ? d = x -nv y
> ? (((d ^ x) & (x ^ y)) < 0) ? trap () : d
>
> ? (b == INT_MIN ? trap () : -nv b)
>
> ? (int)((long long) a * (long long) b) == a *nv b ? trap () : a *nv b
>
> Making sure they are compiled efficiently is another story, but
> especially for the sake of LTO I think this is the way to go.

I agree.  Btw, for the addition case we generate

        leal    (%rsi,%rdi), %eax
        xorl    %eax, %esi
        xorl    %eax, %edi
        testl   %edi, %esi
        jns     .L2
                .value  0x0b0f
.L2:
        rep
        ret

which isn't too bad.

Richard.