Re: INT_MIN % -1

[Paul Schlie <schlie@comcast.net>]

With reference to the below:

I would guess that rather than attempting to expose a check for a / or % by
-1 to the compiler, I would assume that it's best handled by the back end
during code generations, as the most efficient solution is target specific:

- for targets which have hardware / and/or % instruction support and handle
  things properly, nothing special is required other than a correctly coded
  .md file.

- for targets which have hardware/ and/or % instruction support, but don't
  handle things properly, it's likely sufficient to incorporate a test for
  a -1 divisor into the .md description of the operations (which fortunately
  will likely only minimally affect performance, as / and % functions tend
  to often be multi-cycle to begin with, if supported at all).

- for targets which don't have hardware / and/or % instruction support,
  it's likely simplest to trasparently incorporate correct behavior into
  the libgcc2 built-in / and/or % function definitions.

Independently, in circumstances where either the dividend or divisor are
constants, the middle-end may always choose to eliminate the / or %
operations altogether, if it's result can be determined statically.


> From: Robert Dewar <dewar@gnat.com>
> 
>> Paul Schlie wrote:
>> 
>> (where I'm assuming: (-INT_MIN / -1) => -1 isn't reasonably correct or
>> necessary, nor is a run-time exception/trap)
>> 
>> Although possibly naive, I'd like to think that the C standard won't be used
>> as a crutch, but that GCC may rise above many of the unspecified behaviors,
>> and establish instead, well defined logically consistent useful ones, which
>> others may aspire to emulate.
> 
> One possibility would be to have an optional divide by zero trap that sets
> the right result and continues. Then there is a special compiler switch
> that sets up this trap routine if you really really really want this not
> very useful marginal behavior (a real division by zero is undefined, so
> it is fine to do anything you like). That being said, GNAT (the Ada front
> end) does go to the trouble of doing this right. Given the source program:
> 
>> procedure K is
>>    X, Y : Integer;
>>    function Id (X : Integer) return Integer is begin return X; end Id;
>> begin
>>    X := Id (Integer'First);
>>    Y := Id ( - 1);
>>    X := X mod y;
>> end;
> 
> (the Id function is to stop the compiler from doing optimizing value tracing
> :-)
> 
> The generated code (-gnatG output) (with checks off to simplifty) looks like:
> 
> procedure k is
>     x : integer;
>     y : integer;
> 
>     function k__id (x : integer) return integer is
>     begin
>        return x;
>     end k__id;
> begin
>     x := k__id (-16#8000_0000#);
>     y := k__id (-1);
>     x := (if y = -1 then 0 else x mod y);
>     return;
> end k;
> 
> The test for y = -1 is precisely to handle this case, and it is only one test.
> Note that the cost in Ada is generally smaller than in C, because we know more
> about the range of variables. For example, the Ada program:
> 
> procedure K is
>     X, Y : Integer range -1000 .. +1000;
>     function Id (X  : Integer) return Integer is begin return X; end Id;
> begin
>     X := Id (1);
>     Y := Id ( - 1);
>     X := X mod y;
> end;
> 
> Does not generate the check for -1, because it knows that X cannot be
> Integer'First.
>