On Thu, Jul 5, 2018 at 9:35 AM Aldy Hernandez <al...@redhat.com> wrote:
>
> The reason for this patch are the changes showcased in tree-vrp.c.
> Basically I'd like to discourage rolling our own overflow and underflow
> calculation when doing wide int arithmetic. We should have a
> centralized place for this, that is-- in the wide int code itself ;-).
>
> The only cases I care about are plus/minus, which I have implemented,
> but we also get division for free, since AFAICT, division can only
> positive overflow:
>
> -MIN / -1 => +OVERFLOW
>
> Multiplication OTOH, can underflow, but I've not implemented it because
> we have no uses for it. I have added a note in the code explaining this.
>
> Originally I tried to only change plus/minus, but that made code that
> dealt with plus/minus in addition to div or mult a lot uglier. You'd
> have to special case "int overflow_for_add_stuff" and "bool
> overflow_for_everything_else". Changing everything to int, makes things
> consistent.
>
> Note: I have left poly-int as is, with its concept of yes/no for
> overflow. I can adapt this as well if desired.
>
> Tested on x86-64 Linux.
>
> OK for trunk?
looks all straight-forward but the following:
else if (op1)
{
if (minus_p)
- {
- wi = -wi::to_wide (op1);
-
- /* Check for overflow. */
- if (sgn == SIGNED
- && wi::neg_p (wi::to_wide (op1))
- && wi::neg_p (wi))
- ovf = 1;
- else if (sgn == UNSIGNED && wi::to_wide (op1) != 0)
- ovf = -1;
- }
+ wi = wi::neg (wi::to_wide (op1));
else
wi = wi::to_wide (op1);
you fail to handle - -INT_MIN.
Given the fact that for multiplication (or others, didn't look too close)
you didn't implement the direction indicator I wonder if it would be more
appropriate to do
enum ovfl { OVFL_NONE = 0, OVFL_UNDERFLOW = -1, OVFL_OVERFLOW = 1,
OVFL_UNKNOWN = 2 };
and tell us the "truth" here?
Hopefully if (overflow) will still work with that.
Otherwise can you please add a toplevel comment to wide-int.h as to what the
overflow result semantically is for a) SIGNED and b) UNSIGNED operations?
Thanks,
Richard.
